| 1D & 2D dot arrays (3+ dots) |
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Charge sensing in a FDSOI dual-nanowire hole-qubit device
Victor Millory1, Boris Brun-Barriere1, Xavier Jehl1, Romain Maurand1, Benoit Bertrand2, Heimanu Niebojewski2, Louis Hutin2, Silvano De Franceschi1 1Univ. Grenoble-Alpes, CEA, Grenoble INP, IRIG-Pheliqs, Lateqs, Grenoble, France, 2Univ. Grenoble-Alpes, CEA LETI, Minatec Campus, Grenoble, France Abstract: The scalability of silicon manufacturing technologies and the ease of manipulation via electric-dipole spin resonance [1] make hole spin qubits in Si MOS quantum dots an attractive physical platform for the development of quantum processors. Here we report on a novel, dual-nanowire device geometry for the realization of linear arrays of hole spin qubits based on 300-mm fully-depleted silicon-on-insulator (FDSOI) technology. It consists of two parallel, closely-spaced silicon nanowires, fully isolated from each other by a deep dry-etching process. A two-layer gate stack enables independent charge control in the two nanowires (see Figs. (a),(b),(c)). In this novel device geometry, we demonstrate charge control down to the few-hole regime, as well as the tunability of the coupling between adjacent quantum dots along the same nanowire. We also investigate an operation mode where the accumulation and control of few-hole quantum dots in one nanowire is probed by means of a charge-sensing dot formed in the facing nanowire. We validate this operation mode by directly comparing the performance (including signal-to-noise ratio) of such a remote charge sensor (bottom nanowire in (a)) to that of a collinear charge sensor along the same nanowire (upper nanowire in (a)). Representative charge-sensing measurements are shown in Figs. (d) and (e). In both cases, charge readout is performed via source- or drain-coupled rf reflectometry. This work marks an important step toward the realization of scalable one-dimensional arrays of hole-spin qubits based on FDSOI technology [2]. [1] A. Crippa et al. Gate-reflectometry dispersive readout and coherent control of a spin qubit in silicon. Nat Commun 10, 2776 (2019). [2] A. Aprà et al, Dispersive vs charge-sensing readout for linear quantum registers, 2020 IEEE International Electron Devices Meeting (IEDM), 38.4. 1-38.4. 4 |
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Local control of interface charge density in Si/SiGe quantum dot devices through in-situ illumination
Jared Benson1, Sanghyeok Park1, Owen Eskandari1, M. Wolfe1, Brighton Coe1, J. Dodson1, S. Coppersmith2, Mark Friesen1, M. Eriksson1 1University of Wisconsin - Madison, 2University of New South Wales Abstract: Semiconductor quantum dot qubits often require time-consuming tune-up procedures to account for the variations resulting from small imperfections in device fabrication. Here, we present a method by which we can controllably and repeatably alter the trapped charge distribution at the oxide-semiconductor interface in a Si/SiGe quantum dot device, and as a result, we can make the device behave much more uniformly. The method relies on illumination with near-infrared light in the presence of applied gate voltages, and it enables the tuning of the device operating point on a gate-by-gate basis. For example, Fig. 1(a) shows pinch-off voltages for each gate. The blue diamonds show pinch-offs that have been made very uniform. To show the power of the technique, the green circles show that each pinch-off voltage can be tuned independently. We present an explanation of the underlying physics with the help of self-consistent Schrödinger-Poisson simulations (Fig. 1(b)). These simulations also provide an excellent understanding of the pinch-off curve behavior, as shown in Fig. 1(c). As an application of this method, we tune the triple-dot qubit to have extremely uniform operating voltages in the (1,1,1) charge configuration, as shown in Fig. 1(d). Further, these voltages are small, so that a future integrated digital-to-analog converter would only need to provide small output voltages. Importantly, we also show that shifting the device operating voltages in this way has no significant impact on the measured charge noise. |
| Automated tuning |
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Ray-based classification techniques for scalable quantum-dot autotuning
Justyna Zwolak1 and Donovan Buterakos2 1NIST, 2Joint Center for Quantum Information and Computer Science, University of Maryland, College Park, Maryland 20742, USA Abstract: Tuning quantum dot (QD) devices is a time-consuming task that becomes increasingly difficult as the devices increase in size and complexity. As the number of gates and the size of the parameter space that needs to be explored during tuning grows, traditional measurement techniques, relying on complete or near-complete exploration via two-parameter scans (images) of the device response, quickly become impractical. In recent years, machine learning (ML) techniques have shown the potential to support the automation of the device-tuning process. Using an ML approach dubbed the "ray-based classification (RBC) framework,'' we implement a classifier for QD states for two-, three-, and four-QD systems. The simulation used to generate data to train and benchmark the performance of the RBC relies on the Thomas-Fermi approximation. The data is collected along rays in various configurations, as shown in Fig. 1(a). We demonstrate that for each case, the model can learn the region of parameter space that a device is in (i.e., whether QD is empty / filled / overfilled) using only a small number of local measurements along these rays; see Fig. 1(b) for the case of triple-QD device. This information can then be used to determine how gate voltages need to be adjusted to reach the desired charge state. The reduction in measurement cost is a significant gain for time-intensive QD measurements and is a step toward the scalability of these devices. |
| Control and gates (1 to 2 qubits) |
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Systematic high-fidelity operation and transfer for semiconductor spin-qubits
Maximilian Rimbach-Russ QuTech, TU Delft Abstract: Spin-based semiconductor quantum dots qubits are a promising candidate for long-term applications in quantum information processing [1]. Their similarity to classical transistor allows for lithographic fabrication techniques [2], relevant for scaling to fault-tolerant device sizes. Additionally, using silicon electrons or germanium holes as the host material for the QDs allows for significant longer decoherence times due to the low abundance of nuclear spins. One common feature of such spin qubits, however, is the need for electric control on the nanoscale for operation, which also couples the qubits to electrical noise sources. Recent developments have shown that high-fidelity shuttling, movement of the charge carrier while preserving spin-coherence, can be experimentally realized [3,4]. Such charge carrier shuttling can be used to enable intermediate-distance connections. At the same time, a spin-non-adiabatic implementation can be used to enable power efficient and high-fidelity quantum control [5]. Such spin-non-adiabatic dynamics can be realized through intrinsic and/or artificial spin-orbit interaction. Systematic high-fidelity operations can be achieved by static protected against decoherence through operating in regimes that provide high protection against noise electric noise sources, so-called sweet spots [6]. Furthermore, optimized pulse control allows for an additional dynamic protection against error sources, e.g. electric noise and non-adiabatic errors [7]. [1] G. Burkard, T.D. Ladd, A. Pan, J.M. Nichol, and J.R. Petta, Rev. Mod. Phys. 95, 025003, 2023 [2] A.M.J. Zwerver et al., Nat Electron 5, 3, 3, 2022 [3] F. van Riggelen-Doelman et al., arXiv:2308.02406. [4] C.-A. Wang et al., arXiv:2402.18382. [5] Matsumoto, Y., et al. Bucket brigade and conveyor-mode coherent electron spin shuttling in Si/SiGe quantum dots, Bulletin of the American Physical Society 2024. [6] C.-A. Wang, G. Scappucci, M. Veldhorst, and M. Russ, arXiv:2208.04795. [7] M. Rimbach-Russ, S.G.J. Philips, X. Xue, and L.M.K. Vandersypen, Quantum Sci. Technol. 8, 045025, 2023. |
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Symmetrically controlling exchange coupling with only plunger gate
Alexander Ivlev1, Daniel Crielaard1, Marcel Meyer1, William Lawrie2, Nico Hendrickx1, Amir Sammak3, Giordano Scappucci1, Corentin Déprez1, Menno Veldhorst1 1Delft University of Technology, 2University of Copenhagen, 3TNO Abstract: Ever since the first realisations of coupled quantum dot systems, dedicated barrier gates were implemented to control the interdot tunnel coupling. These gates were intended to adjust the height of the electrostatic energy-barrier between the quantum dots, while ideally having minimal effect on the quantum dot potential (attached figure 1a). While these specialised gates provide a high degree of control over the tunnel coupling, they require additional lines standing in the way of scalability. A second established method is to use the detuning to change the tunnel coupling, but this suffers from increased qubit dephasing due to the departure from the charge symmetry point. Inspired by recent developments towards sparse quantum dot occupation, we have studied a new method of adjusting the tunnel coupling in a symmetric fashion. Instead of altering the height of the interdot barrier directly, we changing the depth of the quantum dots confining potential using the plunger gates (attached figure 1b). We use this principle to alter the exchange coupling in a germanium hole spin qubit array and observe that it is possible to change the exchange coupling over 50 times by pulsing solely on the plunger gate voltages. In the attached figure 2 a fingerprint plot is produced depicting the exchange tunability as a function of detuning epsilon and total on-site energy U. These fingerprint plots look similar to the ones typically made with barrier control, although their voltage direction is flipped. Coherence is maintained as the plunger gate voltages are altered by several hundred millivolt allowing us to perform a controlled phase gate using solely the plungers while staying in a charge symmetry point. This development removes the need for individual barrier control, opening new opportunities for scalable spin qubit architecture. |
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Nonadiabatic geometric gates in a planar Germanium quantum dot device
Yuchen Zhou1, Hai-Ou Li2, Guo-Ping Guo2 1University of Science and Technology of China, 2CAS Key Laboratory of Quantum Information, University of Science and Technology of China Abstract: Achieving high fidelity and robust qubit manipulations is a key requirement of realizing scalable and fault-tolerant quantum computation. Here we demonstrate single spin qubit in a planar strain Germanium quantum dot device and characterize its fidelity using gate set tomography. We find the control fidelity of X/2 gate and Y/2 gate is above 99% whereas that of I gate is only 98.24%. As qubit manipulation more slowly, the control fidelity of I gate become lower which indicates the main noise source in our system is off-resonance error. To resolve the problem caused by noise, we introduce geometric quantum computation to realize high control fidelity according to its inherent error tolerance. In a range of Rabi frequency from 7MHz to 20MHz, the control fidelity of the geometric quantum gates is consistently above 99%. Furthermore, although detuning the qubit frequency by ±2.5MHz (±1.2MHz), the control fidelity of geometric X/2 and Y/2 (I) gates are still able to be above 99%, while dynamical gates are not, which experimentally verify the robustness of geometric quantum gate. These results provide proof of the concept that geometric quantum gate can efficiently alleviate the requirement of qubit frequency calibration and is a potential method to achieving high fidelity qubit control in semiconductor quantum computation. |
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Numerical Noise Simulation Based on Gate Set Tomography
Ao-Ran Li, Ning Chu, Yuchen Zhou, Hai-Ou Li University of Science and Technology of China Abstract: With the development of large-scale fault-tolerant quantum computing, the gate fidelity has reached about 99.9%. However, the benchmarking (RB) method of fidelity analysis is no longer meet the requirement of fast measurement, due to the limitations of the information RB can provide. The gate set tomography (GST) has shown its prospect in qubit operation and noise reduction in the recent few years. In our work, we focus on the stochastic error, which is the main reason for limiting the fidelity of single qubit gate in the single-qubit Ge/GeSi heterostructure device. Due to the relatively short qubit coherence time in Ge/GeSi system, we use the Carr-Purcell-Meiboom-Gill (CPMG) sequence to obtain the noise spectrum and Bayesian estimation to generate corresponding qubit frequency noise for GST sequence simulation. When imposing noise with the same amplitude on the qubit, we observe different fidelity trends for the I-gate and X-gate as the frequency of the noise varies. By selecting appropriate parameters, our simulation fits the experiment results well. Turning our attention to two-qubits in silicon quantum dots, our emphasis is on analyzing Hamiltonian errors. By simulating of the two-qubit gates with always-on exchange coupling and comparing the PTM (Pauli transfer matrix) errors from the GST report, we can estimate errors in 9 physical parameters: the Lamour frequency errors for qubits 1 and 2, microwave amplitude calibration errors, crosstalk, J-measurement errors, dEz-measurement errors, phase accumulation in two subspaces, and virtual Z-gate phase errors. By analyzing the noise spectrum of single qubit gate, we believe that path 2 of the geometric gate will greatly improve the gate fidelity. Meanwhile, more than 75% of the Hamiltonian errors in CNOT gate can be effectively suppressed, if we can correct the 9 physical parameters introduced above, even without calibrating any other phases. |
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A diverse set of two-qubit gates for spin qubits in semiconductor quantum dots
Hai-Ou Li, Ming Ni, Guo-Ping Guo University of Science and Technology of China Abstract: To realize large-scale quantum processes, device structure designs and quantum circuit compilations prefer the ability to implement diverse types of two-qubit gates. For spin qubits in semiconductor quantum dots, the Heisenberg interaction between spin qubits empowers the realization of two-qubit gates under specific parameter requirements. However, how to simultaneously realize a diverse set of two-qubit gates in an easily available parameter region remains an intriguing problem. Here, taking advantage of the inherent exchange interaction between spin qubits, we propose and verify a fast composite two-qubit gate scheme to extend the available two-qubit gates as well as reduce the requirements for device tunability. Apart from the formerly proposed CPhase (controlled-phase) gates and SWAP gates, theoretical results indicate that the iSWAP-family gate and Fermionic simulation (fSim) gate set are additionally available for spin qubits, and the device tunability requirements for them are easily available. Furthermore, experimental results perfectly match the simulation results, verifying our composite two-qubit gate scheme and indicating the flexible two-qubit gate operation brought by the gate scheme. With this fast composite gate scheme, broad-spectrum two-qubit operations allow us to efficiently utilize the hardware and the underlying physics resources, helping accelerate and broaden the scope of the upcoming noise intermediate-scale quantum (NISQ) computing. |
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Benchmarking Single-Qubit Gate with 99.99% Fidelity via Pulse Shaping in a Five-Qubit Spin Qubit Device
Yi-Hsien Wu1, Leon Camenzind1, Patrick Bütler1, Akito Noiri1, Kenta Takeda1, Takashi Nakajima1, Takashi Kobayashi1, Ik Kyeong Jin1, Amir Sammak2, Giordano Scappucci3, Seigo Tarucha1 1RIKEN, 2QuTech and the Netherlands Organisation for Applied Scientific Research (TNO), 3QuTech and Kavli Institute of Nanoscience Abstract: Implementing fault-tolerant code is essential for achieving large-scale quantum computing. To implement fault-tolerant codes, we need qubits with gate operation fidelity above a certain threshold, usually 99%, under a common decoding scheme. Improving this gate fidelity will reduce the number of physical qubits required to achieve the same logical error rates. Here, we investigate the fidelity of single-qubit gates for five qubits in a five-qubit device in Si/SiGe with micromagnets. We compare single-qubit gates with different control pulses and achieve single-qubit gate fidelities >99.99% with a Kaiser envelope pulse for all five qubits consistently. We investigate the effect of driving speeds and interval times on the gate fidelities with both rectangular and Kaiser shaped pulses. One of the proposed benefits of the Kaiser envelope pulse is that it reduces the crosstalk error between neighboring qubits. While Qubit Q3 achieves >99.99% fidelity with pulses of both shapes, the other qubit pairs show single qubit randomized benchmarking (RB) gate fidelities of 99.9% for rectangular and 99.99% for Kaiser shaped pulse. We conclude that the rectangular pulse introduces a small crosstalk error, which causes gate-dependent PSB readout error. This small crosstalk error rotates the spin state of the qubit used for PSB projection and induces decay in the RB sequence fidelity, causing the gate fidelity measured by RB to be negatively biased. We use PSB to read parity information of the outer qubit pairs and this discrepancy is observed for qubit Q1, Q2, Q4 and Q5. Qubit Q3, on the other hand, remains unaffected by this crosstalk as it is read out with QND. We expect the shaped envelope pulse to also improve fidelity of simultaneously driven single qubit gates. These ultra-high-fidelity gates will improve the prospect of implementing fault-tolerant codes with silicon spin qubits. |
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Fidelity of Strongly Driven Electric Dipole Spin Resonance
Yasuhiro Tokura University of Tsukuba Abstract: The electric dipole spin resonance (EDSR) is a versatile method for the coherent manipulation of electron spins confined in quantum dots (QDs) by the microwave electric field. This requires mechanisms to couple the orbital motion with the spin degree of freedom. One of the proposed mechanisms is to utilize the slanting magnetic field, which had enabled many recent experimental studies. Another dominating mechanism uses intrinsic or extrinsic spin-orbit interactions (SOIs). In a standard analysis of EDSR, the Rabi frequency at resonant condition is proportional to the strength of spin-orbital coupling and the amplitude of the applied electric field.This relation holds in the lowest (linear) order in the linear slanting magnetic field and linear-to-momentum SOI system, for example for electron spins with Rashba and linear Dresselhaus SOI in GaAs/AlGaAs heterostructures. In contrast, the behavior of the EDSR Rabi frequency for non-linear slanting field, cubic-to-momentum SOI or with higher-order spin-orbital couplings shows non-linear microwave amplitude dependence. In this report, we have studied the fidelity of the EDSR for such non-linear conditions. We assume the orbital motion can be modeled in Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equation and is maintained in a non-equilibrium steady state by the microwave excitation and relaxation to the environment. Regarding large difference in the time scales for the spin dynamics and orbital dynamics, we formulate the Redfield master equation for the spin dynamics in the second order perturbation theory in the residual coupling between the electron spin and orbital motion in the steady state condition. This perturbation is more enhanced for larger microwave amplitudes and degrades the spin manipulation fidelity. We would also like to discuss the effect of excitation to different valley by the larger microwave amplitude. Part of this work is supported by JSPS Kakenhi (23H05455, 23H05458) and JST's Moonshot R¥&D (Grant No. JP-MJMS2061). |
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Measurement Techniques for Silicon Qubits
Chih-Hwan Yang UNSW Sydney Abstract: Since Bruce Kane's seminal proposal of a silicon-based quantum computer in 1998[1], significant advancements have been made, including the realisation of donor-based[2] and quantum dot[3] qubits. Today, silicon-based spin qubits are progressing towards scalability, potentially up to millions of qubits. This poster will dive into some aspects of qubit operations that are sometimes overlooked in experimental settings, providing insights into the complexity of qubit measurement techniques in silicon based qubits. [1] Kane, B.E. A silicon-based nuclear spin quantum computer. Nature 393, 133 (1998). [2] Pla, J. J. et al. A single-atom electron spin qubit in silicon, Nature 489, 541 (2012). [3] Veldhorst, M. et al. An addressable quantum dot qubit with fault-tolerant control-fidelity, Nature Nanotechnology 9, 981 (2014). |
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Dressed singlet-triplet qubit in planar germanium
Alexei Orekhov1, Kostas Tsoukalas2, Uwe Lüpke3, Felix Schupp2, Matthias Mergenthaler3, Stephan Paredes3, Gian Salis3, Andreas Fuhrer2, Patrick Harvey-Collard4 1IBM Research Europe - Zurich, 2IBM Research, 3IBM Research - Zurich, 4IBM Research Zurich Abstract: Gate-defined hole spin qubits in germanium are promising candidates for quantum computing. Here we demonstrate two-axis control of a dressed singlet-triplet (S-T0) qubit. We investigate different gate implementations, including frequency modulation, and discuss fidelity metrics. We observe an increase in coherence times of the dressed qubit compared to the bare S-T0 qubit, as expected from the dynamical decoupling intrinsic to the dressed driving scheme. Operating in the S-T basis enables different types of non-microwave and low-power control techniques. |
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Controlled rotations of hot hole spin qubits
Taras Patlatiuk1, Miguel Carballido1, Simon Svab1, Stefano Bosco2, Pierre Chevalier Kwon1, Rafael Eggli1, Erik Bakkers3, J. Egues4, Daniel Loss1, Dominik Zumbühl1 1University of Basel, 2QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2628 CJ Delft, Netherlands, 3Eindhoven University of Technology, 4Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, BR Abstract: We report on the first realization of conditional rotations of two exchange-coupled hole spin qubits, at the elevated temperature of 1.5 K. To this end, we tuned a double quantum dot (DQD) formed inside a Ge/Si core/shell nanowire into the regime where both hole spins can be individually addressed by two different microwave frequencies. Tuning the interdot tunnel barrier allowed us to operate the device in the regime with exchange coupling changing from 50 MHz to almost 300 MHz for different DQD detunings - a basic requirement to perform exchange-based two-qubit gates. As a proof of principle, we performed conditional rotation(s) of two spins at 1.5K. Limited coherence resulted in qubit resonances with the linewidth larger than the exchange splitting J. Thus, in order to extract the magnitude of J we performed a two-tone spectroscopy, allowing us to transfer the spin-occupation of the DQD between the four possible spin states. Using Pauli spin blockade, the weakly coupled DQD is initialized in one of the two spin-polarized Triplet states with equal probabilities. The other two states with antiparallel spin orientations are not blocked and produce current through the DQD during the readout. We also developed a simple theoretical model that includes spin-orbit interaction and isotropic exchange which was able to capture our experimental observations. Our experimental results lay the foundation for the next required step towards large-scale quantum computation with hole spins. Funding Acknowledgement: Supported by NCCR SPIN of the SNSF, SNI, EMP Nr. 824109, FET TOPSQUAD Nr. 862046 and G. H. E. Foundation |
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Characterization of SiMOS quantum dots fabricated by advanced semiconductor fab
Petar Tomic1, Wei Huang1, Artem Denisov1, Bart Raes2, Clement Godfrin2, Stefan Kubicek2, Kristiaan de Greve2, Thomas Ihn1, Klaus Ensslin1 1ETH Zürich, 8093 Zürich, Switzerland, 2IMEC, 3001 Leuven, Belgium Abstract: Fault-tolerant quantum computers based on silicon quantum dot (Si QD) spin qubits will require building reliable and reproducible large-scale quantum dot arrays [1,2]. Due to small length scales, the fabrication in university cleanrooms is pushed to the edge of capabilities, resulting in small yield and poor uniformity. However, the compatibility of Si QD structures with the modern CMOS fabrication industry offers a solution to scalability and improving yield and uniformity [3]. In this work, we characterize a Si MOS triple quantum dot array fabricated in an IMEC 300 mm state-of-the-art cleanroom [4]. Single and double dot regimes were investigated using two charge detectors. Excellent charge stability is achieved, requiring minimal retuning only on weekly basis. [1] Vandersypen, Lieven MK, and Mark A. Eriksson. "Quantum computing with semiconductor spins." Physics Today 72 (2019): 8-38. [2] Burkard, Guido, et al. "Semiconductor spin qubits." Reviews of Modern Physics 95.2 (2023): 025003. [3] Neyens, Samuel, et al. "Probing single electrons across 300-mm spin qubit wafers." Nature 629.8010 (2024): 80-85. [4] Lorenzelli, Francesco, et al. "Wafer-Scale Electrical Characterization of Silicon Quantum Dots from Room to Low Temperatures." 2023 IEEE ITC. IEEE, 2023. |
| Cryo-electronics |
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22FDX® as a platform for qubit devices
Giselle Elbaz1, Pierre-Louis Julliard1, Mikael Cassé2, Heimanu Niebojewski2, Benoit Bertrand2, Gregoire Roussely2, Valentin Labracherie2, Maud Vinet1, Tristan Meunier3, Bruna Cardoso Paz1 1Quobly, 2CEA-LETI, 3Institut Néel/Quobly Abstract: In comparison to other industry-compatible, Si-based qubit technologies, Fully Depleted Silicon-On-Insulator (FDSOI) has a unique back gate which offers very strong control over the transistor threshold voltage (VTH), particularly with forward back biasing (FBB). This is an important advantage considering that the power restrictions imposed on quantum System-on-Chip (SoC), where qubits are co-integrated with cryoelectronics in 22FDX®, necessitates low operating VTH for qubits and control cryoelectronics alike. To date, quantum dot (QD) formation in 300mm FDSOI has been achieved in industrial-grade devices but in commercial FDSOI technologies it has been limited to simple transistors; attempts to recreate this within a qubit have thus far been unsuccessful. In this work we take the first steps to extend 22FDX® CMOS towards a quantum platform by using FBB. On the commercial 22FDX® platform, we show for the first time the formation of two coupled quantum dots in devices expressly designed for spin qubits (see Fig. 1). This was achieved through an extensive study of integrated test structures, with a focus on MOS characteristics as a function of front-gate equivalent oxide thickness (EOT), back bias, and temperature. Using standard MOS parameters, we study three important figures of merit for generating high-quality qubits: body factor, gate leakage, and mobility. In combination with room- and low-temperature TCAD simulations, this systematic study provides insights into potential technology optimization, showing the advantages of leveraging FBB in a 22FDX® SoC qubit platform, and sheds light into some of the transport mechanisms at play in both cryoCMOS MOSFETs and qubits alike at cryogenic temperatures. |
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Toward the approach of passive photonic link in quantum computers
Patrick Vliex, Santosh Mutum, Mario Schlösser, Stefan van Waasen Forschungszentrum Juelich GmbH Abstract: In Quantum computing, ensuring the stability of qubits is crucial due to their extreme temperature sensitivity. To achieve this, qubits are kept at millikelvin temperatures to minimize thermal disruptions, while high frequency microwave signals with broad data bandwidth are necessary to drive them effectively. Currently, the prevalent approach involves using robust coaxial cables to transmit signals between room temperature electronics and the cryogenic environment where qubits reside. However, as quantum computers scale up with more qubits, coaxial cables face limitations due to cabling bottlenecks and thermal issues, prompting exploration of photonic links as a promising alternative. RF photonics has shown advanced capabilities at room temperature, offering high signal quality, low noise, and significant bandwidth compared to coaxial cables. Therefore, investigating the performance of RF photonics in cryogenic conditions becomes imperative. This study presents findings on the performance of a photonic link utilizing laser photodiodes, demonstrating the behavior of Silicon and InGaAs photodiodes when subjected to high frequency signals in a cryogenic environment. Importantly, this photonic link has the capability to directly drive both qubits and electronics in the 4K stage passively without any biasing. Acknowledgements: This work was funded by the German Federal Ministry of Education and Research (BMBF), funding program "Quantum technologies - from basic research to market", project QSolid (Grant No. 13N16149). |
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Distributed Bragg Reflectors (DBRs) for cryo-packaging using cryo-electronics
Isabelle Sprave1, Denny Duetz2, Sebastian Kock2, Hendrik Bluhm2 1Forschungszentrum Juelich GmbH, 2RWTH Aachen University Abstract: Scaling the number of qubits in spin qubit architectures is challenged by the need for multiple individually addressable gates for electrical control, resulting in considerable connection overhead. A promising solution involves integrating cryogenic electronics close to the qubits to manage the control of 10^6-10^8 physical qubits necessary for practical quantum computing. However, semiconductor spin qubits, which operate best below 1 K, present operational challenges due to the limited cooling capacity of modern cryostats, restricting the power budget of control electronics to a few milliwatts. This constraint could be mitigated by operating electronics at slightly higher temperatures. To achieve this, we propose using a thin thermally insulating layer, connected via superconducting through-vias, to thermally isolate the control electronics from the qubits. This allows the electronics to operate at higher temperatures without affecting the qubit environment. Our design uses thin alternating layers of materials with high acoustic impedance mismatch, similar to a photonic Bragg reflector. It functions by optimizing the destructive interference of coherent phonons, akin to a band stop filter for phonon frequencies typical at the operational temperatures of the control electronics. Our simulations suggest that an optimized Distributed Bragg Reflector (DBR) could reduce thermal power transmission to below 1 mW/cm^2, with a thickness under 10 µm, assuming phononic transport dominates thermal conduction. Initial experiments on a 600 nm thick DBR, composed of 10 alternating double layers of Ta and SiO2, show significant thermal insulation improvements compared to a standard Si reference. Despite some variability in the data, all DBR samples substantially outperformed the Si references, indicating the DBR's potential as an effective thermal insulator. |
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A qubit chip implementation technique with an active silicon interposer enabling signal control and acquisition near silicon qubits
Ryozo Takahashi1, Takuji Miki1, Yusuke Kanno2, Nobuhiro Kusuno2, Misato Taguchi1, Hiroyuki Mizuno2, Makoto Nagata1 1Kobe University, 2R&D Group, Hitachi Ltd. Abstract: A cryogenic qubit chip packaging structure has been proposed that allows control signal generation and monitoring very close to silicon spin qubits. The long and complicated signal wiring required to control silicon qubits from outside a dilution refrigerator degrades signal integrity, which limits the control fidelity of qubits. Flip-chip mounting a qubit chip on an active silicon interposer is expected to realize various functions such as signal switching and wave monitoring near the qubits by utilizing CMOS circuit embedded on the interposer. This enables the generation of pulse signals with lower noise and latency owing to reduced parasitics on the signal routes, compared to signals generated by external instruments. Furthermore, monitoring the state of the control signal in proximity to the qubits has significant potential to establish a feed-back scheme for further improving qubit control fidelity. In this work, we develop a prototype active silicon interposer equipped with signal switching circuits and analog-to-digital converters (ADCs) for pulse generation and signal acquisition, respectively. The signal switching circuits can select two input voltages and output them alternately. For the ADCs, we employ the successive approximation register (SAR) architecture, which has low power consumption thanks to its specific topology with no static current, and connect the output node of the signal switching circuits to the input of the ADCs. The interposer was installed in a dilution refrigerator at cryogenic temperature below 100 mK, and the operating waveforms of the signal switching circuits were successfully acquired by the ADCs. |
| Dopants & nuclear spin qubits |
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Quantum Fredkin gate in nuclear spin registers: a native implementation
Hermann Edlbauer, Cameron Brown, Mathis Moes, Angus Sutherland, Ian Thorvaldson, Junliang Wang, Saiful Haque Misha, Michael Jones, Fabian Pena, Yousun Chung, Charles Hill, Joris Keizer, Ludwik Kranz, Michelle Simmons Silicon Quantum Computing Abstract: Scanning tunneling microscopy (STM) enables fabrication of phosphorus registers in silicon at sub-nm length scales which allow for coherent control of nuclear and electron spins above the fault-tolerant threshold [1]. Electrically driven spin resonance (EDSR) is naturally provided between the electron and nuclei and has been used coherently, in a “flip-flop” qubit [2], and adiabatically, for nuclear spin initialisation [3]. Here, we drive coherent EDSR, for the first time, in a four-qubit nuclear spin register and assess its applicability as quantum-Fredkin gate. Our results pave the way towards all-to-all connectivity in a nuclear spin register and open up new avenues for enhanced execution of algorithms in a silicon quantum processor. Figure 1: Coherent electron spin resonance (ESR; left) drive vs. coherent EDSR (right) on a 4-qubit spin register in silicon. References: [1] Thorvaldson et al., arXiv:2404.08741 (2024) [2] Savytskyy et al., Sci.Adv. 9, 9408 (2023) [3] Reiner et al., Nat. Nanotechnol. (2023) |
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Scalable donor-based electron spin qubit unit in silicon
Shihang Zhang, Yu He, Peihao Huang Southern University of Science and Technology Abstract: Large-scale donor-based spin qubit device in silicon is promising due to its long coherence time and the technical progress of nanoscale placement of donors [1-2]. However, even the scalability of state-of-the-art devices is insufficient. There are two critical obstacles including achieving tunability of the two-qubit coupling and ensuring the addressability of computing qubits. High tunability of the two-qubit coupling can be achieved through an indirect coupling known as 'superexchange' involving a middle donor [3-4]. And the addressability of the qubit can be achieved by tuning the hyperfine interaction using an ancilla donor. Thus, an ancilla donor between two computing qubits could be used for both tunable two-qubit coupling and addressability. However, achieving addressability demands the overlap of the electron wavefunctions between the qubit donor and ancilla donor, which limits the tunability of the two-qubit coupling. In this work, we propose a scalable computing unit for donor-based electron spin qubits that incorporates an ancilla donor. In particular, we introduce an asymmetric structure that exhibits great compatibility between the tunability of two-qubit coupling and the addressability. And the fidelity of single-qubit and two-qubit gates can exceed the fault-tolerant threshold. Additionally, the asymmetric scheme can effectively resist the valley oscillation of the tunneling, with a nanoscale placement precision of donors. Consequently, the proposed scheme is a promising prototype for the large-scale fault-tolerant spin-based quantum processor. In this work, we also propose a scalable spin qubit architecture based on the asymmetric scheme. [1] J.T. Muhonen et al., Nature Nanotechnology, 9, 986 (2014). [2] Y. He et al., Nature, 571, 371 (2019). [3] V. Srinivasa et al., Physical Review Letters, 114, 226803 (2015). [4] T.A. Baart et al., Nature Nanotechnology, 12, 26 (2017). |
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Excitation Spectra of Phosphorus-, Arsenic- and Antimony-Bound Excitons in Strained 28Si
David Vogl1, Noah Braitsch1, Başak Özcan1, Martin Brandt1, M. Thewalt2 1Technische Universität München, 2Simon Fraser University Abstract: The spin properties of donors in silicon such as hyperfine coupling and quadrupole splitting depend on strain. This is particularly important for single-donor devices, where metallic gate structures can generate local stress fields, potentially varying from donor to donor. To provide reference data, we study the electronic states of P, As and Sb donors in isotopically pure 28Si crystals under uniaxial stress. The stress is applied via a linear motor controlled by a piezoelectric force gauge, allowing a precise variation of the strain generated. High resolution excitation spectra of the respective donor-bound excitons (DBE) are then measured as a function of stress and detected by capacitively observing the Auger decay-induced conductivity of the crystals. This contactless measurement technique ensures that mechanical stress is only applied to the sample by the linear motor. In this contribution, we report the dependence of the DBE spectra on stress applied along the [100] and [110] crystallographic axes at selected magnetic fields up to 1.7 Tesla and analyze the spectra based on the dependence of the valence and conduction band energies on strain, as described by the Pikus-Bir Hamiltonian and the valley repopulation model, respectively. This work is supported by the Munich Center for Quantum Science and Technology (MCQST). |
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Double P dopants in Si: Wave functions and spatial metrology from STM images
Piotr Rozanski1, Garnett Bryant2, Michal Zielinski1 1Nicolaus Copernicus University, 2NIST Abstract: The design and implementation of dopant-based silicon nanoscale devices rely heavily on knowing precisely the locations of phosphorous dopants in their host crystal. One potential solution combines scanning tunneling microscopy (STM) imaging with atomistic tight-binding simulations to reverse-engineer dopant coordinates. This work shows that such an approach may not be straightforwardly extended to double-dopant systems. We find that the ground (quasi-molecular) state of a pair of coupled phosphorous dopants often cannot be fully explained by the linear combination of single-dopant ground states. Although the contributions from excited single-dopant states are relatively small, they can lead to ambiguity in determining individual dopant positions from a multi-dopant STM image. To overcome that, we exploit knowledge about dopant-pair wave functions and propose a simple yet effective scheme for finding double-dopant positions based on STM images. |
| Fabrication |
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Locally Etched Back-gate Contact for Spin Qubit Devices in Si/SiGe
Lucas Marcogliese1, Rudolf Richter2, Ouviyan Sabapathy1, Dominique Bougeard3, Lars Schreiber1 1JARA Institute for Quantum information, 2Institut für Experimentelle und Angewandte Physik, Fakultät für Physik, Universität Regensburg, 3Institut für Experimentelle und Angewandte Physik, Fakultät für Physik, Universität Regensburg Abstract: State-of-the-art Si/SiGe spin qubit devices rely on front-side gates to electrostatically define and manipulate quantum dots. Besides providing greater control over the quantum dot potential landscape, a locally etched back-gate contact would enable tuning of the chemical potential independently of the out-of-plane electric field and could be a means to tune valley splitting Evs as well as strain effects within the heterostructure. By increasing the electron wavefunction overlap with the upper SiGe barrier, the average Evs may be enhanced and the statistical distribution of Evs can be shifted such that a larger percentage of in-plane quantum dot positions are above the required threshold for coherent shuttling [1]. Furthermore, pulses can be applied to avoid regions of low valley splitting while shuttling. The objective is to ultimately measure valley splitting maps and demonstrate tunability of the splitting with back-gate voltage. We propose to experimentally realize this operation mode using a new, locally etched back-gate that is on the order of 100 µm in diameter and located at just less than 2 µm from the quantum well (Fig. 1). The contact is fabricated using anisotropic wet etching on (001) substrates yielding shallow sidewalls compatible with directional metallization techniques such as e-beam evaporation. Owing to an etch rate which drops exponentially with Ge alloy concentration, the 2 µm SiGe virtual substrate behaves as a natural etch stop allowing one to reproducibly fabricate Si/SiGe micro-membranes. 1. Jonas R F Lima and Guido Burkard 2023 Mater. Quantum. Technol. 3 025004 |
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On-chip hybrid Coulomb-blockade thermometry
Armel Cotten, Omid Sharifi Sedeh, James Mingxi Xu, Christian Scheller, Dominik Zumbühl University of Basel Abstract: Coulomb blockade thermometers (CBTs) are very well-established electronic thermometers broadly covering the millikevin to kelvin temperature range. They can be conveniently integrated on-chip and provide accurate and fast electron temperature readout based on a simple two-terminal device not requiring any tuning. Furthermore, CBTs can operate as primary thermometers, thus reading absolute temperature without relying on another thermometer for calibration. When designed as CBT arrays with many junctions in series and in parallel, they can exhibit even better accuracy, strong immunity to voltage noise, resilience to nanofabrication issues, and can be operated even into the very challenging microkelvin temperature range [1][2]. Altogether, this is opening a range of very interesting possibilities. In this work, we establish a CBT array nanofabrication process for integration with other devices on-chip. We have fabricated Al-AlOx-Al tunnel junctions using a standard angle evaporation process and characterised them electrically as well as using atomic force microscopy. We concluded that the fabrication process achieves high-quality reproducible tunnel junctions, with a good agreement between their dimensions and electric properties. Furthermore, we have demonstrated easy and reliable operation of the CBTs down to low millikelvin temperatures. Finally, we are integrating the CBTs with a 2D electron gas to make hybrid Coulomb blockade thermometers, accessing directly the 2D gas electron temperature. By equilibrating the 2D electron gas with the well understood metallic part of a Coulomb blockade thermometer and integrating adiabatic demagnetization techniques, we are aiming to proceed to ultralow temperatures in the microkelvin range. Supported by the SNSF Grant No. 215757, the Swiss Nanoscience Institute SNI, and the EU-H2020 European Microkelvin Platform EMP. Figure caption: Topography of Al-AlOx-Al junction obtained with atomic force microscopy References: [1] CP Scheller et al., Applied Physics Letters 104(21), 211106 (2014). [2] M. Samani et al., Phys. Rev. Res. 4(3), 033225 (2022). |
| Large-scale characterization |
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Coherent errors in spin-qubit quantum error correction caused by quasistatic phase damping
David Pataki1, Áron Márton1, János Asbóth2, András Pályi3 1Budapest University of Technology and Economics, 2Budapest University of Technology and Economics, HUN-REN Wigner Research Centre for Physics, 3Budapest University of Technology and Economics, HUN-REN-BME Quantum Dynamics and Correlations Research Group Abstract: Quantum error correction is a key challenge for the development of practical quantum computers, a direction in which significant experimental progress has been made in recent years. For semiconductor spin qubits, one of the leading information loss mechanisms is dephasing, usually modeled by phase flip errors. Here, we introduce quasistatic phase damping, a more realistic error model that describes the effect of Larmor frequency fluctuations due to 1/f noise. We show how this model differs from a simple phase flip error model, in terms of multi-cycle error correction. For the surface code, utilizing the Fermionic Linear Optics simulation framework, combined with phenomenological readout errors, we provide numerical evidence for an error threshold, in the presence of quasistatic phase damping and readout errors. We discuss the implications of our results for near-term quantum error correction experiments carried out with germanium spin qubits. Namely, we predict that the break-even for the 9-qubit surface code is achievable when the data qubits have inhomogeneous dephasing time of 7 μs, and the readout error is at the 5% level using measurement time 430 ns. This research was supported by the Quantum Information National Laboratory of Hungary, and the Horizon Europe projects IGNITE and OpenSuperQPlus100. |
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Fast cryogenic probing of quantum dot spin qubit devices
Toni Berger1, Rafael Eggli1, Simon Geyer1, Felix Schupp2, Matthias Mergenthaler2, Richard Warburton1, Andreas Kuhlmann3 1Uni Basel, 2IBM Zurich, 3University of Basel Abstract: Fast feedback from cryogenic electrical characterization measurements is key for the successful development of scalable quantum computing technology. At room temperature, high-throughput device testing is accomplished with a probe-based solution, where electrical probes are repeatedly positioned onto devices for acquiring statistical data. In this work, we present a probe station that can be operated from room temperature down to 1.4 K [de Kruijf et al., Rev. Sci. Instrum. 94, 054707 (2023)]. It is designed for 2x2 cm2 chips, that are moved with respect to a multi-contact probe card using closed-loop piezo-based positioners. This prober is compact enough to fit inside a standard cryogenic magnet system and is compatible with both direct-current and radio-frequency signals, thereby making it a versatile tool perfectly suited for gathering statistical data on qubits. A large variety of electronic devices can be tested. We showcase the performance of the prober by characterizing silicon fin field-effect transistors as a host for quantum dot spin qubits [Camenzind and Geyer et al., Nat. Electron. 5, 178 (2022)]. Such a tool can massively accelerate the design-fabrication-measurement cycle and provide important feedback for process optimization toward building scalable quantum circuits. |
| New directions |
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Design and Testing of an Optimized Experimental Setup for Global Control of SiMOS Spin Qubits
Nicola Meggiato1, Ensar Vahapoglu2, Wee Han Lim2, Arne Laucht2, Jarryd Pla1, Andrew Dzurak2 1UNSW Sydney, 2UNSW Sydney / Diraq Abstract: Recent work has demonstrated the use of an off-chip potassium tantalate dielectric resonator to coherently control spin qubits in silicon by means of a global microwave magnetic field, making its way as an alternative to most traditional approaches. Even though the Rabi frequencies achieved in this demonstration (∼ 1MHz) is comparable to that achieved with an on-chip microwave antenna, advanced global control schemes envisioned to be employed in large scale devices would require higher Rabi frequencies. Prior work indicates that the measured Rabi frequency is limited not by the microwave control setup, but by the microwave losses within the qubit device. In this work, we extensively use electromagnetic simulations with a realistic 3D model of the existing experimental setup in order to pinpoint and eliminate the losses in the system. Inspired by these simulations, we design and fabricate a more robust and reliable experimental setup, as well as a less lossy microelectronic structure for the qubit devices. These improvements are expected to enhance the resonator quality factor, which ultimately determines the Rabi frequencies. New experimental results at 4 Kelvin reveal an additional source of microwave loss within our SiMOS quantum dot qubit devices resulting from the fabrication of highly doped regions in the silicon. This discovery has the potential to not only explain the limitations of the Rabi frequency achievable with a dielectric resonator but also further explain the noise profile seen by our qubits, irrespective of the microwave control technique used. Our findings further clarify the path to building large-scale silicon spin-based devices with fast and coherent qubits. |
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Two-stage magnetic shielding for hybrid quantum devices in an adiabatic demagnetization refrigerator
Lino Visser1, Marc Neis2, Jéferson Guimarães2, Markus Jerger2, Pavel Bushev2, Rami Barends2, Vincent Mourik3 1PGI-11 Forschungszentrum Jülich, 2Peter Grünberg Institut 13 - Institute for Functional Quantum Systems, Forschungszentrum Jülich GmbH, 3PGI-11, Forschungszentrum Jülich Abstract: Adiabatic demagnetization refrigeration (ADR) is a promising cooling technique for future quantum technology applications. Cooling units for ADRs are cheap and reliable while enabling base temperatures comparable to those obtained in dilution refrigerators. A challenge is the presence residual magnetic fields originating from the magnet used for recharging the paramagnetic salts, as these lower the operation fidelity of superconducting circuits. With the advance of spin qubits and the recent demonstration of long-range coupling by superconducting resonators[1,2], controlling the magnetic environment is crucial. Further, controlling this is beneficial to operate spin qubits at low fields[3] or to implement superconducting-semiconducting hybrid devices in Germanium quantum wells[4,5]. Here, we present the design of a 4 Kelvin two-stage mu-metal and Niobium magnetic shield[6] with ports for 4 superconducting RF wires, and 48 DC lines. The lowest temperature stage enters the magnetic shield through a feedthrough and contains an additional Copper radiation shield[7] around the sample space. Using finite element simulations, we quantify the magnetic shielding factor before manufacturing. To benchmark the ADRs shielding performance, we characterize a set of Niobium resonators, measuring their quality factors. First results indicate a competitive performance of these resonators in our customized set-up. To operate spin qubits, we plan on implementing a small superconducting magnet to control the field locally. We aim to achieve a reduced background field, magnetic field noise and avoid field exposure while recharging the salt pill. [1] P. Harvey-Collard et al. Phys. Rev. X 12, 021026 [2] F. Borjans et al. Nature 577, 195–198 (2020) [3] D Jirovec et al. Nat. Mater. 20, 1106–1112 (2021) [4] O. Sagi et al. arXiv:2403.16774 [5] A. Tosato et al. Commun Mater 4, 23 (2023) [6] A. Bergen et al. Rev Sci Instrum. 2016 Oct;87(10):105109 [7] R. Barends et al. Appl. Phys. Lett. 99, 113507 (2011) |
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Realization of hybrid devices in strained germanium quantum well heterostructures, proximitized by in-situ grown epitaxial aluminum
Christian Olsen1, Vera Weibel1, Luigi Ruggiero1, Pauline Drexler2, Marcus Wyss3, Alexander Vogel3, Dominique Bougeard4, Andrea Hofmann1 1Departement Physik, Universität Basel, 2Institut für Experimentelle und Angewandte Physik, Fakultät für Physik, Universität Regensburg, 3Nano Imaging Lab, Swiss Nanoscience Institute, Universität Basel, 4Institut für Experimentelle und Angewandte Physik, Fakultät für Physik, Universität Regensburg Abstract: Hybrid semiconductor-superconductor devices have shown great potential as building blocks for novel quantum applications. These range from gate controlled Transmon qubits (Gatemon) and Andreev spin qubits, to Josephson diodes and topological devices[1-4]. The fundamental blocks at the center of the aforementioned hybrid quantum devices are the Josephson junction (JJ) and superconducting microwave resonator. Recently, efforts to combine holes in germanium with superconductivity have received much attention[5-9]. The main approach has been focused on an ex-situ amalgamation of the superconducting material with the semiconducting germanium/silicon heterostructure, thereby increasing the probability of introducing extra impurities and defects that can hamper the performance of the device. We present experimental investigations of hybrid JJ's and superconducting coplanar waveguide (CPW) resonators based on a two dimensional hole gas embedded in a germanium/silicon heterostructure with an in-situ grown epitaxial aluminum film on top, see Fig. 1b. Measurements of single JJ's reveal full gate control over the critical current supported by the junction and evidence of an induced superconducting gap of ∼175μeV. Embedding two such junctions in an asymmetric SQUID let us probe the current-phase relation (CPR) of a single junction, revealing a moderately skewed CPR, see Fig. 1a. Finally, we present measurements of CPW resonators with internal quality factors up-to 10k and good in-plane magnetic field resilience. [1] Nature Nanotechnology 13, 915 (2018) [2] Science 373, 430 (2021) [3] Phys. Rev. Research 5, 033131 (2023) [4] Nature 614, 445 (2023) [5] Phys. Rev. Research 3, L022005 (2021) [6] Nature Communications Materials 4, 23 (2023) [7] ACS Nano 13, 14145 (2019) [8] Nano Lett. 19, 1023 (2019) [9] Nature Communications 15, 169 (2024) |
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Prospects for Strong Optical Coupling between Single Erbium Ions and Silicon Photonic Cavities
Justin Brown1, Alexey Lyasota2, Ian Berkman3, Gabriele de Boo2, John Bartholomew4, Shao Qi Lim5, Brett Johnson6, Jeffrey McCallum5, Bin-Bin Xu2, Shouyi Xie2, Rose Ahlefeldt7, Matthew Sellars7, Chunming Yin8, Sven Rogge2 1Centre of Excellence for Quantum Computation and Communication Technology, School of Physics, University of New South Wales, 2Centre of Excellence for Quantum Computation and Communication Technology, School of Physics, University of New South Wales, Sydney, New South Wales 2052, Australia, 3Centre of Excellence for Quantum Computation and Communication Technology, School of Physics, University of New South Wales, Sydney, New South Wales 2052, Australia - Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, 02139, Massachusetts, USA, 4Centre for Engineered Quantum Systems, School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia - The University of Sydney Nano Institute, The University of Sydney, Sydney, New South Wales 2006, Australia, 5Centre of Excellence for Quantum Computation and Communication Technology, School of Physics, University of Melbourne, Victoria 3010, Australia, 6School of Science, RMIT University, Victoria 3001, Australia, 7Centre of Excellence for Quantum Computation and Communication Technology, Research School of Physics, Australian National University, Canberra, Australian Capital Territory 0200, Australia, 8Centre of Excellence for Quantum Computation and Communication Technology, School of Physics, University of New South Wales, Sydney, New South Wales 2052, Australia - Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Microscale Magnetic Resonance and School of Physical Sciences, University of Science and Technology of China, Hefei 230026, China Abstract: Optically active spins in solids are promising for many applications in quantum Information science, such as entanglement distribution nodes in quantum networking, single photon sources for linear optical quantum computing, and as a platform for cluster state quantum computing. Their optical connectivity could also be leveraged to implement low-density parity check (LDPC) error correction codes. Erbium ions implanted in silicon are a particularly promising system, due to erbium's emission in the Telecom C band, where attenuation in optical fibres is at a minimum, and where optical components are mature. Furthermore, the maturity of silicon fabrication allows for the fabrication of optical resonators with high Q/V. We have undertaken bulk ensemble measurements of erbium in silicon, exploring how the properties of implanted erbium ions vary with the concentration of erbium, concentration of background dopants, and proximity to surfaces. By doing optical comb-based spectral hole burning measurements, we have determined an upper bound for the homogeneous linewidth of several erbium sites. Furthermore, we have extracted the dipole moment of the optical transition from power dependent spectral hole measurements. Obtained homogeneous linewidths and dipole moments of two sites have been used to predict cavity QED parameters in the single ion regime (shown in Figure 1), based on the quality factors and mode volumes of different types of photonic crystal (PC) cavities reported in the literature. This indicates that high co-operativity coupling and potentially strong coupling to a single ion should be possible using state-of-the-art silicon photonic crystal cavities. |
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Reinforcement Learning in Bayesian Hamiltonian Tracking for Dephasing-Limited Spin Qubits
Jan Krzywda and Evert van Nieuwenburg Leiden University Abstract: Quantum algorithms often involve estimation of expectation values through multiple runs of the same control procedure. However, many of the current implementations face challenges due to fluctuations in the Hamiltonian parameters caused by the presence of an uncontrolled environment. To tackle this issue, one could track and correct for varying Hamiltonian parameters in real-time. Such tracking methods are particularly efficient in Si spin qubits, where decoherence is often dominated by low-frequency noise. For nuclear noise-limited GaAs spin qubits, tracking of the Overhauser field allowed for a three-orders-of-magnitude improvement in phase gate performance [1]. However, achieving a similar effect in charge noise-limited Si-based devices would require further progress due to fast noise dynamics. Towards this goal, we have proposed a memory-efficient tracking algorithm [2] and have participated in experimental endeavours resulting in demonstration of noise driven Singlet-Triplet oscillations in the absence of a magnetic field gradient [3], and physics-informed, adaptive tracking that reduced estimation time to 0.1ms [4]. Motivated by the above advances and the possibility of running small neural networks directly on the FPGA [5], in this study, we numerically employ a reinforcement learning approach for developing a noise-informed strategy for qubit tracking in Silicon. Our method dynamically allocates resources between the estimation and execution phases, aiming to optimise the balance between data acquisition time and algorithm fidelity (See Fig. 1). The results demonstrate that machine learning can enhance the performance of quantum algorithms through learning tracking strategies that exploit correlations, filtering, and forward prediction. [1] M. Shulman et al., Nat Commun 5, 5156 (2014) [2] J. Benestad, J. A. Krzywda, E. Nieuwenburg, J. Danon, arXiv:2309.15014 (2023) [3] F. Berritta, T. Rasmussen, J.A. Krzywda et al., Nat Commun 15, 1676 (2024). [4] F. Berritta, J.A. Krzywda et al., arXiv:2404.09212 (2024) [5] K. Reuer, et al. Nat Commun 14, 7138 (2023) |
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Transport measurements in p/n doped hexagonal SiGe nanowires
Esther van de Logt1, Pim van den Berg1, Claudius Müller1, Jonas Kareem1, Denny Lamon2, Marvin Jansen2, Joost Ridderbos1, Erik Bakkers2, Floris Zwanenburg1 1University of Twente, 2Eindhoven University of Technology Abstract: In recent years, extensive research has been performed to achieve a direct bandgap in group IV semiconductors. Particularly for silicon and germanium, which are widely used is the cubic crystal phase. Achieving a direct bandgap in these materials opens exciting possibilities to combine conventional semiconductor techniques with photonics and to investigate spin photon interfaces. The hexagonal (2H) crystal phase of Si/Ge alloys, known as hex-GeSi, has emerged as a promising material. This phase exhibits a direct bandgap, in contrast to the cubic direct bandgap crystal phase [1-3]. Within the ONCHIPS and the Qumat consortium, we perform electrical transport measurements on hex-GeSi nanowires. The nanowires are grown epitaxially in a branch configuration using molecular beam epitaxy, with diameters down to 10nm [4]. An example of a device can be seen in the Figure. Furthermore, we investigate how the transport properties are affected by doping the nanowires with Ga and As. The extremely small diameters of these nanowires as well as the known properties of holes in Ge make this material platform an ideal candidate to investigate spin qubit physics [5-7]. Combined with the direct band gap in hex-GeSi we are moving closer to optically addressable spin qubits. [1] C. Rödl et al., 2019, doi: 10.1103/PhysRevMaterials.3.034602. [2] T. Kaewmaraya et al., 2017, doi: 10.1021/acs.jpcc.6b12782. [3] E. M. T. Fadaly et al., 2020, doi: 10.1038/s41586-020-2150-y. [4] A. Li et al., 2022, doi: 10.1088/1361-6528/ac9317. [5] S. Conesa-Boj et al., 2017, doi: 10.1021/acs.nanolett.6b04891. [6] F. N. M. Froning et al., 2018, doi: 10.1063/1.5042501. [7] M. Brauns, 2016, doi: 10.1063/1.4963715. |
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Characterizing GHz surface acoustic wave resonators as a quantum bus
Aldo Tarascio1, Taras Patlatiuk1, Pasquale Scarlino2, Dominik Zumbühl1 1University of Basel, 2EPFL Abstract: A robust quantum bus for efficient qubit connectivity is one of the challenges in quantum computing for scaling-up quantum systems. Recent efforts on coupling semiconductor qubits focus mainly on superconducting resonators and qubit shuttling. Surface Acoustic Wave (SAW) resonators offer promising prospects for qubit scaling due to their resilience to magnetic fields and high temperatures. Unlike conventional superconducting resonators, SAW resonators can maintain high quality factors > 1'000 in the GHz regime in magnetic fields as required for spin qubits and even at high temperatures above 1 K. This could help facilitate integration of CryoCMOS qubit control electronics. Here, we employ GaAs based SAW cavities as a testbed, chosen for their piezoelectric properties and ease of fabrication of SAW resonators through established lithography techniques. Additionally, GaAs heterostructures host high mobility 2D electron gases (2DEGs), enabling integration with quantum dots, advancing towards charge- and/or spin-phonon coupling. The interaction between qubits and SAW phonons follows the formalism of Jaynes-Cummings model used in cavity and circuit quantum electrodynamics. We evaluate cavities Q-factors for various key parameters such as cavity length, as well as transponder and mirror sizes, finding good agreement with theory and operating cavities with large Qs up to ~10k. Further, we place surface gated quantum dots on a mesa containing a 2DEG into the cavity and study their interactions, working towards strong coupling of the SAW to the charge and spin in the dot. In the future, non-piezoelectric substrates like Si or Ge can also be envisioned by applying a thin piezoelectric surface films or by using off chip resonators and wafer bonding. |
| Noise, quality metrics & material growth |
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Impact of interface traps on charge noise and low-density transport properties in Ge/SiGe heterostructures
Leonardo Massai1, Bence Hetenyi1, Matthias Mergenthaler1, Felix Schupp2, Lisa Sommer3, Stephan Paredes1, Stephen Bedell4, Patrick Harvey-Collard5, Gian Salis6, Andreas Fuhrer Janett2, Nico Hendrickx7 1IBM Research - Zurich, 2IBM Research, 3IBM Research Europe - Zurich, 4IBM Research - T.J. Watson Research Center, 5IBM Research Zurich, 6IBM, 7QuTech Abstract: Hole spins in Ge/SiGe heterostructures have emerged as an interesting new qubit platform with favorable properties. By leveraging the highly anisotropic g factor, it's possible to achieve fast high-fidelity electrical control and noise-resilient operation at sweet spots. However, in SiGe heterosturctures, commonly observed gate-induced electrostatic disorder, drifts, and hysteresis hinder reproducible tune-up of quantum dot arrays and have not yet been systematically investigated. Here, we study both Hall bar and quantum dot devices (Fig.1a) fabricated on Ge/SiGe heterostructures and we present a consistent model for the origin of gate voltage hysteresis and its impact on transport metrics and charge noise. We identify five distinct operation regimes associated with the gradual filling of a spatially varying density of charge traps at the SiGe-oxide interface when pushing gate voltages more negative. While the induced trap population initially leads to an upturn in the carrier mobility, we observe a sharp decline of low-density transport metrics as the interface trap density becomes unevenly saturated for more negative gate voltages. With each gate voltage push (Fig.1b), we also find a local activation of transient low-frequency relaxation dynamics, which leads to a Brownian noise component (Fig.1c) that completely vanishes again after 30 hours. We discuss the resilience of the SiGe material platform to both interface-trap-induced disorder and noise. We also present various ways to measure and mitigate these effects by either adapting relevant fabrication steps or by improving tune-up and measurement processes. |
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Optimising the quality of gate dielectrics in Ge hole-spin qubit devices
Simon Robson1, Yona Schell2, Jaime Saez-Mollejo2, Andrea Ballabio3, Daniel Chrastina3, Giovanni Isella3, Georgios Katsaros1 1Institute of Science and Technology Austria, 2ISTA, 3L-NESS, Physics Department, Politecnico di Milano Abstract: Continued advances in the fabrication of Ge/Si1-xGex heterostructures have led to gate-defined hole spins confined in strained germanium layers being identified as a viable qubit platform. Compared to existing spin-based qubit architectures, Ge hole qubits promise some significant advantages. These include a large spin-orbit interaction to allow all-electric control of the spin, a small effective mass to give more relaxed gate layout geometry, and a p-like orbital symmetry giving a reduced hyperfine interaction and thus favouring spin coherence. However, as the number of qubits is scaled up, increased attention to the uniformity of the substrate is required, whereby electrically-active interface defect states or nearby ionised impurity atoms in the bulk can affect transport and introduce significant sources of decoherence. Careful control and optimisation of the device production process, including feedback loops at various fabrication stages, is thus an important task for qubit scale-up. Using test structures fabricated on a Si0.3Ge0.7 substrate, we present the first results of a systematic study into the electrical properties of devices intended for use in gate-defined spin-qubit experiments. In this initial phase of investigation, attention is drawn towards the oxide acting as the primary source of charge noise, stemming from the presence of interface trapped charge and fixed oxide charge. We investigate the properties of various combinations of gate material dielectrics, such as Al2O3 and HfO2, together with additional pre- and post-growth treatment processes, such as the incorporation of a post-metallisation forming gas anneal. Based on the results of multiple electrical characterisation techniques, we show that there is an optimised set of parameters that reflects a minimum in the amount of electrically-active defect states and thus a favourable environment for which to host large-scale qubit arrays in a relatively noise-free setting. |
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Design of voltage-controlled pulses for exchange gate operations on semiconductor spin qubits with optimal fidelities in the presence of 1/f ɑ charge noise using fractional calculus
Bohdan Khromets and Jonathan Baugh University of Waterloo Abstract: Voltage-controlled spin qubits in quantum dots suffer from severe performance losses due to low-frequency 1/f ɑ charge noise. We apply the analytical framework of fractional calculus to achieve the highest average fidelities of noisy quantum gate operations. We specifically address the design of two-spin SWAPk gates via the exponential voltage control of exchange interaction. Due to the ongoing improvement in the material and device design quality, we focus on the regime where electric noises dominate decoherence. In this case, we derive that the ideal pulse in the presence of stationary noise is long, low-amplitude, and broad, with the optimal exchange pulse shape described by the symmetric beta distribution function with a parameter of 1 − ɑ/2. We numerically analyze the dependency of the average SWAPk gate infidelity on pulse shape (Fig. 1 (a)), length (Fig. 1 (b)), and noise color ɑ (Fig. 1 (c)), where we choose k=1, frequency cutoffs fmin =10 kHz, fmax =10 GHz, and fixed noise energy 1 meV2 for all noise processes. The optimal and square pulse shapes perform nearly identically while yielding lower average infidelities (up to a factor of ~4) than the Gaussian voltage pulses. The decrease of infidelity with pulse length (Fig. 1 (b)) highlights that the exponential suppression of exchange noise with low-amplitude voltage pulses outweighs the impact of longer exposure to noise (the interplay with dephasing is expected to yield a sweet spot in pulse length). As follows from Figure 1(c), pulse shape and length optimizations are most effective in mitigating weakly correlated noises with low α values. The conventional fast, high-amplitude exchange pulses are thus optimal only for the non-stationary 1/f ɑ noise processes, as we confirm for fractional Brownian motion. The proposed techniques apply to the characterization and optimization of quantum computation performance of various voltage-controlled qubit architectures. |
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Polarization-assisted QD formation by integrated CMOS-compatible HfO2-based ferroelectric gate oxide: a study at cryogenic temperatures
Niccolò Martinolli1, Fabio Bersano1, Adrian Ionescu2 1EPFL, 2Ecole Polytechnique Fédérale de Lausanne Abstract: In the context of cryogenic electronics, the stable ferroelectric remnant polarization and endurance characteristics of doped-HfO2 thin films appear as both promising and appealing. We propose preliminary investigations based on a fully depleted silicon-on-insulator (FDSOI) single electron/hole transistor (SET/SHT), where the gate stack includes a ferroelectric Si-doped HfO2 (Si:HfO2) layer (fig. 1a). The static polarization of the ferroelectric contributes to inducing electrostatic quantum confinement in the thin Si film (fig. 1b). Several material parameters need to be carefully optimized for the integration of ferroelectrics in quantum devices, through extensive characterization of model systems (fig. 1c). The domain size in Si:HfO2 and the ability of a nanometric gate to locally switch its polarization are studied through piezoresponse force microscopy (PFM). By employing 5-200 μm wide ferroelectric capacitors (FeCap) and ferroelectric field effect transistors (FeFET), we can access information on the leakage current density, remnant polarization, and polarization loss, by a combination of pulsed and quasi-static characterization, namely polarization-voltage (P-V), current-voltage (I-V) and capacitance-voltage (C-V) measurements. We report results for temperatures ranging from 10 K to 295 K. Moreover, the values of the coercive field are evaluated from room to low temperature, to address the feasibility of a dynamic ferroelectric switching at cryogenic condition. Furthermore, C-V measurements at low (1 Hz) and high (1 MHz) frequency are used for extracting the effective density of charge traps (Neff) and the interface traps density (Dit), critical values for charge noise and spin decoherence. These results are compared with those obtained by charge pumping and correlated with flicker noise and cathodoluminescence (CL) measurements. Finally, this study serves as benchmark of ferroelectric cryo-CMOS and memory devices. |
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Effects of optical illumination on Sb-donor quantum devices
Sean Hsu1, Martin Nurizzo2, Danielle Holmes2, Daniel Schwienbacher2, Andrea Morello2 1Centre for Quantum Computation and Communication Technology, School of Electrical Engineering and Telecommunications, UNSW Sydney, Sydney, Australia, 2Centre for Quantum Computation and Communication Technology, School of Electrical Engineering and Telecommunications, UNSW Sydney, Sydney, Australia Abstract: Donor spins are atomic-scale qubits in silicon with 1- and 2-qubit fidelities above 99%, and exceptional coherence times once the effect of magnetic noise from 29Si nuclei is removed by isotopic purification. However, charge noise and charge drift remain a limiting factor, like in all semiconductor quantum devices. Charges can be trapped in and around the oxide dielectric between the metallic gates and the enriched 28Si where the donor is located. These charges can tunnel between random potentials, or drift due to the electric fields within the device, causing slow variations of the electric field on the donor, which in turn affects hyperfine coupling, g-factor, and readout position in gate space. Similar phenomena have been observed in gate-defined quantum dots in Si/SiGe and GaAs/AlGaAs heterostructures. In these devices it has been demonstrated that optical illumination can be used to flush trapped charges. The light creates electron-hole pairs that acts as free carriers and help establishing a new charge equilibrium within the device. The effects of optical illumination Si-MOS devices in cryogenic environments, however, have yet to be reported. In this work, we present the results of optical illumination on Sb-donor devices. We investigated the effects of two different optical sources, one in the visible light range (473 nm) and one in the infrared (780 nm), of these Sb-donor devices in a dilution refrigerator at 20 mK and 1.2 K. We assessed the shift in the threshold voltage of the SET used for single shot electron spin readout via both biased and unbiased optical illumination. We also examined if the change in threshold voltage was caused by the temperature increase due to illumination. Work is underway to correlate illumination with changes in charge noise and, ultimately, spin coherence and gate fidelity in donor spin qubits after illumination. |
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Description of Non-Markovian Nosie via Instrument Get Tomography
Wei-Zhu Liao and Hai-Ou Li University of Science and Technology of China Abstract: Recently, gate set tomography (GST) has become a powerful protocol for characterization of quantum logic gates, which focuses on the reconstruction of the entire set of logic operation and is almost entirely calibration free. In our work, we have realized a single-qubit gate which fidelity can be up to 99.3% in a silicon-metal-oxide-semiconductor (SiMOS) quantum dots device. And we notice that a remarkable stochastic error exists in our devices with the GST error generation model. However, in addition to this Markovian noise, another pending problem is the numerous non-Markovian noises in our system which will cause the bad fitness to the GST estimation. The non-Markovian correlations (past operations influence the current operations) can significantly impact our quantum devices. To solve this issue, we are supposed to take system-environment (SE) correlations into account. Here we will propose a new self-consistent framework called instrument get tomography (IST) to describe the non-Markovian noises in our devices. The instrument set consists of instruments, SE unitaries and initial SE states. Furthermore, IST can, like GST, model the instruments and SE correlations with the statistical experiment results based on maximum likelihood estimation (MLE). This method is called MLE-IST. Also, there are some constraints introduced to guarantee the experiment results to be physical. To measure the effectiveness of this method, we introduce the square error of probabilities (SEP). Comparing with MLE-GST, the MLE-IST obtains a significant average SEP reduction by order of -6.00 in simulations. At last, we reconstruct the system instruments and SE unitaries in our real quantum devices. They can help us to analyze the non-Markovian noises. Next, we expect to improve our qubit with the results of IST. |
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Optimization of Ge/Si core/shell nanowire qubit devices
Artemii Efimov1, Nicolas Forrer1, Yang Liu1, Pierre Chevalier Kwon1, Miguel Carballido1, Simon Svab1, Alicia Ruiz1, Taras Patlatiuk1, Ang Li2, Erik Bakkers3, Ilaria Zardo1, Dominik Zumbühl1 1University of Basel, 2TU Eindhoven, 3Eindhoven University of Technology Abstract: Ge/Si core/shell nanowires (NW) [1] are a promising platform for hole spin qubits. These quasi-1D nanostructures particularly benefit from a strong and electrically tunable direct-Rasba spin-orbit interaction (DRSOI) arising from the mixture of heavy- and light-hole states on account of the strong biaxial confinement. By leveraging DRSOI, ultrafast, all-electrical spin driving can be implemented [2]. Moreover, due to the p-type wave function the contact hyperfine interaction is reduced and the absence of valley degrees of freedom, provides a great foundation for building spin qubits. In our system, the NW core-to-shell ratio (CSR) is a crucial parameter that dictates the compressive strain and consequently the strength of DRSOI. To this end, we grow and compare NWs with different growth parameters, such as CSR as well as surface oxidation, to optimize material parameters for fast and coherent hole spin qubits. Additionally, strong enhancement of the Hahn-echo coherence time over the Ramsey decay time suggests the presence of low-frequency charge noise as a dominant source of decoherence. Efforts to improve the coherence, point to the native oxide on the Si shell, to be the main source of charge traps. [3, 4]. Our work is aimed at improving the NW material to facilitate hosting spin qubits with excellent properties and coherence for future generations of Ge/Si qubit devices. Supported by NCCR SPIN of the SNSF, the Swiss Nanoscience Institute SNI, the EU-H2020 European Microkelvin Platform EMP Grant Nr. 824109, and EU-H2020 FET TOPSQUAD Nr. 862046. [1] Conesa-Boj et al., Nano Lett. 17 (2017) [2] F. N. M. Froning et al., Nature Nanotechnology 16, 308 (2018) [3] M. J. Carballido et al, arxiv: 2306.xxxxx (in preparation) (2023) [4] A.P. Higginbotham et al, Nano Letters 14, 3582–3586 (2014) |
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Modeling a charge fluctuator in Si/SiGe quantum dots
Dylan Albrecht1, N. Tobias Jacobson2, Rohith Vudatha2, Feiyang Ye3, Ammar Ellaboudy3, John Nichol3 1Sandia National Laboratories NM, 2Sandia National Laboratories, 3University of Rochester Abstract: We present a phenomenological model of single two-level fluctuator (TLF) forward and reverse rates compatible with experiments probing the rates' voltage, temperature, and local heating dependence. The figure shows the device and some of the data and analysis used to extract transition rates. A scanning electron micrograph of the Si quadruple-quantum-dot device is shown in (a). Sensor-dot conductance peaks measured by rf reflectometry are in (b). We use a factorial hidden Markov model (FHMM) to fit the corresponding reflectometry time series traces (c) and extract the transition rates. The red curves in (c) overlaid on the noisier blue data are the most likely fluctuator noise trajectories, as estimated from the FHMM using the Viterbi algorithm. We also estimate the Allan variance to extract switching times as shown in (d). The green regions in (d) are the Allan variances for FHMM-generated data, and the blue regions are the Allan variances for the experimental data. In addition to the widely varying charge noise environment showcased across the panels in (c), focusing on the large TLF shown in the first panel of (c), we find, from a series of measurements, that TLF switching times depend on gate voltages, temperature, and the current through a nearby quantum dot. We develop a phenomenological model capturing the experiments targeting this behavior. From the model fits we show, for example, that the TLF is predominantly coupled to the P gate electrode with a lever arm of magnitude 5 μeV/mV, coupled to the T2 gate with a lever arm of opposite sign of magnitude 1 μeV/mV, and only weakly coupled to the other gates. This work shows the power of detailed noise characterization combining measurement, analysis, and modeling. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525 |
| Quantum simulation |
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Probing a quantum phase transition and triplet propagation in Ge quantum dot ladder
Elizaveta Morozova1, Xin Zhang2, Utso Bhattacharya3, Alexander Nico-Katz4, Maximilian Rimbach-Russ1, Daniel Jirovec5, Pablo Cova Fariña1, Stefan Oosterhout6, Amir Sammak6, Sougato Bose4, Eugene Demler7, Giordano Scappucci8, Menno Veldhorst1, Lieven Vandersypen2 1QuTech, TU Delft, 2QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 3Institute for Theoretical Physics, ETH Zurich, Zurich, Switzerland, 4Department of Physics and Astronomy, University College London, United Kingdom, 5TU Delft/ QuTech, 6QuTech, Delft University of Technology and Netherlands Organisation for Applied Scientific Research (TNO), Delft, The Netherlands., 7Institute for Theoretical Physics, ETH Zurich, Switzerland, 8TU Delft, QuTech Abstract: In recent years, leading platforms for quantum simulations have been cold atoms and superconducting qubits. However, semiconductor quantum dots offer the advantage of individual qubit and inter-qubit coupling addressability, as well as intrinsic local and long-range Coulomb interactions. Many important milestones regarding quantum simulations have already been achieved in quantum dots, such as the metal to Mott insulator transition, Nagaoka ferromagnetism, antiferromagnetic spin chains and resonating valence band states. In our previous research, we obtained a fully working and tuned 2x4 Ge quantum dot ladder, where we formed singlet-triplet qubits and provided extensive characterization of the device (fig. 1a). We are now using this quantum processor to investigate physical phenomena naturally arising from the Hamiltonian of such a system. First, we focus on a quantum phase transition that features two competing orders within the quantum ladder: one driven by exchange interaction along the rungs (J_perp) and the other by the Zeeman energy (E_z). By changing the J_perp/E_z from low to high, we experimentally observed the ground state of the system changing from paramagnetic state to unpolarized state by counting the number of triplets in the system (fig. 1b). Interestingly, in the preliminary results, we can observe sophisticated behaviour at the transition point as we increase the exchange interaction strength along the rail of the ladder (J_par). This behaviour can be attributed to the intermediate phase between the paramagnetic and unpolarized states. Another insightful experiment we performed is triplet propagation in the system, where we tune ST_ qubit energies to be the same and allow the triplet to propagate in the system (fig. 1c). We attempted to inject 1 or 2 triplets into the array and observed correlated quantum walk behaviour. These results should highlight the quantum dot platform as promising for probing quantum magnetism and observing quantum phase transitions. |
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Running a six qubit algorithm on a silicon spin qubit array
Eline Raymenants1, Irene Fernandez de Fuentes1, Brennan Undseth2, Oriol Pietx-Casas2, Sergey Amitonov1, Larysa Tryputen1, Amir Sammak1, Albert Schmitz3, Shavindra Premaratne3, Anne Matsuura3, Giordano Scappucci4, Lieven Vandersypen4 1QuTech, 2Delft University of Technology, 3Intel Labs, 4TU Delft, QuTech Abstract: Quantum simulators in the NISQ era play a vital role in exploring complex quantum phenomena and validating physical platforms for multiqubit operations. To this end, spin qubits in gate-defined Si/SiGe quantum dots have showcased significant promise, with the recent demonstration of a high-fidelity six-qubit quantum processor[1]. However, the full operational mode of this device remained elusive. Expanding upon this progress, we use the six-spin array to implement a digital quantum simulation mimicking quenching Hamiltonian dynamics[2]. Tunable two-qubit interactions allow us to implement the circuit in Fig. 1 as a function of system size, revealing the characteristic scaling behavior of the quenching Hamiltonian, where the return probability (i.e. the overlap between the initial and the final state) as a function of time relates to the number of qubits, Fig. 1. Using a modified initialization sequence from [1], we can prepare and read out with high fidelity the |000000> state. Operating the device at 200mK helps mitigate microwave- and pulse-induced frequency shifts[3]. We benchmark the algorithm using state tomography on 3 qubits, for a fixed evolution time where we expect maximal entanglement. We were able to identify dephasing as the limiting factor to the achieved state fidelities. By using experimentally obtained dephasing times, we numerically run the algorithm with a noise model where decoherence is introduced through phase flip operators with a damping coefficient set by T2*. We find that the experimental results are in good agreement with predictions. The results of this digital simulation showcase the suitability of semiconductor dot devices for multiqubit control and to investigate many-body physics. Future work will focus on mitigating decoherence effects through, among others, simultaneously driven qubit operations, refocusing pulses, and micromagnet and device design optimizations. [1] Nature 609.7929 (2022): 919-924. [2] Physical review letters 119.8 (2017): 080501 [3] Physical Review X 13.4 (2023): 041015 |
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Extracting information from analog quantum simulation: Probing quantum states and dynamics one electron at a time
Garnett Bryant1, Yan Li2, Keyi Liu2 1NIST, 2University of Maryland Abstract: Extracting information from analog quantum simulations done using quantum dot and dopant arrays raises significant challenges that must be addressed. For now, transport is used to probe simulations. Adjacent charge sensors can provide additional information about local charge configurations. Signatures must be identified that can reveal the nature of the states of the simulator. We use an extended Hubbard model to investigate many-body states in quantum dot and dopant arrays. The array is tunnel-coupled to a charge pump to add electrons one-by-one and drain to extract electrons at chosen sites. We probe ferromagnetic states of the array that occur when the on-site repulsion is larger than the critical value for one hole in a half-filled band of square arrays (Nagaoka ferromagnetism) or for three electrons in a loop array. In the ferromagnetic state, all electrons spin-polarized in the same direction and on-site repulsion is strongly suppressed. For short times, transport through a ferromagnetic state becomes spin-polarized opposite to the array polarization. Electrons polarized opposite to the array polarization move easily because interaction is suppressed, while electrons polarized parallel to the array polarization are Pauli blocked. When the on-site repulsion is small, the array is not ferromagnetic, and transport becomes weakly spin-polarized parallel to the array partial polarization. Drain position can play a critical role in revealing the dynamics and nature of the quantum states, providing spatial and time signatures to discriminate the character of the simulator states. In few-electron simulations, effects induced by quantum fluctuations due to hopping and interaction are revealed, the evolution and onset of quantum jumps can be understood, and exotic dynamics can be observed. Developing fast probing is a new challenge that must be addressed to fully extract information about quantum states and dynamics in few-electron quantum simulators. |
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Stabilizing a discrete time crystalline phase in an array of silicon spin qubits.
Irene Fernandez de Fuentes, Eline Raymenants, Sugeiva Kisnapavan, Sander de Snoo, Lieven Vandersypen QuTech Abstract: Simulating the dynamics of quantum many-body systems is a suitable prospect for practical applications of near intermediate scale quantum devices. Specifically, non-equilibrium quantum phenomena are well-suited for systems that exhibit respectable qubit coherence times, high fidelity initialization, control, and readout. To this end, arrays of spins confined in 28Si/SiGe quantum dots are suitable candidates to use as programmable quantum simulators. In this work, we investigate the stabilization of a discrete time crystalline (DTC) phase utilizing a chain of six spins previously characterized in [1]. A DTC phase represents a novel state of matter wherein an interacting many-body localized (MBL) closed quantum system, subjected to periodic driving via a Floquet operator, avoids thermalization and exhibits persistent spatiotemporal order [2]. Experimentally, operating in the MBL regime is possible since the exchange-coupled spin chain can be treated as an effective Ising chain in the presence of the device micromagnet field gradient [3]. Crucially, we explore simultaneous single-qubit control with multiplexed microwave signals to construct the Floquet operator and bring the system periodically out of equilibrium. Our aim is to minimize idling times and thus reduce thermalization with the external bath, thereby extending the window for probing the prolonged coherence of our qubits within the DTC phase. Implementing this method alone would represent a significant advancement for parallel single qubit operation, streamlining the process for operating larger arrays of qubits. [1] Philips, Stephan GJ, et al. "Universal control of a six-qubit quantum processor in silicon." Nature 609.7929 (2022): 919-924. [2] Randall, J., et al. "Many-body–localized discrete time crystal with a programmable spin-based quantum simulator." Science 374.6574 (2021): 1474-1478. [3] Li, Bikun, et al. "Discrete time crystal in the gradient-field Heisenberg model." Physical Review B 101.11 (2020): 115303. |
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Measuring long-range spin correlation in spin wave modes prepared in gate-defined quantum dot arrays
Tzu-Kan Hsiao, Tien-Ho Chang, Chia-Hao Wei National Tsing Hua University Abstract: Spins in gate-defined quantum dot arrays are a naturally candidate for studying quantum magnetism in Heisenberg model. In particular, the development of initialization, control and readout techniques has facilitated realizations of Nagaoka ferromagnetism, Heisenberg antiferromagnetic spin chains, and resonating valence bond states. Nevertheless, the observation of long-range spin order in magnetic phases remains elusive in quantum dot systems. Here we report our progress in studying quantized spin wave states in a quantum dot ring. We will describe experimental methods for preparing and probing the spin waves in an intermediate-scale quantum dot array. The long-range spin order can be confirmed by measuring the spin correlation as a function of distance. From the spin correlation we can further obtain the spin structure factor and the spin wave dispersion relation. Finally, we will show our measurement progress for this project. |
| Qubit array (3+ qubits) |
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Tuning of multi-quantum-dot devices in SiMOS
Andreas Nickl1, Tuomo Tanttu2, Arindam Saha3, Santiago Serrano Ramirez2, Wee Han Lim2, Fay Hudson2, Andre Saraiva2, Andrew Dzurak2, Aaron Tranter3, Arne Laucht2 1University of New South Wales, 2School of Electrical Engineering and Telecommunications, University of New South Wales, NSW 2052, Sydney, NSW, Australia, 3Department of Quantum Science & Technology, The Australian National University, ACT 2601 Canberra Abstract: Electron spins in Silicon-MOS devices are a promising platform to realize qubits for a large-scale quantum processor. Their small footprint, combined with the well-established fabrication methods from the classical nano electronics industry and research, allows dense packing of qubit arrays [1]. Single and two-qubit gate fidelities above the error correction threshold have been reported at temperatures above 1K, enabling operation in cryostats with high cooling power compared to the mK regime [2]. So far, demonstrations in SiMOS are restricted to the single and two qubit regimes. In this work we show recent advances of tuning and calibration as well as simultaneous control of single and two qubit gates in larger linear spin qubit arrays. We use a reinforcement-learning model to find and optimize initialization and readout of parity spin states by Pauli spin blockade [3]. This automated method accelerates the calibration process at least tenfold and gives access to a higher parameter space than comprehendible by a human experimentalist. [1] Veldhorst, M., Yang, C., Hwang, J. et al. A two-qubit logic gate in silicon. Nature 526, 410–414 (2015). https://doi.org/10.1038/nature15263 [2] Huang, J.Y., Su, R.Y., Lim, W.H. et al. High-fidelity spin qubit operation and algorithmic initialization above 1 K. Nature 627, 772–777 (2024). https://doi.org/10.1038/s41586-024-07160-2 [3] Seedhouse, A. E., Tanttu, T., Leon, R. C. C. et al. (2021). Pauli Blockade in Silicon Quantum Dots with Spin-Orbit Control. PRX Quantum, 2(1), 010303. https://doi.org/10.1103/PRXQuantum.2.010303 |
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High-fidelity two-qubit gate and teleportation with distant spin qubits in silicon
Yuta Matsumoto1, Maxim De Smet1, Larysa Tryputen1, Sander de Snoo1, Alexandra Meerovici Goryn1, Maximilian Rimbach-Russ1, Amir Sammak2, Giordano Scappucci1, Lieven Vandersypen1 1QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands, 2QuTech and Kavli Institute of Nanoscience, Delft University of Technology, 2600 GA Delft, The Netherlands; QuTech and Netherlands Organisation for Applied Scientific Research (TNO), 2628 CK Delft, The Netherlands Abstract: In quantum computing architectures, the connectivity between individual qubits is crucial for the performance of quantum error correction codes [1]. For spin qubits in semiconductor heterostructures, two-qubit interactions based on the exchange coupling are inherently short-distance, typically limited to nearest-neighbour qubit coupling. To couple distant spins, approaches such as using a superconducting resonator [2] have been demonstrated. Another promising strategy is to physically displace, or shuttle, the single charge using conveyor-style shuttling [3,4,5], where phase-shifted sinusoidal gate voltages propagate electrons trapped in moving dots. In this work, we report on a high-fidelity shuttling controlled-Z (CZ) gate between electron spins in a linear array of six quantum dots confined in a Si/SiGe heterostructure. Each of the two electrons is shuttled using a conveyor to the middle of the array, where they meet. We observe a saturation of the exchange coupling when both electrons are confined in a single, large potential minimum. Randomized benchmarking of the CZ gate yields a fidelity of 99.4%. As a practical demonstration of this novel shuttling two-qubit gate, we perform the first quantum state teleportation protocol using semiconductor spins. In this demonstration, the quantum state of the spin in dot 6 is conditionally teleported to the spin in dot 2. References: [1] Xu et al., Nature Physics (2024) [2] Dijkema et al., arXiv.2310.16805 (2023) [3] Struck et al., Nature Communications 15 1325 (2024) [4] Xue et al., Nature Communications 15 2296 (2024) [5] De Smet, Matsumoto et al., arXiv.2406.07267 (2024) |
| Readout |
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An inductively coupled NbTiN resonator for gate dispersive charge sensing of SiMOS quantum dots
Tim Botzem1, Dario Denora2, Santiago Serrano3, Wee Han Lim4, Arne Laucht4, Jarryd Pla5, Andrew S. Dzurak4 1UNSW/DIRAQ, 2ETH Zürich, Zurich, Switzerland, 3School of Electrical Engineering and Telecommunications, University of New South Wales, NSW 2052, Sydney, NSW, Australia & Diraq, Sydney, NSW, Australia, 4School of Electrical Engineering and Telecommunications, University of New South Wales, NSW 2052, Sydney, NSW, Australia, Diraq, Sydney, NSW, Australia, 5School of Electrical Engineering and Telecommunications, University of New South Wales, NSW 2052, Sydney, NSW, Australia Abstract: Leveraging the existing gate electrode architecture of gate defined quantum dot devices for qubit readout allows for omitting dedicated on-chip sensing circuitry and opens the pathway to more compact qubit chips. In this poster, I characterize an off-chip gate-coupled RF reflectometry setup (see figure below) consisting of an inductively coupled NbTiN resonator and RF filter arrays, for the dispersive readout of a SiMOS double quantum dot. With this approach, charge sensing at a signal-to-noise ratio of two is achieved within 5 μs. The maximum internal quality factor of the superconducting resonator is of order 2000 and is currently limited by RF losses in the silicon sample. I will discuss mitigating strategies and further improvements to the readout circuitry. |
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Parametric amplifier of high kinetic inductance multiple frequency cavity
Yongqiang Xu, Gang Cao, Guo-Ping Guo University of Science and Technology of China Abstract: The parametric amplifier has the significant applications on the qubit readout in circuit quantum electrodynamics (circuit QED). Based on the necessary nonlinearity provided by high kinetic inductance (HKI) thin films, the parametric amplifiers could work under large magnetic field with large gain. Here, we realize a HKI multiple frequency cavity, and characterize the properties of eight cavity modes, including cavity frequency and quality frequency. We achieve the non-degenerate parametric amplification of the eight different cavity modes, with more than 20 dBm multi-mode gain. The optimized work point of pump powers and the impact of frequency differences are demonstrated as well. The multiple frequency cavity parametric amplifier contributes to the frequency division multiplexing readout, and may facilitate the exploration of entangled photons generation. |
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Theory of charge-sensing-based noisy qubit readout of semiconductor qubits
Domonkos Svastits1, Gábor Széchenyi2, Daniel Loss3, Stefano Bosco4, Andras Palyi1 1Budapest University of Technology and Economics, 2Eötvös University, 3University of Basel, 4QuTech, TU Delft Abstract: Qubit readout experiments are noisy, deviating from textbook projective measurements. Typically, readout is implemented by doing measurements on a second quantum system, the ‘meter', that is coupled to the qubit. An example is a semiconductor charge qubit in a double quantum dot, whose readout is performed by measuring the current through a nearby quantum point contact (QPC) [see Fig. a)]. A microscopic model of that process provides a characterisation of the noisy readout in terms of so-called ‘measurement operators', which describe the post measurement state conditioned on the measured current. In this work, we propose a model for such a noisy measurement of a semiconductor-based charge qubit. In the model, the QPC is replaced by an ancilla qubit, that is coupled to the data qubit [see Fig. b)]. Measurement of the data qubit is achieved by subsequent projective measurements of the ancilla qubit. The model yields the measurement operators as functions of the parameters, that is, double-dot energy detuning ε and tunnelling t, coupling strength δγ, and the integration time. We obtain the measurement operators numerically exactly for realistic, experimentally relevant parameter values. We demonstrate that our model describes three fundamental processes: (1) Dephasing of the qubit, (2) The emergence of different values of the charge-sensor current, depending on the initial qubit state. (3) Measurement-induced relaxation of the qubit. Furthermore, we use the generalization of our model to describe Pauli-blockade-based noisy measurements of spin qubits incorporating spin-orbit effects as well as the effect of measurement-induced leakage. Further potential applications of our model include the optimisation of the readout quality in the multi-dimensional space of benchmarks (readout fidelity, quantum-non-demolition-ness of the readout, purity of the post-measurement state, leakage, etc.); optimisation of qubit-state inference based on soft information; and optimisation of adaptive circuits, including those used for quantum error correction. |
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Improving Radio-Frequency Readout of Hole Spins in a Ge/SiGe Linear Array with a Screening Layer Approach
Eoin Kelly1, Leonardo Massai1, Alexei Orekhov1, Marta Pita-Vidal1, Cornelius Carlsson1, Matthias Mergenthaler1, Felix Schupp1, Stephan Paredes1, Steve Bedell2, Patrick Harvey-Collard1, Andreas Fuhrer1, Gian Salis1 1IBM Research Europe - Zurich, 2IBM Quantum Abstract: Fast and high-fidelity readout of quantum dot spin qubits in Ge/SiGe can be achieved by using a radio-frequency single-hole transistor (RF-SHT) capacitively coupled to the qubits as a charge sensor together with spin-to-charge conversion. The RF-SHT is composed of a resonant LC matching circuit attached to its source. However, when designing such matching circuits in our Ge/SiGe heterostructure, we observe large RF dissipation which reduces the LC resonator quality factor to very low values that inhibit the achievement of good matching. We have found an effective approach to overcome such dissipation by screening the electric field of the resonant LC circuit from the substrate by using a grounded metallic layer between the device bondpads and the heterostructure (panel (a)). We utilize this screening layer approach together with a superconducting nanowire inductor to form an RF-SHT in a linear Ge/SiGe quantum dot array. We simulate for the charge sensitivity of this RF-SHT with a screening layer as a function of the matching circuit parameters (panel (a) inset), and then experimentally investigate both charge-state determination (panel (b)) and hole-spin state readout. An example of spin-state readout in the form of singlet-triplet oscillations as a function of in-plane magnetic field is presented in panel (c). We furthermore explore the limiting factors for high-fidelity spin-state preparation and readout such as the magnetic field orientation, ramp times between charge states, and exchange interaction. |
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Characterizing different readout schemes in Si:P donor/dot devices
Fan Fei1, Brian Courts2, FNU Utsav1, Jonathan Wyrick1, Pradeep Namboodiri1, Joshua Pomeroy1, Richard Silver1 1NIST, 2University of Maryland College Park Abstract: Si:P monolayer quantum devices fabricated using STM based hydrogen lithography are a strong candidate for spin-based quantum computing. While DC current readout may be sufficient for high conductance, low noise, slow speed low tunnel rate devices, increased noise sensitivity, line capacitance, and local charge noise can significantly influence the need for improved measurement bandwidth. On the other hand, other approaches such as RF reflectometry can be tuned to be less sensitive to noise sources with the tradeoff of using AC excitation at the device to sense the device state. Additionally, the signal contrast is directly proportional to the impedance change of the device between its on/off states. This results in a choice as to whether DC or RF readout is preferred, having a higher bandwidth at SNR=1. In this presentation we characterize measurement bandwidth and sensitivity to noise using different readout schemes (DC current, RF reflectometry, cryo HEMT transimpedance amplifier) and determine a criterion for choosing the best method for specific device attributes. We also evaluate improved impedance matching using a tunable tank circuit for improved SNR when using RF. We evaluate these spin readout methods on a donor-dot SET device and characterize T1 times as a function of magnetic field. |
| Shuttling |
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Towards 2D connectivity in a scalable electron spin qubit architecture in Si/SiGe
Max Beer1, Ran Xue2, Inga Seidler3, Simon Humpohl3, Jhih-Sian Tu4, Stefan Trellenkamp4, Tom Struck3, Lino Visser3, Tobias Offermann5, Hendrik Bluhm3, Lars Schreiber3 1JARA Institute for Quantum Information - RWTH Aachen University, 2RWTH Aachen University, Aachen, Germany, 3JARA Institute for Quantum Information, 4Helmholtz Nano Facility (HNF), Forschungszentrum Jülich, Jülich, Germany, 5- Abstract: In recent time, we demonstrated linear many-quantum dot (QD) arrays performing conveyor-mode shuttling for both highly controlled multi-electron charge transfer over several micrometers [1] as well as coherent spin-entangled electron pair separation [2] using a distance independent number of control lines. These achievements lay the groundwork for conveyor-mode shuttling as a basis for a truly scalable quantum computing architecture by simultaneously reducing the complexity of wiring and control pulses while also providing sufficient distance between qubit gate operation sites for effective isolation during gate operation as well as the integration of on-chip cryo electronics [3]. Following this success, the demonstration of electron shuttling in two dimensions is paramount. We aim to achieve this by fabricating T-Junction devices that, in addition to a linear QD array, feature a junction area from which another, perpendicular linear QD array originates (see figure). Such designs present several challenges compared to previous devices, the most pressing stem from the complexity of the immediate junction area. Here, gates need to enable charge transfer both in a linear motion as well as orthogonally while providing sufficient electron confinement to suppress accidental orbital excitation. By now this is well understood due to extensive electrostatic simulations carried out by us [4], with current work investigating the influence of charged interface defects on device operation. This and similar obstacles meet the overarching challenge of fabricating such a large device featuring over 200 electrostatic gates in an academic clean room environment, as this commands tight process control and careful optimization of existing techniques. Figure: Scanning electron micrograph of a T-Junction device with 3 metal layers. [1] Xue et al. Nat. Commun. 15, 2296 (2024) [2] Struck et al. Nat. Commun. 15, 1325 (2024) [3] Otten et al., 29th IEEE ICECS (2022) [4] Künne et al., arXiv:2306.16348 (2023), accepted for Nat. Commun. |
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Toward shuttling a hole spin across a germanium five quantum dot linear array
Yona Schell1, Jaime Saez-Mollejo1, Simon Robson1, Andrea Ballabio2, Daniel Chrastina2, Giovanni Isella2, Georgios Katsaros1 1ISTA, 2L-NESS, Physics Department, Politecnico di Milano Abstract: Ge spin qubits offer promising prospects for realizing quantum processors due to the low effective mass of holes confined in Ge, small hyperfine interaction, and strong spin-orbit interaction, which enables efficient electrical spin manipulation. However, despite advances in 2x2 and 3-4-3 qubit devices [1,2] scaling up a single array encounters challenges. A suggested solution is to create smaller qubit cells and connect them by transferring the spin information through a quantum link [3]. Such a quantum link can be a superconducting resonator, coupling two spin qubits. Alternatively, the spin can be shuttled through a 1D channel or through quantum dots. The latest approach has been demonstrated in a linear quantum dot array in silicon [4] and, more recently, in 2D Ge arrays operating in accumulation mode [2]. Here, we report on a five quantum dot linear array formed in a Ge/SiGe heterostructure (as shown below in the scanning electron microscopy picture) operating in depletion mode. Our efforts focus on the characterization of these quantum dot devices in terms of charging energy, lever arm and sensitivity of the two charge sensors. We will report on the challenges of depleting all the quantum dots to achieve single hole occupation necessary for conducting shuttling experiments. [1] Hendrickx, Nico W., et al., Nature 591.7851 (2021) [2] Wang, Chien-An, et al., arXiv:2402.18382 (2024). [3] Vandersypen, L. M. K., et al., npj Quantum Information 3.1 (2017) [4] Zwerver, A. M. J., et al., PRX Quantum 4.3 (2023) |
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Optimal Control of Conveyor-Mode Electron Shuttling in a Si/SiGe Quantum Bus in the Presence of Charged Defects
Markus Kantner1, Lasse Ermoneit1, Burkhard Schmidt1, Arnau Sala2, Nils Ciroth2, Lars Schreiber2, Thomas Koprucki1 1Weierstrass Institute for Applied Analysis and Stochastics (WIAS), 2JARA-FIT Institute for Quantum Information, FZ Jülich and RWTH Aachen University Abstract: Scaling of quantum processors to large qubit numbers is one of the major challenges on the route to the realization of universal fault-tolerant quantum computers. Spin qubits in Si/SiGe quantum dots provide excellent prospects for scalability, but the lithographic processing, signal routing and wiring of large qubit arrays at a small footprint poses a significant problem. A possible remedy to this wiring problem is a modular architecture, where small-scale qubit arrays are interconnected by a quantum bus. The latter enables coherent transfer of quantum information between distant arrays by physically moving electrons along a channel. Conveyor mode shuttling is a particularly robust and scalable mode of operation for such a quantum bus, where only a few control pulses (independent of the channel length) are required. A major limitation in qubit shuttling is the interaction of the electron with charged defects within the channel, which can cause non-adiabatic transitions to excited states in a sequence of Landau–Zener transitions. The associated change of the effective g-factor induces spin-dephasing via a randomization of the accumulated phase and thereby degrades the shuttling fidelity. In this contribution, we theoretically explore the capabilities for circumventing defect centers using optimally engineered control pulses. To this end, we minimize the accumulated energy uncertainty to enable quasi-adiabatic (nearly deterministic) electron shuttling without reducing the shuttling velocity. Our approach is based on (open loop) quantum optimal control theory and 3D numerical Schrödinger wave packet propagation using realistic potential landscapes. The optimization approach is universally applicable to a broad range of scenarios, e.g., orbital excitations in complex defect potential landscapes and valley excitations in disordered alloys. |
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Dynamics of spatially separated spin singlet: from double quantum dot to long-distance shuttling
Lukasz Cywinski1, Mats Volmer2, Tom Struck2, Lars Schreiber3 1IF PAN, 2JARA-FIT Institute for Quantum Information, Forschungszentrum Jülich GmbH and RWTH Aachen University, Germany, 3JARA Institute for Quantum Information Abstract: Singlet-triplet mixing in double quantum dots occurs due to interdot difference in Zeeman splittings, but when Zeeman energy is close to valley splitting in one of the dots, mixing with polarized triplets via spin-valley coupling becomes relevant, allowing for measurement of valley splittings in the two dots [1,2,3]. Spin-coherent conveyor-mode shuttling [4,5] of one of the dots allows for sensing of spin-valley resonances (and thus local valley splittings) as a function of position of the shuttled dot [6]. We analyze theoretically the dynamics of charge separation, singlet-triplet mixing, dephasing due to spin and valley splitting fluctuations, and Pauli spin blockade measurement of singlet return probability after shuttling one of electrons back and forth. Not fully adiabatic character of charge separation process is necessary to explain the measured singlet return probability oscillations [5]. [1] Cai et al. Nat. Phys. 19, 386 (2023). [2] Jock et al. Nat. Commun. 13, 641 (2022). [3] Liu et al. Phys. Rev. Appl. 16 024029 (2021). [4] Langrock, Krzywda ea. PRX Quantum 4, 020305 (2023). [5] Struck et al. Nat. Commun. 15, 1325 (2024). [6] Volmer, Struck et al. arXiv:2312.17694 (2023). |
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Protecting quantum information during spin shuttling against the noise produced by random sheet.
Yuning Zhang1, Aleksandr Mokeev1, Viatcheslav Dobrovitski2 1QuTech, Delft University of Technology, 2Delft University of Technology Abstract: Spin shuttling has recently emerged as a pivotal technology for large-scale semiconductor quantum computing. By transporting qubits between quantum dots, spin shuttling enables entanglement between non-neighboring qubits, which is essential for many tasks (such as quantum error correction). However, the spin qubit becomes decohered by magnetic noise during the shuttling process. Since the noise varies in time and space in a correlated manner, the associated dephasing in a system of several entangled spins often cannot be treated using the standard theory of random processes, and requires more advanced mathematical tools. In this work, we employ the concept of random sheets in order to study the dephasing of electronic spins during shuttling. We develop an explicit model for spatially correlated 1/f noise and analyze two methods of protecting quantum information, namely (i) encoding of a logical qubit in a singlet-triplet (ST) decoherence-free subspace of two entangled spins, and (ii) suppression of dephasing of a single unencoded spin using the spin echo. We investigate the performance of both methods under various scenarios, including the sequential and parallel shuttling of the ST qubits, as well as one-way and forth-back shuttling of a single qubit with spin echo. While both methods can mitigate dephasing under experimentally realistic conditions, we find that their performance is nontrivially affected by the shuttling trajectories and the underlying noise. In particular, stronger correlations, while deteriorating the shuttling fidelity for unprotected qubits, can enhance the performance of noise mitigation protocols. We identify the advantageous regimes for each noise mitigation strategy. Our work demonstrates how quantum information can be better protected during shuttling by exploiting correlations in qubit noises and choosing shuttling trajectories. Our results provide heuristics for a broader range of encodings and dynamical decoupling protocols, helping develop schemes for high-fidelity spin shuttling in semiconductor quantum computing. |
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Simulation of spin shuttling in Si-MOS quantum dots
Chu Wang and Hai-Ou Li University of Science and Technology of China Abstract: Towards fault-tolerant operation of silicon spin qubits, achieving interconnectivity between spatially separated qubits is a challenging topic since the exchange coupling widely utilized to execute two-qubit gates is only effective between nearest-neighbor quantum dots. Several methods for quantum state transfer have been demonstrated in semiconductor quantum dots, such as SWAP operation and spin shuttling, during which electron spin is transported between quantum dots. Coherent spin transport can pave the way to scalability of spin-based quantum computing. Here we theoretically study a simplified model of spin shuttling between two quantum dots. Spin-orbit interaction and effects of valley coupling are included in this model, which are both possible sources of error occurring in experiments. Besides, tunnel coupling and Zeeman splitting induced by external magnetic field are vital parameters since they determine the energy scale in spin transfer process, which is relevant to the allowed transport velocity. We investigate the spin fidelity loss due to these effects during the spin shuttling process and can further give a suitable parameter space of ideal spin shuttling sufficient for the fault-tolerant threshold. We also explore our previous experiment results of the shuttling process, in which experiment parameters are not good enough. We analyze errors in the experiment and consider the main cause of the error, which is the prospect of further improvements. |
| Spin-photon coupling |
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Singlet-triplet Ge hole spin qubit quantum measurement and entangling rates via dynamical longitudinal coupling
Rusko Ruskov (Hristov)1 and Charles Tahan2 1Laboratory for Physical Sciences, University of Maryland, Physics, 2University of Maryland, Physics Abstract: Based on a recently developed theory [1] of the dynamical longitudinal coupling of a qudit to a superconducting resonator, we perform a feasibility study of the quantum measurement rate and the geometric phase entangling rate mediated by such a coupling for the case of a singlet-triplet (S-T0 and S-T-) Ge hole spin qubits. The possibility to overcome typical dephasing mechanisms such as a charge noise, phonon relaxation, and coupling to TLS fluctuators is discussed at various quantum dots' detunings. [1] R. Ruskov, C. Tahan, Longitudinal (curvature) couplings of an N-level qudit to a superconducting resonator at the adiabatic limit and beyond, arXiv:2312.03118 |
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Optimizing quantum dot configurations for enhancing spin-photon interaction
Valerii Kozin1, Marina Obramenko1, Jelena Klinovaja1, Daniel Loss1, Stefano Bosco2 1University of Basel, 2QuTech Abstract: The exploration of spin-photon interaction in quantum dots presents significant insights for quantum computing and communication technologies. This work delves into the theoretical models governing these interactions within various quantum dot configurations, focusing on single and double well potentials across one-dimensional and two-dimensional systems involving spin-orbit interactions. Through a comparative analysis of these configurations, our findings reveal distinct behaviors in spin-photon coupling, which are critically influenced by the quantum dot's spatial dimensionality and potential landscape. It is demonstrated that by optimizing the potential shape one may drastically increase the spin-photon coupling strength. The results highlight the potential for optimizing quantum dot designs to enhance spin-photon interactions, offering a pathway to more efficient quantum devices. |
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Silicon color center electroluminescence and prospects for photonic readout of silicon spin qubits
Joshua Pomeroy1, Nikki Ebadollahi2, Runze Li3, V Veetil4, M Pelton4, A Katzenmeyer5, Pradeep Namboodiri1, Kartik Srinivasan1, Marcelo Davanco5 1National Institute of Standards and Technology, 2NIST/University of Maryland, College Park, 3University of Maryland, 4University of Maryland, Baltimore County, 5NIST Abstract: Photoluminescence and electroluminescence from W- and G-type color centers (CCs) in silicon will be presented, along with MOS (metal-oxide-semiconductor) based quantum dot charge sensing measurements, which are components of an effort to realize photonic spin readout. While silicon can be a near defect- and spin- free environment for qubits, these attributes also correspond to limited options for introducing or extracting quantum information, or even performing projective measurements with minimal overhead. We report on a potential alternative approach to electrical readout, where spin-to-charge conversion could be detected via a change in an optical property (e.g., via Stark shift) of a capacitively coupled silicon CC whose primary emission is in the telecom band. Toward this end, we have synthesized W- and G- centers in silicon, performed photoluminescence spectroscopy, incorporated CCs into p-i-n junctions operated as CC-LEDs (light emitting diodes) and measured electroluminescence of these novel devices. This talk will discuss the concepts, present color center luminescence data, MOS quantum dot devices, and discuss the progress on developing hybrid electronic-photonic devices for harnessing color centers for quantum light and projective spin read out. Figure 1: (left) Electroluminescence from a W-center LED device taken at 6 K with 50 µA of electrical current. Inset is a photo of a color center chip mounted for installation in the optical cryostat. (Right) AFM image of the MOS gate array for a quantum dot chain with two independent charge sensor channels being developed for spin qubit devices. Our long-term goal is to integrate these systems into a hybrid architecture. |
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Drive-enabled tunable entanglement between spatially separated spin qubits
Vanita Srinivasa1, Jacob Taylor2, Jason Petta3 1University of Rhode Island, 2Joint Quantum Institute/Joint Center for Quantum Information and Computer Science/University of Maryland/National Institute of Standards and Technology, 3Princeton University Abstract: The generation of robust and tunable entanglement between spatially separated spin qubits is central to enabling scalable spin-based quantum information processing via modularity. For silicon spin qubits in quantum dots, candidate microwave cavity photon-based long-range links have been demonstrated [1–3]. While these results provide one path to modular spin qubit architectures, practical challenges remain for applying this approach to more than two qubits due to the need for simultaneous resonance between multiple qubit and cavity frequencies, as well as the precise positioning of micromagnets required in silicon for spin-charge coupling and qubit-qubit interaction. Our recent work [4] addresses this challenge by providing a theoretical framework for tunable entanglement via the sidebands of driven spin qubits coupled via microwave cavity photons. By transferring the tuning from dc pulses and micromagnets to ac driving fields, we show that tailored spin qubit entanglement can be achieved with mutually off-resonant qubit and cavity frequencies and a suppressed sensitivity to cavity photon loss, providing tunability and spectral flexibility. We investigate this approach in detail for multiple types of driven spin qubit systems, including both systems involving micromagnets and those with intrinsic spin-dependent electric dipole moments, and identify optimal operation regimes for implementing drive-tailored entangling gates. These results suggest a promising route to modularity for spin qubits. [1] F. Borjans, X. G. Croot, X. Mi, M. J. Gullans, and J. R. Petta, Nature 577, 195 (2020). [2] P. Harvey-Collard, J. Dijkema, G. Zheng, A. Sammak, G. Scappucci, and L. M. K. Vandersypen, Phys. Rev. X 12, 021026 (2022). [3] J. Dijkema, X. Xue, P. Harvey-Collard, M. Rimbach-Russ, S. L. de Snoo, G. Zheng, A. Sammak, G. Scappucci, and L. M. K. Vandersypen, arXiv:2310.16805. [4] V. Srinivasa, J. M. Taylor, and J. R. Petta, arXiv:2307.06067, to appear in PRX Quantum (2024). |
| Valleys or hole physics |
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Multiscale and multiphysics simulation of hole spin qubits in silicon: A pathway to model realistic devices
Pratik Chowdhury1, Scott Liles1, Aaquib Shamim1, Wee Lim2, Fay Hudson2, Andrew Dzurak2, Alexander Hamilton1, Rajib Rahman1 1School of Physics, The University of New South Wales, Sydney 2052, Australia, 2School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney NSW 2052, Australia. Diraq, Sydney NSW, Australia. Abstract: Hole qubits have received considerable attention in recent times owing to their strong spin-orbit coupling (SOC) enabling fast, electrical control through electric dipole spin resonance (EDSR). Hole quantum dots (QDs) are multi-band systems with strong quantum confinement and complex SOC which are affected by device specific details such as strain, interfaces, and disorder. Moreover, thermal strain from the deposited metal gates penetrates the amorphous oxide layer and affects the silicon lattice where the qubits are hosted. The atomistic tight-binding (TB) technique is a full-band and full-Brillouin Zone method where atomic granularity of the lattice, and disorder are inherently accounted for. SOC also falls out naturally from the atomically constructed lattices and interfaces, without needing additional symmetry constraints and assumptions. Therefore, this method captures the intricacies of the hole qubits present in experiments. The long-range thermal strain from the metal gates, however, is problematic to model, as accurate tight-binding models of the amorphous oxides are difficult to develop. Hence, we employ continuum elasticity theory based on finite-element modelling (FEM) to capture the thermal strain and translate this into atomic displacements of the silicon lattice. The atomistic TB Hamiltonian (sp3d5s* basis), which represents strain from atomic bond length and orientation changes, is used to compute the resultant qubit wavefunctions and electronic properties, such as the g-factors. This framework based on COMSOL and NEMO3D is used to investigate hole qubit building blocks spanning over ∼ 100 nm, thereby creating a pathway to model realistic devices. We use the above-mentioned simulation framework to model a prototype device (figure 1a) and calculate strain with three different gate stack materials, Aluminium, Palladium and Polysilicon (figure 1b). We show how g-factor magnitudes and anisotropy of the lowest-two spin manifolds are affected by the device details (figure 1c) and compare our results with recent experimental measurements (figure 1d). |
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Effect of roughness in planar Ge hole spin qubit: device specific theoretical modelling of hole quantum dot in Ge/SiGe heterostructures
Abhikbrata Sarkar, Pratik Chowdhury, Andre Saraiva, Andrew Dzurak, Rajib Rahman, Dimi Culcer UNSW Abstract: Strained germanium quantum dots in Ge/SiGe heterostructures exhibit fast and coherent hole qubit control in experiments. Strain inhomogeneity in planar Ge hole system arises from the random alloy disorder and gate electrode contraction. This roughness leads to significant structural and bulk inversion asymmetry (SIA and BIA). To study the effect of roughness, we calculate the displacement field due to the random alloy disorder with atomic resolution using the valence force field (VFF) method in NEMO3D (fig.1a). Next the effect of gate electrode contraction at cryogenic temperature is modelled via finite element method in COMSOL(fig.1b). Based on the k.p formalism, the single hole qubit Hamiltonian is given by: HQD=HLK+HZ+Vconf+eFzz+Hε. The top-gate field Fz induces k3 SIA Rashba(αR) in the quasi-2d limit, while the cumulative inhomogeneous strain contained in Hε induces linear-in-k SIA Rashba(ϵR) and BIA Dresselhaus(ϵD) spin-orbit interaction. For applied in-plane-B operation, an ac electric field(Eac) enables π-rotation of the hole spin via electron dipole spin resonance (EDSR), with Rabi frequency fR=100 MHz in experiments. Typical homogeneous strain assumption in literature underestimates the EDSR Rabi frequency and evaluates fR=4 MHz; whereas our roughness model benchmarks well against experimental result. In the quasi-2d heavy hole qubit subspace, the analytical Rabi frequency is calculated as: fR= (ϵD B×Eac)⁄Δ2 + ((ϵR+ αR)B.Eac)⁄Δ2 This explains that the improvement of EDSR rate comes from the Dresselhaus roughness spin-orbit coupling, since B×E term dominates fR (fig.1c), and offers a selectivity of B and Eac orientation for fast hole qubit operation. Recent research have also revealed the impact of the local device profile on g-factor of Ge hole qubits, estimating as low as g=0.028 with precise in-plane magnetic field. Our model evaluates a similar g-factor and shows an order-of-magnitude variation in the g-factor based on the relative orientation of gate-stack with respect to the applied in-plane B(fig.1d). |
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Towards Optimization of Valley Splitting in Si/SiGe Quantum Wells
Abel Thayil1, Lasse Ermoneit1, Markus Kantner2 1Weierstrass Institute for Applied Analysis and Stochastics, 2Weierstrass Institute for Applied Analysis and Stochastics (WIAS) Abstract: Silicon-germanium (SiGe) heterostructures are a major candidate for realizing fully scalable quantum computers due to their inherently long spin coherence times and compatibility with existing semiconductor fabrication techniques. A critical challenge in strained Si/SiGe quantum wells is the existence of two nearly degenerate conduction band minima that can lead to leakage of quantum information. In the literature, several strategies have been proposed to enhance the energy splitting between the two valleys such as sharp interfaces, oscillating Ge-concentrations (wiggle well) and shear strain engineering. In this work, we formulate the design of the epitaxial profile in the quantum well as an optimization problem and seek for an optimized alloy composition profile that maximizes the valley splitting while respecting several manufacturing limitations. Our theory is based on a recently proposed extended virtual crystal approximation to properly account for the disorder in SiGe alloys. We demonstrate that our approach reproduces existing heuristics such as the wiggle well and the Germanium spike as limiting cases but also finds enhanced epitaxial profiles. Our work thus presents an interesting design tool to tailor the valley splitting in Si/SiGe heterostructures. |
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Germanium quantum wells as a novel material platform for spin qubits
Niels Focke1, Lino Visser1, Spandan Anupam1, Vincent Mourik1, Felix Reichmann2, Alberto Mistroni2, Yuji Yamamoto2, Giovanni Capellini3 1PGI-11 Forschungszentrum Jülich, 2IHP Leibniz-Institut für Innovative Mikroelektronik, 3IHP Leibniz-Institut für Innovative Mikroelektronik and Universita Roma Tre Abstract: Germanium quantum wells emerged in recent years as a promising platform for gate-defined spin qubits. The unique properties of a two-dimensional hole gas in strained Ge, with exceptional carrier mobility, compatibility with silicon-based technologies, intrinsic spin-orbit-coupling, and anisotropic g-tensor are key to this promise. Particularly, the last two properties allow fast all-electrical qubit driving and enable novel approaches for spin qubit control. Additionally, the low effective mass and Fermi level pinning to the valence band simplifies the fabrication requirements of these devices. These considerations make Germanium quantum wells an excellent material choice for spin qubits. However, many of the platform's physical properties are yet to be understood in depth. Our measurements aim to uncover the microscopic behavior of the quantum well stack. The initial focus is on one and two qubit devices, to explore and understand the anisotropy of spin-orbit interaction and g-factor tensor. We report the current progress of our studies regarding these devices. |
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Enhanced sweet spot robustness in hole flopping mode qubits
Esteban Rodriguez-Mena1, Yann-Michel Niquet2, Jose Carlos Abadillo-Uriel3 1CEA Grenoble, 2CEA/IRIG/MEM/L Sim, 3ICMM-CSIC Abstract: Manipulating spin qubits through electric fields is a desirable trait, it requires less power compared to magnetic manipulation and can be done via gate voltages in the nanostructure. However, all-electrical manipulation of spin qubits is challenging due to their magnetic nature. The usual way to overcome this challenge is to induce some form of spin-orbit coupling in the system; with electrons this is done with micromagnets and with holes the spin-orbit comes from free. Once the spin qubit is susceptible to electrical manipulation, it is possible to enhance this manipulability by enlarging the system using a double quantum dot. This type of qubit, named flopping mode [1], has large electric dipoles, allowing even spin-photon coupling of 330MHz in hole devices [2]. However, enlarging the susceptibility of spin qubits to electric fields also leads to charge-noise sensitivity. Flopping modes naturally offer sweet spots where this can be mitigated but large enough magnetic field gradients between dots destroy these sweet spots. For holes, this is currently an issue since the degree of variability between dots may lead to large g-factor differences and, hence, large effective magnetic gradients. In this work we show that realistic effects such as inhomogeneous strain or the vector potential can lead to additional corrections to the flopping mode Hamiltonian which renormalize g-factors as a function of detuning in a way that compensates large g-factor differences, enhancing the sweet spot robustness. Furthermore, we discuss the effects of these detuning-dependent effects on manipulability and spin-photon coupling of flopping modes. [1] P. Mutter and G. Burkard. Phys. Rev. Research 3, 013194 (2021). [2] C. Yu et al., Nature Nanotechnology 18, 741-746 (2023). |
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Coherent hole QD – photon interface for strong charge-photon coupling and probing strongly-correlated states
Franco De Palma1, Fabian Oppliger1, Wonjin Jang1, Stefano Bosco2, Marian Janik3, Stefano Calcaterra4, Georgios Katsaros3, Giovanni Isella4, Daniel Loss2, Pasquale Scarlino1 1École polytechnique fédérale de Lausanne, 2University of Basel, 3Institute of Science and Technology Austria, 4Politecnico di Milano Abstract: Semiconductor quantum dot (QD) qubits represent one of the promising candidates for future quantum information processing architectures. In terms of the scalability of QD qubits, superconductor – semiconductor hybrid circuits may facilitate the scaling of QD qubits to a practical limit. QDs embedded in a superconducting cavity can interact with the microwave photons residing in the cavity, where these photons can allow, for example, long-range interaction between distant QD qubits, and high-fidelity quantum state detections. Recent works have demonstrated that hole QD qubits can offer a favorable route toward large scale quantum computation based on its intrinsic spin-orbit interaction and p-type wavefunction. In the aspect of material for hole QD definition, Ge/SiGe heterostructures provide 2-dimensional hole gas with high mobility and small effective mass which allows scalable QD architectures. However, coherent interaction between a photon and a hole QD in Ge/SiGe heterostructures have not been investigated in detail up to date. Here, we investigate the strong photon coupling to hole charge qubits defined in Ge/SiGe heterostructure. By coupling a tunable, high-impedance superconducting quantum interference device (SQUID) array cavity to a double QD (DQD), we observe a vacuum-Rabi splitting of the resonator mode with a cooperativity C ~ 100 confirming the strong coupling regime. Our tunable SQUID array cavity is exploited to perform spectroscopy of the DQD charge qubits, and to further study quenched energy spectrum of strongly-correlated states. We further find the strongly-correlated states are less susceptible to charge noise due to distinct spin structure, which provides a route toward coherent spin-photon interface based on exchange interaction in a DQD. |
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Random telegraph noise in planar Ge hole qubits: sweet spots versus hot spots
Sina Gholizadeh1, Abhikbrata Sarkar1, Zhanning Wang1, Sankar Das Sarma2, Dimitrie Culcer1 1School of Physics, The University of New South Wales, Sydney 2052, Australia, 2University of Maryland Abstract: Understanding decoherence mechanisms in Ge hole quantum dots is crucial for their application as qubits. Theoretical studies predict contrasting behaviors for in-plane and out-of-plane magnetic fields. We employ symmetry arguments and a simplified Schrieffer-Wolff transformation to explain this. We identify a specific term responsible for noise-induced decoherence, demonstrating its dependence on magnetic field orientation. For out-of-plane fields, the qubit primarily interacts with the z-component of the noise electric field. This leads to a renormalization of the Rashba constant, reduced by operating the qubit at a sweet spot. Conversely, qubit in the presence of in-plane magnetic fields shows sensitivity to the in-plane noise components, primarily through orbital magnetic field terms. We show this explicitly by turning them off in the numerics. So in an in-plane magnetic field, the extremum in the qubit Zeeman splitting does not do much. |
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Electrically Driven Hole Spin Resonance Detected with Charge Sensor in a Planar Si CMOS Structure
Aaquib Shamim1, Scott Liles2, Joe Hillier2, Issac Vorreiter2, Fay Hudson3, Wee Han Lim4, Andrew Dzurak3, Alexander Hamilton5 1University of New South Wales, 2University of New South Wales,Sydney, 3Diraq,Sydney,NSW,Australia, 4Diraq, Sydney, NSW, Australia, 5University of New South Wales,Sydney,Australia Abstract: Hole-spin qubits based on Si CMOS devices have garnered attention due to their intrinsic spin-orbit interaction (SOI), weak hyperfine interaction, and anisotropic g-tensor. The SOI allows all electrical control of qubits via electric dipole spin resonance (EDSR) which removes the need for the micro-magnets or electric spin resonance strip (ESR) lines making devices less bulky. The weak hyperfine interaction increases the coherence times. The planar Si CMOS structure is industry compatible and combined with individual spin addressability via EDSR, is apt for scaling up to a larger number of qubits. This integration has not yet been shown for a known number of holes in Silicon CMOS. We studied a hole double quantum dot (DQD) operating in the Pauli spin blocked (2,8) → (1,9) charge transition regime. We were able to operate it as a singlet (S)-triplet (To) qubit. Characterizing the system in singlet-triplet basis provided insights into energy level diagram, position in detuning where Zeeman energy dominates over exchange energy and enabled determination of the g-factors of the dots. We performed microwave driven EDSR of the spins and coherently rotate the spins with a Rabi frequency up to 50 MHz. We also studied the in-plane g-factor anisotropy which varies by 100%. The result demonstrates the capability of industry compatible Si CMOS structure for operating hole spin qubits and allowing local EDSR spin control leading to rapidly driven spin qubits. |
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Probing g-factor anisotropy and spin-orbit effects towards reproducible hole-spin qubits in silicon
Joseph Hillier1, Ik Jin1, Scott Liles1, Isaac Vorreiter1, Aaquib Shamim1, Ruoyu Li2, Clement Godfrin2, Stefan Kubicek2, kristiaan degreve2, Alexander Hamilton1 1QED group, School of Physics, University of New South Wales, Sydney NSW, 2052, Australia, 2IMEC, B-3001 Leuven, Belgium Abstract: Holes in silicon quantum dots (QDs) show immense promise as spin qubits by offering all electric control via the intrinsic spin-orbit interaction on a readily scalable CMOS platform [1-2]. This has permitted a plethora of hole-spin qubit advancements, including ultra-fast singlet-triplet operation, tunable spin-orbit effects, and universal operation of a six-qubit processor [3-5]. However, the strong spin-orbit interaction is known to produce unwanted variability in the QD g-tensor, which should be addressed for efficiently up-scaling hole-spin qubits. In this work, we investigate the g-tensor sensitivity to changes in hole occupation number of a silicon planar MOS double-QD device fabricated on a 300 mm integrated platform [6]. We use electric-dipole spin-resonance (A) to extract the g-factor anisotropy (B) and measure the spin-leakage current (D) within Pauli-spin blockade to extract the spin-orbit coupling orientation. The g-factor is found to vary by more than 400% depending on orientation, but importantly, the anisotropies are consistent in response at each charge occupancy (E/F). The extracted spin-orbit coupling is aligned in-plane (θ, φ) = (88 °, -16.9 °) and strongly correlated with the difference in the g-factor at each QD site (C). Our results provide insight on the g-factor stability and underlying spin-orbit mechanisms which are crucial to understand for future manipulation and readout of spin-qubits within large hole QD arrays. [1] Maurand, R. et al. Nat Commun 7, 13575 (2016). [2] Piot, N. et al. Nat. Nanotechnol. 17, 1072–1077 (2022). [3] Liles, S. et al. arXiv:2310.09722 (2023). [4] Froning, F.N.M. et al. Nat. Nanotechnol. 16, 308–312 (2021). [5] Philips, S.G.J. et al. Nature. 609, 919-924 (2022) [6] Li, R. et al. IEEE International Electron Devices Meeting (IEDM). pp 38.3.1-38.3.4 (2020). |
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A Qubit with Simultaneously Maximized Speed and Coherence
Miguel Carballido1, Simon Svab1, Rafael Eggli1, Pierre Chevalier Kwon1, Jonas Schuff2, Rahel Kaiser1, Leon Camenzind3, Ang Li4, Natalia Ares2, Erik Bakkers5, Taras Patlatiuk1, Stefano Bosco6, Carlos Egues7, Daniel Loss1, Dominik Zumbühl1 1University of Basel, 2University of Oxford, 3RIKEN Center for Emergent Matter Science (CEMS), Wako, Japan, 4Eindhoven University of Technology, 5Eindhoven University of Technology, 6QuTech, TU Delft, 7Universidade de São Paulo / University of Basel Abstract: Strong spin-orbit interaction (SOI) has attracted major interest in quantum dot spin qubits, as it enables ultrafast, scalable and compact qubit devices. However, the SOI not only facilitates all electrical driving, but at the same time couples the qubit to charge noise, leading to dephasing. This seems to suggest that stronger SOI leads to faster qubit operation but only at the cost of coherence. Here, we provide experimental evidence in a Ge/Si core/shell nanowire demonstrating a qubit which reaches its fastest operation speed exactly at the most coherent point, which coincides with vanishing derivatives of its g-factor with respect to all gate voltages. Such a compromise-free sweet spot combining qubit speed and coherence requires a strong SOI with a maximum in coupling strength. Here, this is likely provided by strong and gate tunable direct-Rashba SOI in the 1D confinement of the Ge/Si nanowire employing iso-Zeeman driving from a remote gate. Our results overturn the conventional paradigm that fast operation speeds imply reduced qubit lifetimes, even at the elevated temperature of 1.5 K. Funding Acknowledgement: This work was supported by the NCCR SPIN of the SNSF, SNI, EMP Nr. 824109, FET TOPSQUAD Nr. 862046 and G. H. E. Foundation |