Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session Z74: Semiconducting Qubits VIFocus
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Sponsoring Units: DQI Chair: Akito Noiri, RIKEN Room: Room 403/404 |
Friday, March 10, 2023 11:30AM - 12:06PM |
Z74.00001: Implementing universal control in encoded exchange-only Si/SiGe spin qubits Invited Speaker: Aaron Weinstein Exchange-only silicon spin qubits are an encouraging platform for quantum computing mainly due to the existing strengths of silicon-based material development and CMOS fabrication techniques. However, this encoding scheme comes at the cost of increased number of spins-per-qubit and increased number of pulses-per-quantum gate in the encoded computational space. Recently, HRL has implemented encoded logic on arrays of six quantum dots in Si/SiGe [1]. This challenging experiment required key advances in fabrication, automation, calibration, software and analysis. In this talk, I will elaborate on several of these advances that enabled exchange-only quantum computing in arrays of dots, including pathways to reduce error and to increase scale. The exchange-only encoding has built-in noise protection and utilizes a single physical control mechanism for all qubit initialization and control, which together form a promising pathway towards fault tolerance. |
Friday, March 10, 2023 12:06PM - 12:18PM |
Z74.00002: Magnetic gradient fluctuation compensation during universal control of a Si/SiGe exchange-only qubit Teresa L Brecht Time-fluctuating local magnetic fields are a significant noise source affecting spin qubits. In exchange-only qubits encoded in a decoherence-free subsystem (DFS), magnetic field gradients cause both error and leakage out of the encoding [1]. For quantum dots in Si/SiGe, the dominant magnetic noise process is the hyperfine interaction between the electron spins and those of the Si-29 and Ge-73 nuclei [2]. Previously, echo sequences [2,3] have proven effective at decoupling the transverse component of these fluctuating gradients. Here, we demonstrate that is possible to construct a complete set of Clifford gates that provides universal control over DFS qubits while simultaneously incorporating gradient compensation echoes. We show blind randomized benchmarking [1] data using this gate-set with baseline single qubit error rates of 1.8e-3 per Clifford gate, evaluated as a function of both the global magnetic field and the idle time between exchange pulses. Despite containing over three times as many pulses, the gradient-compensating Clifford gate-set out-performs our standard Clifford gate-set in a regime of extended idle durations and applied field. These results point toward error compensation of hyperfine effects as a possible avenue for improving operational fidelity in exchange-only quantum computing. |
Friday, March 10, 2023 12:18PM - 12:30PM |
Z74.00003: Driving charge noise sources in semiconductor qubit devices with oscillating electric field Yujun Choi, Susan N Coppersmith, Robert J Joynt Charge noise is thought to be the major decoherence mechanism of qubits in semiconductor qubit devices. It gives rise to 1/f-like noise that has been reported in a wide variety of devices. This produces short coherence times T2 due to the high noise power at low frequencies. It is suspected that two-level systems (TLSs) made up of charge defects or traps are responsible for the 1/f noise. We propose a method to characterize the TLSs and reduce the noise by applying an oscillating electric field to drive the defects. The driving field can shift the power spectral density to higher frequencies. We analyze this phenomenon using the purely stochastic resonance method and also the Lindblad equation method to test the idea that the TLSs can also exhibit some level of quantum coherence. We show that the coherence time of a qubit can be increased substantially when certain conditions on the driving field are satisfied, which is similar to the observations of spin bath driving for spin qubits in diamonds. |
Friday, March 10, 2023 12:30PM - 12:42PM |
Z74.00004: How cryogenic illumination resets gate defined semiconductor quantum devices Michael Wolfe, Brighton X Coe, Tyler Kovach, Thomas McJunkin, Benjamin Harpt, Donald E Savage, Max G lagally, Gabriel J Bernhardt, Robert McDermott, Shimon Kolkowitz, Mark A Eriksson Illumination is widely used to "reset" quantum-dot qubit devices at cryogenic temperatures. While highly effective, this technique has not been well studied and is often discussed informally as poorly understood lore. We present the results of systematic measurements of threshold voltage shifts in gate defined Si/SiGe Hall bars using a near infrared (780 nm) laser diode. We find that illumination under an applied gate voltage can be used to dial in a specific, stable, and reproducible threshold voltage in the range of ±0.5 V. Outside this range the threshold voltage can still be tuned, however the tunability diminishes due to the finite density of interface traps at the oxide interface, eventually saturating near ±1 V. We present a simple and intuitive model that explains both these results and why cryogenic illumination is successful at resetting quantum dot qubit devices after charging events. |
Friday, March 10, 2023 12:42PM - 12:54PM |
Z74.00005: Fast quantum control via adiabatic resonance Xuedong Hu, Xinyu Zhao As a major technique to realize quantum gates in superconducting and semiconductor qubits, adiabatic passage (AP) is robust but slow. Here we propose a universal requirement for all schemes aiming at accelerating AP, that we call the "adiabatic resonance" (AR) condition. In essence, the evolution in the adiabatic frame should be cyclic. Furthermore, we design a scheme to realize fast AP by extending the AR condition to multiple AR, namely letting the evolution path periodically return to the adiabatic path. We have applied the AR condition to two- and three-level examples. In particular, we derive the most general condition of AR for an arbitrary two-level system, and apply it in a double quantum dot system to realize rapid adiabatic Landau-Zener transition. The robustness of the scheme is analyzed for low- and high-frequency noises. We also show an example in three-level system to accelerate the adiabatic manipulation and discuss the possibility of optimizing the pulses and the robustness against noisy pulses. |
Friday, March 10, 2023 12:54PM - 1:06PM |
Z74.00006: High-fidelity CPHASE gate in a pair of capacitively coupled few-electron singlet-triplet qubits Xin Wang, Guo Xuan Chan Due to their limited coupling to charge noises, spin qubits have been the main candidates for robust quantum information processing in semiconductor quantum dot devices. Among the proposed spin qubits, singlet-triplet qubits stand out due to their all electrical control scheme. Although high-fidelity single-qubit operations have been experimentally demonstrated, the fidelities of two-qubit capacitive gates are limited. In contrast to conventional two-electron singlet-triplet qubits, we propose to host the capacitive gates between a pair of four-electron singlet-triplet qubits, each of which is operated in the detuning regime where the electron occupation is asymmetric. Using full configuration interaction calculations, we show that the non-monotonic behavior of the dipole moment of each qubit leads to an optimal operating point where the capacitive coupling is maximal and the effective exchange energies are first-order insensitive to charge noises. Numerical simulations under realistic charge noises and hyperfine noises show that operating CPHASE gates at the optimal point can achieve fidelities above 99% [1]. |
Friday, March 10, 2023 1:06PM - 1:18PM Author not Attending |
Z74.00007: Measurement of correlated charge noise of two coupled silicon quantum dot qubits John Rooney, Will Qinghe Wang, HongWen Jiang Gate-defined quantum dots in silicon have shown to be a promising platform for encoding qubits due to their long coherence times and scalability with current industrial fabrication technology [1]. For these systems, charge noise due to fluctuating electric fields remains an important driver for their decoherence [2]; however, the correlation of this noise between qubits has not been extensively studied. In this talk, we discuss our results regarding this noise affecting two adjacent double quantum dots (DQDs) in a linear array Si/SiGe heterostructure device. We find the charge noise of the two DQDs becomes correlated with sufficiently strong coupling between the DQD pairs and the noise for each DQD is minimized when the detuning of the partner DQD reaches zero. We investigate how this correlation evolves as we tune the inter-dot coupling, and the mechanism behind this behavior is discussed. |
Friday, March 10, 2023 1:18PM - 1:30PM |
Z74.00008: Fidelity assessment and tomography of symmetrically pulsed 2Q gates in MOS quantum dots. Tuomo I Tanttu, Wee Han Lim, Jonathan Y Huang, Nard Dumoulin Stuyck, William Gilbert, MengKe Feng, Jesus D Cifuentes Pardo, Amanda E Seedhouse, Christopher Escott, Kohei M Itoh, Michael Thewalt, Fay E Hudson, Andrea Morello, Arne Laucht, Henry Yang, Andre Saraiva, Andrew S Dzurak Qubits based on silicon are one of the most promising pathways to scalable quantum computing. Recently several groups have demonstrated two-qubit primitive gate fidelities above 99% in Si-SiGe and phosphorus donors in silicon using either interleaved randomized benchmarking or Gate Set Tomography (GST) [1-5]. In this work, we study two-qubit gate fidelities in silicon MOS double quantum dot systems using three similar devices. We use three different quantum characterization, verification, and validation (QCVV) methods; interleaved randomized benchmarking, Fast Bayesian Tomography (FBT) [6] and GST. Using tunable exchange and composite pulses we demonstrate controlled phase gate and decoupled controlled phase gate in two of the three devices and show above 99% fidelity as measured by interleaved randomized benchmarking. |
Friday, March 10, 2023 1:30PM - 1:42PM |
Z74.00009: Dispersively sensed entangling gate in silicon quantum dots fabricated on 300mm wafers Jacob F Chittock-Wood, Ross Leon, Michael A Fogarty, Sofia M Patomäki, Felix von Horstig, Adam Siegel, Hamza Jnane, Julien Jussot, Stefan Kubicek, Bogdan Govoreanu, Simon C Benjamin, Fernando Gonzalez-Zalba, John Morton Silicon-based quantum processors offer scaling advantages by combining a small qubit footprint with advanced semiconductor manufacturing, promising high density, uniform qubit arrays readily integrated with complementary metal-oxide-semiconductor (CMOS) technology. Here we characterise a maximally entangling gate on a two-electron spin state defined in a double quantum dot. The dots are hosted in a planar MOS structure in natural silicon, fabricated using a hybrid 300mm optical and electron beam lithography process. This is paired with fast readout via radio-frequency dispersive measurement, enabled by an off-chip 512 MHz superconducting resonator, allowing projective measurement of the two-electron spin states. We demonstrate coherent control via the exchange interaction to perform a √SWAP gate in ≤ 8 ns within a decay time of T2SWAP ≈ 400 ns, leading to a gate quality factor ≈ 25 at this control point. The combination of this maximally entangling gate with dispersive readout in a device manufactured using 300mm wafer scale processing presents a simultaneous demonstration of many of the key ingredients required for a scalable unit cell for a silicon-based quantum processor. |
Friday, March 10, 2023 1:42PM - 1:54PM |
Z74.00010: Efficient entanglement generation using a spin bus Miguel G Rodriguez, Mark Friesen, Yun-Pil Shim The current state-of-the-art quantum computing model realizes quantum circuits using a sequence of quantum gates from a universal one- and two-qubit gate set. Quantum circuits comprised of only those gate operations typically require a very long sequence of such quantum gates, putting some significant restrictions on the quantum coherence and the quantum circuit complexity. For spin qubits in semiconductor quantum dot systems, the exchange interaction between neighboring pairs of spin qubits is the standard mechanism for the two-qubit gates. The very short-range nature of the exchange coupling leads to a large overhead in implementing two-qubit gates between qubits that are far apart. The spin bus, a strongly coupled spin chain, has been proposed to overcome these shortcomings and to provide long-range, multi-qubit gate operations efficiently. We focus on finding the optimal quantum circuits with the spin bus multi-qubit entangling gates by using numerical optimization methods. We compare our results for quantum circuits for the generation of typical entangled states with the conventional exchange-based approach and show that the spin bus can perform the tasks while considerably reducing the depth of the quantum circuits. This work demonstrates the potential of multi-qubit gates for the efficient implementation of large quantum circuits. |
Friday, March 10, 2023 1:54PM - 2:06PM |
Z74.00011: GPU-Accelerated Simulations of Charge Qubits in Semiconductor Devices. Hugo V Lepage Theoretical modelling of the evolution of wave functions within quantum computing devices can both help design new circuits as well as understand the behaviour of current devices. This is particularly useful when investigating possible sources of error and optimizing the fidelity of quantum operations. In our recent works, we present accurate simulations of the evolution of electron-charge wave functions in semiconductor devices. We present optimal qubit basis definitions that maximize the fidelity of logic operations and put forward pulse sequences that achieve a universal set of operations. We consider experimentally realistic semiconductor qubits with finite pulse rise and fall times and determine the fastest pulse sequence yielding the highest fidelity and show that our protocol leads to improved control of a qubit. Furthermore, we present simulation results for two-qubit interactions in entangling operations, once more optimizing the dynamic confining potential in order to obtain high-fidelity operations. |
Friday, March 10, 2023 2:06PM - 2:18PM |
Z74.00012: Sources of Dephasing in Si/SiGe Quantum Dots Amir Shapour Mohammadi, Adam R Mills, Jason R Petta We utilize a sensor dot positioned near the Si/SiGe quantum dot (QD) to probe the Hz range of the power spectral density (PSD) of electrostatic charge noise. We use data from Ramsey and CPMG experiments to probe sub-mHz and sub-KHz PSD of frequency detuning noise, allowing us to estimate the dominance of charge noise. Discrepancies between charge noise characterizations and frequency detuning noise measurements indicate that other noise sources such as hyperfine interactions could play a considerable role in qubit dephasing. To remove the effect of spatial-separation of the sensor dot from the QD, we measure the QD chemical potential after each Ramsey shot. We see little time-correlation between the chemical potential and frequency detuning, indicating that charge noise may not be the dominant source of dephasing in this device, and we may need better techniques for budgeting noise considerations. |
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