Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session B39: Semiconductor Qubits IFocus Recordings Available
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Sponsoring Units: DQI DCMP Chair: Hannes Bernien, UChicago; Felix Borjans Room: McCormick Place W-196A |
Monday, March 14, 2022 11:30AM - 12:06PM |
B39.00001: Exchange-only CNOT using the SLEDGE quantum dot architecture Invited Speaker: Matthew D Reed Semiconducting quantum dot qubits are a compelling quantum information technology due to their manufacturability, excellent coherence, and high-fidelity SPAM and gate operations. Exchange-only implementations are of particular interest because they enable qubit control using only asynchronous, baseband voltage pulses. Two-qubit entangling gates between encoded qubits can also be implemented in an exchange-only context [1, 2], though experimental demonstrations have remained unrealized since such gates involve roughly ten times the number of pulses in single-qubit operations and place strict requirements on device yield and performance. Here we demonstrate randomized benchmarking and tomography of a two-qubit controlled NOT gate between two exchange-only qubits fabricated using the recently published Single-Layer Etch-Defined Gate Electrode (SLEDGE) architecture [3]. The SLEDGE process can more reliably achieve the device uniformity required for a two-qubit exchange-only gate while maintaining high levels of performance. Relative to prior two-qubit gate results in silicon qubits, this approach simplifies signal generation and device design [4]. |
Monday, March 14, 2022 12:06PM - 12:18PM |
B39.00002: Charge-noise spectroscopy of Si/SiGe quantum dots via dynamically-decoupled exchange oscillations Elliot J Connors Electron spins in silicon quantum dots are promising qubits due to their long coherence times, scalable fabrication, and potential for all-electrical control. However, charge noise in the host semiconductor presents a major obstacle to achieving high-fidelity single- and two-qubit gates in these devices. In this work, we measure the charge-noise spectrum of a Si/SiGe singlet-triplet qubit over more than 13 decades in frequency using a combination of methods, including dynamically-decoupled exchange oscillations with up to 512 π pulses during the qubit evolution. The charge noise is colored across the entire frequency range of our measurements, although the spectral exponent changes with frequency. Moreover, the charge-noise spectrum inferred from conductance measurements of a proximal sensor quantum dot agrees with that inferred from coherent oscillations of the singlet-triplet qubit, suggesting that simple transport measurements can accurately characterize the charge noise over a wide frequency range in Si/SiGe quantum dots. |
Monday, March 14, 2022 12:18PM - 12:30PM |
B39.00003: Limits to two-spin-qubit gate fidelity from vacuum and thermal fluctuations Wenbo Sun, Yifan Wang, Sathwik Bharadwaj, Li-Ping Yang, Yu-ling Hsueh, Ashwin K Boddeti, Rajib Rahman, Zubin Jacob The basic building block in a quantum circuit is the logical qubit which requires high fidelity of physical qubit operation. In spin qubit quantum computing systems, metallic gates and antennas are necessary for qubit operation, initialization, and readout. Thermal and vacuum fluctuations are enhanced in the vicinity of metallic gates, and the gate fidelity is limited by corresponding evanescent wave Johnson noise (EWJN). Here, we use an open quantum system model to describe Markovian spin qubit relaxation and decoherence processes using Lindblad master equation. We study the fundamental limits to two spin-qubit gate fidelity from EWJN in two promising quantum computing systems: NV centers in diamond and silicon quantum dot system. We first consider optimization of control pulse design where we propose an open qubit driving protocol that is more robust against Markovian relaxation and decoherence processes. Then, we perform geometry optimization of metallic gates and antennas to obtain the optimal gate fidelity. Our work provides a rigorous treatment to reach the limits of two-spin-qubit gate fidelity overcoming the effects of vacuum and thermal fluctuations. |
Monday, March 14, 2022 12:30PM - 12:42PM |
B39.00004: Two-qubit logic gates with qubits made by advanced semiconductor manufacturing Guoji Zheng, Thomas F Watson, Elliot J Connors, Lester F Lampert, Samuel Neyens, otto k zietz, Felix Borjans, Florian Luthi, Simon Schaal, Hubert C George, Eric M Henry, Roza Kotlyar, Ravi Pillarisetty, Stephanie A Bojarski, Jeanette M Roberts, Jim S Clarke Silicon spin qubits offer a promising platform for building a scalable quantum processor owing to their compatibility with advanced semiconductor manufacturing [1]. Intel has pioneered the fabrication of SiMOS and Si/SiGe quantum dot devices on an all-optical, 300mm process line, and has successfully produced qubits [2,3]. However, the ability to perform two-qubit logic gates on these industrial devices, which is a key requirement for a multi-qubit processor, has yet to be achieved. In this talk, we report the demonstration of two-qubit gates on Si/SiGe quantum dot devices made with 300mm EUV technology. The spin qubit consists of a single electron contained in a quantum dot. Single-qubit gates are performed via EDSR, with nearby micromagnets providing the necessary artificial spin-orbit coupling as well as the qubit frequency separation. Rapid voltage pulses are applied to control the exchange coupling between two qubits, allowing us to realize various types of two-qubit gates. We detail progress on optimizing control pulses to improve the quality of our two-qubit gates and determining the limiting factor for the gate fidelity. |
Monday, March 14, 2022 12:42PM - 12:54PM |
B39.00005: Multi-level, Multi-photon Landau-Zener-Stuckelberg Interferometry of Two-Electron States in Semiconductor Quantum Dots Yadav P Kandel, Haifeng Qiao, John Nichol Gate-defined quantum dots enable defining few-level quantum systems with charge and spin degrees of freedom of one or many electrons. Such few-level systems are important and robust platforms for encoding and manipulating quantum information. The same few-level systems can also be used to study different aspects of natural and engineered quantum interactions between electrons. We present a study of multiple-photon and multiple-level Landau-Zener-Stuckelberg interference of both charge and spin states of two electrons in a semiconductor double quantum dot. By driving the electrons with a microwave electrical signal, we sweep them periodically through multiple avoided crossings that couple different spin and charge states. Our results agree with theoretical calculations and provide a new perspective on Landau-Zener-Stuckelberg interferometry in quantum systems. |
Monday, March 14, 2022 12:54PM - 1:06PM |
B39.00006: Optimal Readout of Spin Qubits with Correlated Noise Michael J Gullans, Michael D Stewart As estimated fidelities for quantum control, state preparation, and readout of semiconductor spin qubits begin to approach or exceed thresholds for fault-tolerant quantum computing, it is essential to develop rigorous measurement protocols that account for the known sources of noise in spin-qubit devices. Here, we develop a theory of optimal spin readout in the presence of time-correlated noise processes. For white noise, the spin readout fidelity from the optimal Bayesian inference strategy can be obtained from a time-local transfer matrix analysis of the measured current signal, analogous to the calculation of the partition function of a local one-dimensional statistical mechanics model. However, in the presence of noise correlations, the optimal readout strategy becomes non-local in time, requiring one to keep track of an exponentially growing number of trajectories in the analysis. We develop efficient algorithms for this problem and use our readout method to benchmark other commonly employed sub-optimal strategies. Our work unifies the theory of several recently proposed methodologies for spin-readout. Moreover, these results represent a first step towards a rigorous theory of spin-qubit quantum process tomography in the presence of correlated noise. |
Monday, March 14, 2022 1:06PM - 1:18PM |
B39.00007: Parametric longitudinal coupling between a high-impedance superconducting resonator and a semiconductor quantum dot singlet-triplet spin qubit Charlotte Boettcher, Shannon Harvey, Saeed Fallahi, Geoff Gardner, Michael J Manfra, Uri Vool, Stephen D Bartlett, Amir Yacoby Long-distance two-qubit coupling, mediated by a superconducting resonator, is a leading paradigm for performing entangling operations in a quantum computer based on spins in semiconducting materials. Here, we demonstrate a novel, controllable spin-photon coupling based on a longitudinal interaction between a spin qubit and a resonator. We show that coupling a singlet-triplet qubit to a high-impedance superconducting resonator can produce the desired longitudinal coupling when the qubit is driven near the resonator's frequency. We measure the energy splitting of the qubit as a function of the drive amplitude and frequency of a microwave signal applied near the resonator antinode, revealing pronounced effects close to the resonator frequency due to longitudinal coupling. By tuning the amplitude of the drive, we reach a regime with longitudinal coupling exceeding 1 MHz. This demonstrates a new mechanism for qubit-resonator coupling, and represents a stepping stone towards producing high-fidelity two-qubit gates mediated by a superconducting resonator. |
Monday, March 14, 2022 1:18PM - 1:30PM |
B39.00008: Switching between relaxation hotspots and coldspots in disordered spin qubits Guido Burkard, Amin Hosseinkhani We present a valley-dependent envelope function theory that can describe the effects of arbitrary configurations of interface steps and miscuts on the qubit relaxation time. For a given interface roughness, we show how our theory can be used to find the valley-dependent dipole matrix elements, the valley splitting, and the spin-valley coupling as a function of the electromagnetic fields in a Si/SiGe quantum dot spin qubit. We demonstrate that our theory can quantitatively reproduce and explain the result of experimental measurements for the spin relaxation time with only a minimal set of free parameters. Investigating the sample dependence of spin relaxation, we find that at certain conditions for a disordered quantum dot, the spin-valley coupling vanishes. This, in turn, completely blocks the valley-induced qubit decay. We show that the presence of interface steps can in general give rise to a strongly anisotropic behavior of the spin relaxation time. Remarkably, by properly tuning the gate-induced out-of-plane electric field, it is possible to turn the spin-valley hot spot into a "cold spot" at which the relaxation time is significantly prolonged and where the spin relaxation time is additionally first-order insensitive to the fluctuations of the magnetic field. This electrical tunability enables on-demand fast qubit reset and initialization that is critical for many quantum algorithms and error correction schemes. We therefore argue that the valley degree of freedom can be used as an advantage for Si spin qubits. |
Monday, March 14, 2022 1:30PM - 1:42PM |
B39.00009: Optimizing single-qubit and two-qubit gates in spin qubit arrays Irina Heinz Scaling up spin qubit systems requires high-fidelity single-qubit and two-qubit gates. Gate fidelities exceeding 99% were already demonstrated in silicon based quantum dots, whereas for the realization of larger qubit arrays crosstalk effects on neighboring qubits must be taken into account. We analyze qubit and gate fidelities impacted by driving fields when performing single-qubit and two-qubit operations with a simple Heisenberg model. Furthermore we propose conditions for driving fields to robustly optimize qubit gates and avoid crosstalk effects leading to a restricted choice for the driving field strength, exchange interaction, and thus gate time. Considering realistic experimental conditions we propose a set of parameter values to perform an optimal CNOT gate and so open up the pathway to scalable quantum computing devices. |
Monday, March 14, 2022 1:42PM - 1:54PM |
B39.00010: All-silicon double quantum dot architecture for spin qubit Claude Rohrbacher, Clement Godfrin, Stefan Kubicek, Bogdan Govoreanu, Iuliana P Radu, Eva Dupont-Ferrier Silicon spin qubits, with their long coherence time [1] and compatibility with industrial CMOS technology [2-4], hold great promise for large-scale quantum computing. Nevertheless, improvements in device reproducibility and fabrication are still needed to build a scalable spin qubit architecture.
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Monday, March 14, 2022 1:54PM - 2:06PM |
B39.00011: Two-qubit gates on cavity-coupled spin-qubit systems Stephen R McMillan, Guido Burkard A critical element towards the realization of quantum networks is non-local coupling between nodes. Scaling connectivity beyond nearest-neighbor interactions requires the implementation of a mediating interaction often termed a “quantum bus”. Cavity photons have long been used as a bus by the superconducting qubit community, but it has only recently been demonstrated that spin-based qubits in double quantum dot architectures can reach the strong coupling regime [1,2] and exhibit spin-spin interactions via real or virtual photons [3,4]. The capability to perform two-qubit gates is expected in the dispersive regime, where cavity loss plays a less prominent role [5]. Here we explore the potential for non-local quantum gating schemes between single-spin qubits mediated by virtual photons. |
Monday, March 14, 2022 2:06PM - 2:18PM |
B39.00012: Computing with spin qubits at the surface code error threshold Maximilian Russ, Xiao Xue, Nodar Samkharadze, Brennan Undseth, Amir Sammak, Giordano Scappucci, Lieven Vandersypen High-fidelity control of quantum bits is paramount for the reliable execution of quantum algorithms and for achieving fault-tolerance, the ability to correct errors faster than they occur. The central requirement for fault-tolerance is expressed in terms of an error threshold. Whereas the actual threshold depends on many details, a common target is the ~1% error threshold of the well-known surface code. Reaching two-qubit gate fidelities above 99% has been a long-standing major goal for semiconductor spin qubits. These qubits are promising for scaling as they can leverage advanced semiconductor technology. Here we report a spin-based quantum processor in silicon with single- and two-qubit gate fidelities all above 99.5%, extracted from gate set tomography. The average single-qubit gate fidelities remain above 99% when including crosstalk and idling errors on the neighboring qubit. Having surpassed the 99% barrier for the two-qubit gate fidelity, semiconductor qubits are well positioned on the path to fault-tolerance and to possible applications in the era of noisy intermediate-scale quantum (NISQ) devices. In this talk I will focus on the aspects which leads to the realization of the high-fidelity CZ gate and what we can learn from the remaining errors. |
Monday, March 14, 2022 2:18PM - 2:30PM |
B39.00013: Highly tunable exchange-only singlet-only qubit in a GaAs triple quantum dot Jørgen H Qvist, Arnau Sala, Jeroen Danon One of the largest challenges for GaAs-based spin qubits is decoherence caused by the random nuclear field of the host material. A potential solution to mitigate this source of decoherence is to encode the qubit in a singlet-only subspace, where the qubit states are insensitive to the nuclear field to lowest order. We propose an implementation of such a singlet-only qubit in a GaAs-based triple quantum dot with a (1, 4, 1) charge occupation. In the central multielectron dot, the interplay between Coulomb interaction and an out-of-plane magnetic field creates an energy spectrum with a tunable singlet-triplet splitting, which can be exploited to create a six-particle singlet-only qubit with a qubit splitting that can straightforwardly be tuned over tens of μeV by adjusting the external magnetic field. We confirm the full exchange-based electric control of the qubit and demonstrate its superior coherence properties due to its singlet-only nature. |
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