Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session N41: Noise Characterization, Mitigation and Engineering in Superconducting QubitsFocus Recordings Available
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Sponsoring Units: DQI DCMP Chair: Matthew McEwen, UCSB Room: McCormick Place W-196C |
Wednesday, March 16, 2022 11:30AM - 11:42AM |
N41.00001: Decreased Transmon T1 and Increased Pe from High Resonator Drive Power Yizhou Huang, Zachary Steffen, Haozhi Wang, Frederick C Wellstood, Benjamin Palmer We discuss detrimental effects on the lifetime T1 and initial excited state population (Pe) of a 6 GHz Al/AlOx/Al transmon when pumping the read-out resonator at large powers. In our device, the qubit is coupled to an 8 GHz Al resonator that is in turn coupled to an input-output transmission line for measuring the resonator. Using a low power dispersive readout, we find T1 ~ 30us and Pe ~ 0.36%. On the other hand, when pumping the resonator at high powers, slightly above the Jaynes-Cummings nonlinear readout point[1], T1 decreases to 2 us and the excited state population increases more than an order of magnitude to Pe ~ 10%. After a high power pulse, T1 recovers on a time scale that is consistent with quasiparticle tunneling through the junction being the dominant loss mechanism[2]. On the other hand, as we will discuss, Pe has a non-trivial dependence on the detuning of the drive from the qubit transition frequency. |
Wednesday, March 16, 2022 11:42AM - 11:54AM |
N41.00002: Diagnosing errors in superconducting two-qubit gates using continuous measurements John Steinmetz, Debmalya Das, Karthik Siva, Gerwin Koolstra, William P Livingston, Larry Chen, Christian Juenger, Noah J Stevenson, Ravi K Naik, David I Santiago, Irfan Siddiqi, Andrew N Jordan Improving the fidelity of two-qubit gates is essential for performing quantum algorithms on superconducting quantum computers. By using continuous weak measurements of superconducting qubits throughout a two-qubit gate, we can reveal applied pulse shapes and coherent gate errors with high time resolution. We treat potential coherent gate errors as unknown time-dependent parameters in the Hamiltonian and estimate them by fitting the measured voltage records to a master equation. We experimentally demonstrate this method on imperfect single-qubit and parametric entangling two-qubit gates, and show that we can accurately reconstruct errors such as over-rotations and leakage out of the computational subspace. The gate fidelity can be improved by designing a correction pulse that cancels out the reconstructed error. |
Wednesday, March 16, 2022 11:54AM - 12:06PM |
N41.00003: Resolving catastrophic error bursts in large arrays of superconducting qubits Matthew J McEwen, Lara Faoro, John Martinis, Lev Ioffe, Rami Barends Scalable quantum computing can become a reality with error correction, provided coherent qubits can be constructed in large arrays. The key premise is that physical errors can remain both small and sufficiently uncorrelated as devices scale, so that logical error rates can be exponentially suppressed. However, energetic impacts from cosmic rays and latent radioactivity violate both of these assumptions. We use the scale of Google's Sycamore processor to directly observe impacts of high-energy rays and identify large bursts of quasiparticles that simultaneously and severely limit the energy coherence of all qubits, causing chip-wide failure. We track the events from their initial localised impact to high error rates across the chip. Our results provide direct insights into the scale and dynamics of these damaging error bursts in large-scale devices, and highlight the necessity of mitigation to enable quantum error correction at scale. |
Wednesday, March 16, 2022 12:06PM - 12:42PM |
N41.00004: Correlated charge noise and relaxation errors in superconducting qubits Invited Speaker: Chris D Wilen The central challenge in building a quantum computer is error correction. Unlike classical bits, which are susceptible to only one type of error, quantum bits ("qubits") are susceptible to two types of error, corresponding to flips of the qubit state about the X- and Z-directions. While the Heisenberg Uncertainty Principle precludes simultaneous monitoring of X- and Z-flips on a single qubit, it is possible to encode quantum information in large arrays of entangled qubits that enable accurate monitoring of all errors in the system, provided the error rate is low. Another crucial requirement is that errors cannot be correlated. Here, we characterize a superconducting multiqubit circuit and find that charge fluctuations are highly correlated on a length scale over 600~μm; moreover, discrete charge jumps are accompanied by a strong transient suppression of qubit energy relaxation time across the millimeter-scale chip. The resulting correlated errors are explained in terms of the charging event and phonon-mediated quasiparticle poisoning associated with absorption of gamma rays and cosmic-ray muons in the qubit substrate. Robust quantum error correction will require the development of mitigation strategies to protect multiqubit arrays from correlated errors due to particle impacts. |
Wednesday, March 16, 2022 12:42PM - 12:54PM |
N41.00005: Characterizing and mitigating crosstalk errors of simultaneous entangling gates in a superconducting circuit Youngkyu Sung, Amy Greene, Leon Ding, Agustin Di Paolo, Gabriel O Samach, Bharath Kannan, Antti Vepsalainen, Jochen Braumuller, Roni Winik, Elaine Pham, David K Kim, Alexander Melville, Bethany M Niedzielski, Kyle Serniak, Mollie E Schwartz, Jonilyn L Yoder, Jeffrey A Grover, Joel I Wang, Terry P Orlando, Simon Gustavsson, William D Oliver The ability to perform simultaneous two-qubit gates with low error rates is a key requirement to run a quantum algorithm with as short of a circuit depth as possible. However, crosstalk errors in a superconducting qubit system, mostly originating from stray electromagnetic coupling between circuit elements, make it challenging to implement simultaneous entangling gates with high fidelities. Here, we characterize and mitigate crosstalk errors in a 2x2 array of superconducting qubits, where adjacent qubit-qubit interactions are mediated by tunable couplers. We benchmark the fidelities of simultaneous two-qubit gates (CZ-CZ, CZ-iSWAP, or iSWAP-iSWAP) applied on qubit pairs and identify dominant error sources on a case-by-case basis. |
Wednesday, March 16, 2022 12:54PM - 1:06PM |
N41.00006: Characterization of a superconducting metamaterial quantum many-body simulator Xueyue Zhang, Eun Jong Kim, Oskar Painter Superconducting quantum circuits are an emerging hardware platform for directly simulating quantum many-body systems with precise control and read-out at the single qubit (spin) level. Microwave metamaterial circuits introduced into the wiring of a superconducting quantum processor can serve both as a quantum bus for mediating tunable, long-range coupling among qubits, and as a read-out bus for multiplexed qubit read-out. In this talk, we describe our work to realize and characterize a metamaterial-based quantum processor with 10 qubits. We characterize both single-qubit properties including coherence and anharmonicity, and the long-range coupling among qubits. Using the metamaterial as a Purcell filter, we demonstrate multiplexed high-fidelity single-shot readout for all 10 qubits. The many-body Hamiltonian is probed as a function of qubit detuning from the two photonic bandedges of the metamaterial, revealing subtle differences in the accessible quantum phases of the Bose-Hubbard model. Based on the measured system properties, we also propose several new opportunities for many-body simulation on such a processor. |
Wednesday, March 16, 2022 1:06PM - 1:18PM |
N41.00007: Towards Disorder-Induced Dynamics of Flatband States in Superconducting Circuits Jeronimo G Martinez, Christie S Chiu, Basil M Smitham, Andrew A Houck The physics of strongly correlated many-body systems can be explored in systems with dispersionless energy bands: flat bands. Superconducting circuits provide a flexible architecture to simulate tight-binding Hamiltonians by placing qubits in specific geometries to generate photonic lattices; the non-linearity of qubits generate effective interactions for microwave photons in the lattice. In this talk, I present our recent progress towards generating a flat band state in a quasi-1D lattice made out of transmon qubits. By introducing disorder in the qubit energies, we study the dynamics and thermalization of flat bands. |
Wednesday, March 16, 2022 1:18PM - 1:30PM |
N41.00008: Stabilizing two-qubit remote entanglement with engineered synthetic squeezing Aashish Clerk, Andrew Lingenfelter, Luke C Govia It is well known that qubits immersed in a squeezed vacuum environment exhibit many exotic phenomena, including dissipative entanglement stabilization. Here, we show that these effects only require interference between excitation and decay processes, and can be faithfully realized without using non-classical light; instead, one uses simple classical temporal modulation. We present and analyze two schemes that harnesses this idea to stabilize entanglement between two remote qubits coupled via a transmission line or waveguide, where either the qubit-waveguide coupling is modulated, or the qubits are directly driven. Our protocols are especially well suited to state of the art circuit cavity QED systems featuring tuneable coupling elements (e.g. Ref. [1]), as well as more general circuit waveguide QED systems (e.g Ref. [2]). |
Wednesday, March 16, 2022 1:30PM - 1:42PM |
N41.00009: Flux noise in superconducting qubits: A second principles theory José Alberto Nava Aquino, Rogério de Sousa Impurity spins randomly distributed at the surfaces and interfaces of superconducting wires are known to cause flux noise in Superconducting Quantum Interference Devices (SQUIDs), providing a dominant mechanism for decoherence and Hamiltonian noise in all flux-tunable superconducting qubits [1]. While flux noise is well characterized experimentally, the microscopic model underlying spin dynamics remains a great puzzle. Available first-principles theories are too computationally expensive to capture spin diffusion over large length scales, hindering comparisons between microscopic models and experimental data. In contrast, third principles approaches lump spin dynamics into a single phenomenological spin-diffusion operator D∇2, preventing connection to microscopic models and the impact of different disorder scenarios such as the presence of spin clusters. Here we propose an intermediate "second principles" method to describe spin diffusion and flux noise. It is based on a discrete version of the diffusion operator that connects directly to microscopic models, and becomes the usual Laplacian in the continuum limit. We apply the method to Heisenberg models in two dimensional square lattices with a random distribution of vacancies, with nearest-neighbor spins coupled by constant ferromagnetic exchange. At high frequencies ω our results reveal the regime of quantum 1/ω flux noise, with amplitude determined by inhomogeneity (spin cluster formation) and confining effects such as the presence of wire edges. The method establishes a connection between flux noise experiments and microscopic Hamiltonians with the goal of guiding strategies for reducing flux noise. |
Wednesday, March 16, 2022 1:42PM - 1:54PM |
N41.00010: Simulation of Readout in Circuit QED using Tensor Processing Units Ross Shillito, Alexandru Petrescu, Joachim Cohen, Alexandre Blais, Guifré Vidal Tensor Processing Units (TPUs) are application specific integrated circuits (ASICs) developed by Google to support large-scale machine learning tasks, but they can also be used for other computational tasks. In this work, we repurpose TPUs for classically simulating superconducting devices used as quantum hardware. Specifically, we consider the simulation of a transmon coupled to a resonator in a number of parameter regimes. The ability of TPU's to multiply extremely large matrices allows us to explore the structural instabilities of the transmon in the presence of hundreds of energy quanta, outside the capability of other traditional classical hardware. We establish the onset of this structural instability and subsequently establish bounds on readout drive power. |
Wednesday, March 16, 2022 1:54PM - 2:06PM |
N41.00011: Drive-induced dephasing in circuit QED Joachim Cohen, Alexandru Petrescu, Ross Shillito, Alexandre Blais The transmon succeeded the Cooper-pair box by trading a smaller anharmonicity for an exponential reduction of the sensitivity to charge noise. Despite this reduction, a number of experiments show that strongly driving the transmon leads to a decrease in T2. As numerous qubit manipulation schemes rely on the use of strong drives, it urges the need for understanding the sources of this decrease. |
Wednesday, March 16, 2022 2:06PM - 2:18PM |
N41.00012: Composite bosons in transmon arrays: a perturbative approach Sami Laine, Olli Mansikkamäki, Atte Piltonen, Matti Silveri Capacitively coupled transmon arrays form a splendid platform to study the intricacies of few-body and many-body quantum physics in a precisely controllable environment. From a theoretical point of view, these arrays realise the disordered Bose–Hubbard model with attractive interactions. |
Wednesday, March 16, 2022 2:18PM - 2:30PM |
N41.00013: Composite bosons in transmon arrays: numerical approach Olli Mansikkamäki, Sami Laine, Atte Piltonen, Matti Silveri The dynamics of bosonic many-body systems are notoriously slow to simulate due to the exponential scaling of the Hilbert space size. Analogue quantum simulation of the Bose-Hubbard model can be realised with arrays of capacitively coupled transmons. In experimental realisations the transmon anharmonicity, that is, the on-site interaction, dominates the hopping of the bosons. This results in approximate conservation of the interaction energy. |
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