Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L33: Fault-Tolerance, Gates, and LeakageFocus Session Live
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Sponsoring Units: DQI Chair: Guanyu Zhu, IBM TJ Watson Research Center |
Wednesday, March 17, 2021 8:00AM - 8:12AM Live |
L33.00001: Towards fault-tolerant quantum error correction with spin qubits in diamond Mohamed Abobeih, Yang Wang, Joe Randall, Sjoerd Loenen, Conor Bradley, Barbara Terhal, Tim Hugo Taminiau Quantum error correction (QEC) is essential for reliable large-scale quantum information processing. Pioneering experiments have demonstrated QEC codes that could only correct specific types of errors using various physical platforms [1,2]. However, full experimental demonstration of a fault-tolerant QEC code that can correct any type of single-qubit error remains an open challenge. Here, I will present our results towards the implementation of a fault-tolerant QEC code using a solid-state spin register in diamond. Recently, we have demonstrated that such a register can hold up to 10 qubits with high-fidelity universal control, coherence times up to one minute, and genuine multipartite entanglement [3,4]. Building upon these results, I will show how we can use multiple non-destructive parity measurements to encode logical states in C13 nuclear-spin qubits in diamond. These parity measurements might be further used to detect and correct arbitrary single-qubit errors on the encoded states, and are therefore an important step towards fault-tolerant quantum information processing. |
Wednesday, March 17, 2021 8:12AM - 8:24AM Live |
L33.00002: Focus Beyond Quadratic Speedup for Error-Corrected Quantum Advantage Ryan Babbush, Jarrod McClean, Craig M Gidney, Sergio Boixo, Hartmut Neven We discuss conditions under which it would be possible for a modest fault-tolerant quantum computer to realize a runtime advantage by executing a quantum algorithm with only a small polynomial speedup over the best classical alternative. The challenge is that the computation must finish within a reasonable amount of time while being difficult enough that the small quantum scaling advantage would compensate for the large constant factor overheads associated with error-correction. We compute several examples of such runtimes using state-of-the-art surface code constructions for superconducting qubits under a variety of assumptions. We conclude that quadratic speedups will not enable quantum advantage on early generations of such fault-tolerant devices unless there is a significant improvement in how we would realize quantum error-correction. While this conclusion persists even if we were to increase the rate of logical gates in the surface code by more than an order of magnitude, we also repeat this analysis for speedups by other polynomial degrees and find that quartic speedups look significantly more practical. |
Wednesday, March 17, 2021 8:24AM - 8:36AM Live |
L33.00003: Magic State Distillation for Surface Code for Biased Noise Qubits Shraddha Singh, Shruti Puri Ultra-high fidelity magic states are required for universal fault-tolerant quantum computing with surface code. However, the error probability of the injected magic state on the surface code lattice is proportional to the physical error rate and does not reduce with code distance. Consequently, several noisy states have to be distilled via magic state distillation to get one high fidelity state, thereby increasing the overhead cost for universal, fault-tolerant computation. Here we show how the structure of noise in the qubits and entangling gates can be exploited to suppress errors in the injected magic state and reduce the overall resource overheads for magic state distillation. |
Wednesday, March 17, 2021 8:36AM - 8:48AM Live |
L33.00004: Scalable, pipelined stabilizer measurement scheme and high-fidelity logical operations in a distance-2 surface code Jorge Marques, Miguel S Moreira, Hany Ali, Nandini Muthusubramanian, Wouter Vlothuizen, Marc Beekman, Chris Zachariadis, Nadia Haider, Alessandro Bruno, Leonardo DiCarlo The ability to perform simultaneous multi-round stabilizer measurements and high-fidelity logical operations are necessary to realize quantum error correction. We present the realization and comparison of high-fidelity fault-tolerant and non-fault tolerant logical operations (initialization, gates and measurement) in Surface-7, the distance-2 surface code, implemented in a superconducting quantum processor. We demonstrate superior performance of the pipelined [Versluis et al., Phys. Rev. Applied 8, 034021, (2017)] implementation of multi-round error detection over the parallel alternative, a key step enabling scalability to larger surface codes. |
Wednesday, March 17, 2021 8:48AM - 9:00AM Live |
L33.00005: Correlation matrix tool for error diagnostics in QEC experiments Juan Atalaya, Dvir Kafri, Matthew McEwen, Zijun Chen, Rami Barends, Julian Kelly, Yu Chen, Vadim Smelyanskiy, Alexander N. Korotkov Identification and mitigation of nonconventional errors such as leakage and cross-talk in repetition and surface code experiments is essential to achieve exponential suppression of logical errors with increasing the code distance. In this talk, we introduce an error-diagnostic tool that allows us to characterize long-range as well as long-time errors on the error graph caused by, e.g., cross-talk or leakage to non-computational states. The probability p_ij of an error involving arbitrary nodes i and j of the error graph is extracted from correlation of the error detection events at these nodes. The matrix p_ij can be used to identify particular error mechanisms and their strengths. In addition, these probabilities can provide accurate edge weights for minimum-weight-perfect-matching decoders. |
Wednesday, March 17, 2021 9:00AM - 9:12AM Live |
L33.00006: Toward a topological CNOT between two Kerr-cat qubits: part 1/2 Rodrigo Cortiñas, Nicholas Frattini, Shruti Puri, Owen Duke, Chan U Lei, Steven Girvin, Michel Devoret Schrödinger cat states, superpositions of coherent states in an oscillator, can encode a noise-biased qubit that is naturally protected against one Pauli error channel. Such a protected "cat qubit" has the ability to significantly reduce the overhead associated with quantum error correction in, for instance, a surface-code-style architecture. This overhead reduction relies on the ability to perform any gate in a manner that preserves the noise bias. Unlike pure two-level systems, exchanging coherent states in one oscillator conditioned on the second oscillator's state generates a noise-biased CNOT. Such an exchange-based topological gate does not depend on the path or the speed, but only presence or absence of exchange. This exchange can also be understood as correlated motion of 4 coherent states in a 4D phase space. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L33.00007: Toward a topological CNOT between two Kerr-cat qubits: part 2/2 Nicholas Frattini, Rodrigo Cortiñas, Shruti Puri, Owen Duke, Chan U Lei, Steven Girvin, Michel Devoret Schrödinger cat states, superpositions of coherent states in an oscillator, can encode a noise-biased qubit that is naturally protected against one Pauli error channel. Such a protected "cat qubit" has the ability to significantly reduce the overhead associated with quantum error correction in, for instance, a surface-code-style architecture. This overhead reduction relies on the ability to perform any gate in a manner that preserves the noise bias. Unlike pure two-level systems, exchanging coherent states in one oscillator conditioned on the second oscillator's state generates a noise-biased CNOT. Such an exchange-based topological gate does not depend on the path or the speed, but only presence or absence of exchange. This exchange can also be understood as correlated motion of 4 coherent states in a 4D phase space. In part two, we focus on the experimental design and preliminary results. |
Wednesday, March 17, 2021 9:24AM - 9:36AM Live |
L33.00008: A hardware-efficient leakage-reduction scheme for a transmon-based surface code Francesco Battistel, Boris Varbanov, Barbara Terhal Leakage in superconducting transmon qubits poses a threat to quantum error correction (QEC) as leakage errors cannot be decomposed into standard Pauli errors. Furthermore, leakage can last for many QEC cycles, propagating many correlated errors through the code. Leakage-reduction units can shorten the average leakage lifetime by bringing the qubit back to the computational subspace. However, many of the units investigated so far for transmons either require changes in hardware or increase the QEC-cycle time. |
Wednesday, March 17, 2021 9:36AM - 10:12AM Live |
L33.00009: Practical Quantum Error Correction with Surface-Cats Invited Speaker: Shruti Puri In quantum error correction, fault-tolerant circuits limit the ways in which errors spread in a system and are essential for reliable execution of quantum algorithms using unreliable devices. Unfortunately, fault-tolerance comes at the cost of large resource overheads. In fact, the penalty in the overhead can be so serious that it can suppress potential speedups in many quantum algorithms. Therefore, we need to find ways to achieve fault-tolerance in a hardware-efficient manner. Most of the work towards fault-tolerance relies on generic noise models for qubit operations and is agnostic to realistic errors in specific hardware platforms. In this talk I will show how we can achieve substantial reductions in the overheads for fault-tolerant quantum error correction by exploiting the underlying structure of noise in qubits encoded in bosonic degrees of freedom. I will focus on the planar surface code realized using the bosonic Kerr-cat qubit and present numerical results to demonstrate improvements in the resource requirements for fault-tolerance. |
Wednesday, March 17, 2021 10:12AM - 10:24AM Live |
L33.00010: Path-independent quantum gates: general formalism and algebraic structure Wen-Long Ma, Liang Jiang Ancilla systems are often indispensable to universal control of a nearly isolated central system. However, ancilla systems are typically more vulnerable to environmental noise, limiting the performance of such ancilla-assisted control. To address this challenge, we propose a general class of path-independent (PI) quantum gates [1], which integrate quantum error correction and quantum control and therefore can be resilient to ancilla noise. Furthermore, we reveal the underlying algebraic structure for such PI gates, which we call the PI matrix algebra. The PI matrix algebra is defined on both the ancilla and central systems but isomorphic to the ordinary matrix algebra defined on the ancilla system alone. With such an algebraic structure, we provide a unifying criterion for PI gates against general ancilla errors, with ancilla dephasing and relaxation errors as typical examples. |
Wednesday, March 17, 2021 10:24AM - 10:36AM Live |
L33.00011: Removing leakage-induced correlated errors in superconducting quantum error correction - Theory Dvir Kafri, Matthew McEwen, Zijun Chen, Juan Atalaya, Kevin Satzinger, Chris Quintana, Paul V Klimov, Daniel Sank, Craig Gidney, Austin G Fowler, Yu Chen, Vadim Smelyanskiy, John Martinis, Hartmut Neven, Julian Kelly, Alexander N. Korotkov, Andre Petukhov, Rami Barends Quantum computing can become scalable through error correction, but logical error rates only decrease with system size when physical errors are sufficiently uncorrelated. During computation, unused high energy levels of superconducting qubits can become excited, creating leakage states that are long-lived and mobile. Here, we report a multilevel reset protocol that returns a transmon superconducting qubit to the ground state from all relevant higher level states. The protocol is based on an adiabatic transfer of photons from each transmon to its readout resonator. We develop a three-phase semiclassical model describing the protocol and find good agreement with experiment. We then discuss application of the reset gate to the bit flip repetition code. |
Wednesday, March 17, 2021 10:36AM - 10:48AM Live |
L33.00012: Removing leakage-induced correlated errors in superconducting quantum error
correction - Experiment Matthew McEwen, Dvir Kafri, Zijun Chen, Juan Atalaya, Kevin Satzinger, Chris Quintana, Paul V Klimov, Daniel Sank, Craig Gidney, Austin G Fowler, Yu Chen, Vadim Smelyanskiy, John Martinis, Hartmut Neven, Julian Kelly, Alexander N. Korotkov, Andre Petukhov, Rami Barends Removing excitations from non-computational states is an essential challenge in achieving stable quantum error correction. We present the experimental realisation of a multilevel reset protocol that produces the ground state with an error below 5e-3 within 250 ns, starting from the qubit being in any of the first three excited levels. We deploy this gate in the context of the bit-flip stabilizer code, and demonstrate a significant reduction in the population of leakage built up over time while running the code. We show that the removal of leakage reduces the incidence of time-correlated errors, and significantly improves the logical error rate as well as the error suppression factor Λ. This provides the first demonstration of error suppression that is stable over large numbers of rounds. |
Wednesday, March 17, 2021 10:48AM - 11:00AM On Demand |
L33.00013: Universal Fault-Tolerant Quantum Computing with Stabiliser Code Families Paul Webster, Michael Vasmer, Thomas Scruby, Stephen D Bartlett Scalable universal quantum computing requires fault-tolerant implementations of a universal set of logical operators. Several important results constraining this goal exist in specific contexts, along with particular methods for overcoming these constraints, but no broadly applicable framework has been developed. We address this by defining a general notion of fault tolerance of quantum channels on scalable quantum error-correcting code families. With this definition, we present a no-go theorem that precludes a universal set of unitary fault-tolerant logical operator implementations for a wide range of stabiliser code families, including concatenated codes and conventional topological stabiliser codes such as surface and colour codes. Deriving this theorem also illuminates a general approach for how non-unitary channels can circumvent its constraints, which we show is manifested in a range of apparently distinct universal, fault-tolerant schemes. |
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