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 W72: Fault-Tolerance and Optimization |
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Sponsoring Units: DQI Chair: David Layden, IBM Research - Almaden Room: Room 406 |
Thursday, March 9, 2023 3:00PM - 3:12PM |
W72.00001: Flag Gadgets based on Classical Codes Benjamin Anker, Milad Marvian Fault-tolerant quantum error correction is essential for full-scale quantum computing due to high noise levels; however, conventional fault-tolerant schemes require high overhead. Recently it has been shown that by using flag gadgets it is possible to perform fault-tolerant syndrome extraction, a key subroutine of quantum error correction, with less overhead. Although flag gadgets have already been used in several experiments, a framework that applies to general quantum codes and does not require fast physical operations to achieve a resource reduction has been missing. |
Thursday, March 9, 2023 3:12PM - 3:24PM |
W72.00002: Sampling-based quasiprobability simulation for fault-tolerant quantum error correction on the surface codes under coherent noise Shigeo Hakkaku, Kosuke Mitarai, Keisuke Fujii We propose a sampling-based simulation for fault-tolerant quantum error correction under coherent noise. A mixture of incoherent and coherent noise, possibly due to over-rotation, is decomposed into Clifford channels with a quasiprobability distribution. Then, an unbiased estimator of the logical error probability is constructed by sampling Clifford channels with an appropriate postprocessing. We characterize the sampling cost via the channel robustness and find that the proposed sampling-based method is feasible even for planar surface codes with relatively large code distances intractable for full state-vector simulations. As a demonstration, we simulate repetitive faulty syndrome measurements on the planar surface code of distance 5 with 81 qubits. We find that the coherent error increases the logical error rate. This is a practical application of the quasiprobability simulation for a meaningful task and would be useful to explore experimental quantum error correction on the near-term quantum devices. |
Thursday, March 9, 2023 3:24PM - 3:36PM |
W72.00003: High-threshold fault-tolerance in measurement-based error correction with tailored fusion circuits Kaavya Sahay A practical approach to implement measurement-based error correction (MBEC) is by stitching together many copies of few-body resource states into a larger entangled state, called the cluster state, using fusions or Bell measurements. Fusions measure Z⊗Z and X⊗X correlations. We propose new four- and six- body resource states that can be input to fusion circuits to build the RHG cluster state and the recently discovered XZZX cluster state. Our construction offers a practical advantage in realistic platforms in which the outcome of an X⊗X measurement in a fusion is more likely to be unreliable than the outcome of a Z⊗Z measurement. This is because the resource states are tailored so that errors introduced in these cluster states due to faulty X⊗X measurements obey a parity conservation law, which gives rise to a two-dimensional system symmetry. This considerably simplifies the decoding problem and can lead to substantial improvement in thresholds. We study the applicability of fusion-based MBEC using these resource states in (i) two specific qubit platforms: dual-rail photonic qubits in linear optics and Yb atoms and (ii) a broad class of qubits in which dephasing errors are dominant. In all cases, we find a simplification in the requirements for fault-tolerant error correction. |
Thursday, March 9, 2023 3:36PM - 3:48PM |
W72.00004: Adaptive Shor Scheme for fault tolerance on repetition code Yaniv Kurman, Yonatan Cohen, Lior Ella Quantum error correction schemes are essential for reaching fault-tolerant quantum computing. However, fault-tolerant schemes require significant overhead that by itself introduces additional errors. The Shor scheme tackles this problem by repeating syndromes, which can reach (t+1)^2 measurement rounds for a stabilizer code that can correct up to t errors. A recent proposal of adaptive syndrome measurement for Shor schemes [1] suggests a protocol that requires no more than (t+3)^2/4-1 measurement rounds, significantly less than the regular Shor scheme. The adaptive measurement determines the syndrome for error correction based on the difference between consecutive rounds while keeping fault tolerance criteria. Thus, the adaptive scheme reaches its superiority by including classical calculation and decision-making during quantum calculation. In this work, we present the first simulation results of the adaptive Shor scheme and verify that it is indeed fault tolerant. We exemplify these results with a repetition code and indicate the requirements for implementing such fault-tolerant schemes in a physical system. |
Thursday, March 9, 2023 3:48PM - 4:00PM |
W72.00005: Improved quantum error correction with randomized compiling Aditya Jain, Pavithran Iyer, Stephen D Bartlett, Joseph V Emerson Optimizing fault-tolerance schemes has been a central goal in the field of quantum error correction. In this paper, we explore the role and effectiveness of using noise tailoring techniques to improve the performance of error correcting codes. Noise tailoring methods such as randomized compiling (RC) improve the fidelity of quantum circuits by converting coherent noise to an effective stochastic noise. Combined with quantum error correction schemes, they effectively reduce the number of features of the physical noise process that impact a code's performance. Of particular interest is the class of coherent errors, where RC has the maximum effect. For these errors, we show that RC can offer an improvement in performance of the concatenated Steane code by several orders of magnitude. We also show that below a threshold rotation angle, the gains in logical fidelity can be arbitrarily magnified by increasing the size of the codes. These results suggest that using randomized compiling can lead to a massive reduction in the resource overhead required to achieve fault tolerance. |
Thursday, March 9, 2023 4:00PM - 4:12PM |
W72.00006: A highly accurate decoding of the surface code using Ising model solver Yusaku Takeuchi, Keisuke Fujii, Shintaro Sato, Hirotaka Oshima, Jun Fujisaki Quantum computers hold the promise of solving computational problems that are unsolvable using conventional computers.However, without a fault-tolerant quantum computer based on quantum error correction, theoretically proven quantum speedup cannot be achieved. One of the most important problems for implementation of quantum error correction is fast decoding. While, for the surface code, minimum weight perfect matching (MWPM) has been applicable for polynomial time decoding, it is neither optimal for correlated X and Z errors (depolarizing noise) nor simple to be implemented on a middleware for controlling quantum computing devices. Here we propose to perform near-optimal decoding by mapping the decoding problem to the energy minimization problem of the Ising model. Then the Ising model can be solved by a heuristic algorithm, such as simulated annealing or special purpose hardware designed for it. We performed a series of numerical simulations and show that the proposed method provides a high accuracy than MWPM under depolarizing noise is near-optimal comparing it with the exact solution calculated by CPLEX (an exact inter programing solver). |
Thursday, March 9, 2023 4:12PM - 4:24PM Author not Attending |
W72.00007: Erasure qubits: Overcoming the T1 limit in superconducting circuits Aleksander M Kubica, Arbel Haim, Yotam Vaknin, Fernando Brandao, Alex Retzker The amplitude damping time, T1, has long stood as the major factor limiting quantum fidelity in superconducting circuits, prompting concerted efforts in the material science and design of qubits aimed at increasing T1. In contrast, the dephasing time, Tφ, can usually be extended above T1 (via, e.g., dynamical decoupling), to the point where it does not limit fidelity. In this article we propose a scheme for overcoming the conventional T1 limit on fidelity by designing qubits in a way that amplitude damping errors can be detected and converted into erasure errors. Compared to standard qubit implementations our scheme improves the performance of fault-tolerant protocols, as numerically demonstrated by the circuit-noise simulations of the surface code. We describe two simple qubit implementations with superconducting circuits and discuss procedures for detecting amplitude damping errors, performing entangling gates, and extending Tφ. Our results suggest that engineering efforts should focus on improving Tφ and the quality of quantum coherent control, as they effectively become the limiting factor on the performance of fault-tolerant protocols. |
Thursday, March 9, 2023 4:24PM - 4:36PM |
W72.00008: Engineering error-detectable gates on logical qubits Jacob C Curtis, Takahiro Tsunoda, William D Kalfus, Luigi Frunzio, Robert J Schoelkopf Superconducting cavities coupled to transmon ancillae is a promising platform for fault-tolerant quantum computation. This pairing produces universal control over a long-lived bosonic mode, in whose many levels we encode a logical qubit. The resources required for fault-tolerance are high and likely require multiple layers of error correction. In this work, we construct a universal set of SNAP-based (Selective Number-dependent Arbitrary Phase [1]) quantum gates on a logical qubit whose dominant transmon errors are detectable. These gates use three levels of the ancilla whose final state heralds the success of the logical operation. Single ancilla X and Z errors correspond to unique ancilla states at the end of the SNAP gate. Our ancilla readout is single-shot and high fidelity, enabling us to accurately detect and discard shots with an error. Postselection increases the fidelity of resulting gate sequences, at the cost of reduced success rate. We show that this cost is small and substantially improves logical gate fidelity. |
Thursday, March 9, 2023 4:36PM - 4:48PM |
W72.00009: Minimum energy decoding for multimode Gottesman-Kitaev-Preskill (GKP) codes Mao Lin, Christopher Chamberland, Kyungjoo Noh Quantum error correction (QEC) plays an essential role in fault-tolerantly realizing quantum algorithms of practical interest. Among different approaches to QEC, encoding logical quantum information in harmonic oscillator modes has been shown to be promising and hardware efficient. In this work, we consider multimode Gottesman-Kitaev-Preskill (GKP) codes (encoding a qubit in many oscillators) from a lattice perspective and study a minimum-energy decoding strategy for correcting random Gaussian shift errors. For a given GKP code, we first identify its corresponding lattice, followed by considering the Voronoi cell structure of its symplectic dual lattice. The minimum-energy decoder works by finding the closest point in the dual lattice to a given shift error. We also use the same method to compute the code distances of a multimode GKP code. While minimum energy decoding incurs exponential time cost in the number of modes for general unstructured GKP codes, we give several examples of structured GKP codes where the minimum energy decoding can be performed in polynomial time. |
Thursday, March 9, 2023 4:48PM - 5:00PM |
W72.00010: Squeezed Kerr quantum oscillator with multiple spectral degeneracies Diego Ruiz, Ronan Gautier, Jérémie Guillaud, Mazyar Mirrahimi Kerr-nonlinear oscillators driven by a two-photon process have been recently considered as an interesting system to encode quantum information and to ensure a hardware-efficient scaling towards fault-tolerant quantum computation. In this talk, we show that an extra control parameter, the detuning of the two-photon drive with respect to the oscillators resonance, plays a crucial role in the properties of the defined qubit. At specific values of this detuning, we benefit from strong symmetries in the system, leading to multiple degeneracies in the spectrum of the effective confinement Hamiltonian. Overall, these degeneracies lead to a stronger suppression of bit-flip errors. We also study the combination of such Hamiltonian confinement with colored dissipation to suppress leakage outside of the bosonic code space. We show that the additional degeneracies allow us to perform fast and high-fidelity gates while preserving a strong suppression of bit-flip errors. |
Thursday, March 9, 2023 5:00PM - 5:12PM |
W72.00011: Low Depth Parity Check Gate Set for Quantum Error Correction GOZDE USTUN, Andrea Morello, Simon Devitt Designing QEC schemes with high error threshold reduces the demands on the physical hardware, and makes practical quantum computing more feasible. In this work, we exploit the insight that a QEC code needs not use universal logic gates, but can be simplified to perform solely the task of error detection and correction. By building gates that are fundamental to QEC rather than universal computation, we can boost the threshold and ease the experimental demands on the physical hardware. We call these gates low-depth parity check gates, since they perform a two-body parity check measurement in a single operation, instead of requiring a sequence of 1-and 2-qubit gates. We insert the low-depth parity check gates within a 'widget' that yields directly the measurement of two-body Pauli Product operators (Mpp2), namely the XX and ZZ operators. We present a rigorous formalism for constructing and verifying the low-depth parity check gate set, designed specifically for QEC, which can also be used for N-qubit parity check circuits. We then proceed to apply this technique to the two-body parity check circuits which are necessary for the implementation of the Honeycomb code, which requires only two-body parity check circuits instead of the four-body parity checks needed in the surface code |
Thursday, March 9, 2023 5:12PM - 5:24PM |
W72.00012: Quantum error correction in cat-code qubits: analysis and minimization of failure rates Danyang Chen, Sean van Geldern, Tali Shemma, Andrew A Houck, Chen Wang, Jens Koch A bosonic mode coupled to an ancillary nonlinear circuit can be used to encode a logical qubit. One choice studied in the context of quantum error correction is the cat code. Repeated measurement of photon-number parity was shown by Ofek et al. to protect qubit from the dominant error (single-photon loss). However, a number of undetectable errors and failure modes remain. For cat qubits using either a transmon or fluxonium as the ancilla, we survey and categorize the major failure modes, providing efficient methods to estimate failure rates. By minimizing the total failure rate, we obtain optimal parameters for both the physical system and the protocol. |
Thursday, March 9, 2023 5:24PM - 5:36PM |
W72.00013: Quantum error correction with metastable states of trapped ions using erasure conversion Mingyu Kang, Wes Campbell, Kenneth R Brown Erasures, or errors with known locations, are a more favorable type of error for quantum error-correcting codes than Pauli errors. Converting physical noise into erasures can significantly improve the performance of quantum error correction. Here we apply the idea of performing erasure conversion by encoding qubits into metastable atomic states, proposed by Wu, Kolkowitz, Puri, and Thompson [Nat. Comm. 13, 4657 (2022)], to trapped ions. We suggest an erasure-conversion scheme for metastable trapped-ion qubits and develop a detailed model of various types of errors. We then compare the logical performance of ground and metastable qubits on the surface code under various physical constraints and discuss the trade offs. |
Thursday, March 9, 2023 5:36PM - 5:48PM |
W72.00014: Protecting Expressive Circuits with a Quantum Error Detection Code Chris N Self, Marcello Benedetti, David Amaro Quantum error correction opens the way for quantum computers to speed up relevant tasks like simulating quantum systems. However, fully fault-tolerant quantum error correction imposes heavy demands on quantum hardware, making it practically unreachable for existing quantum computers. In this context we develop the $[[k+2,k,2]]$ quantum error detection code towards implementation on existing trapped-ion computers. Encoding $k$ logical qubits into $k+2$ physical qubits, this code provides fault-tolerant state initialisation and syndrome measurement circuits that can detect any single-qubit error. The code has a universal set of local and global logical rotations that, notably, have physical support on only two qubits. A high-fidelity -- though non fault-tolerant -- compilation of this universal gate set is possible thanks to the two-qubit physical rotations present in trapped-ion computers with all-to-all connectivity. Given the particular structure of the logical operators, we nickname it the Iceberg code. On the 12-qubit Quantinuum H1-2 hardware we demonstrate the protection of circuits of 8 logical qubits with up to 256 layers, saturate the logical quantum volume of $2^8$, and show the positive effect of increasing the number of syndrome measurements. These results demonstrate the practical usefulness of the Iceberg code for early fault-tolerant computation. |
Thursday, March 9, 2023 5:48PM - 6:00PM |
W72.00015: Fault-tolerant quantum computation in bosonic systems with ancillas Daohong Xu, Qian Xu, Pei Zeng, Liang Jiang The bosonic system is a promising candidate for quantum computing due to the hardware efficiency of its corresponding quantum error correction (QEC) scheme. One practical way to achieve the universal control of a bosonic mode is to couple it to additional discrete-level (DV) ancillas for nonlinear operations. However, such ancillas are typically more prone to environmental noise, limiting the fidelity of ancilla-assisted quantum operations. To address this challenge, we develop a general framework for fault-tolerant quantum computation (FTQC) on a bosonic system with DV ancillas composed of fault-tolerant operations against noises from both parts. We define the fault-tolerance criteria in this hybrid system and consider employing both rotation-symmetric codes and the Gottesman-Kitaev-Preskill (GKP) code. Several FTQC toolkits are fitted into our protocol: we generalize the path-independent (PI) gate design such that up to an acceptable error, the gate implemented on the bosonic mode depends only on the initial and final state of the ancilla; we also embed the "flag qubit" QEC protocol as an autonomous error indicator to further improve the performance. Numeric simulations show suppressions of higher-order errors. Our general framework can guide the design of FTQC with noisy bosonic-ancilla hybrid systems. |
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