Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L27: Quantum Error Correction Theory and Experiment IFocus

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Sponsoring Units: DQI Chair: Barbara Terhal Room: BCEC 160C 
Wednesday, March 6, 2019 11:15AM  11:27AM 
L27.00001: Faulttolerant magic state preparation with flag qubits Christopher Chamberland, Andrew Cross Despite considerable effort, magic state distillation remains one of the leading candidates to achieve universal faulttolerant quantum computation. However, when analyzing magic state distillation schemes, it is often assumed that gates belonging to the Clifford group can be implemented perfectly. In many current quantum technologies, twoqubit Clifford gates are amongst the noisiest components of quantum computers. In this talk I will present a new scheme for preparing magic states with very low overhead that uses flag qubits. I will then compare our scheme to leading magic state distillation methods and show that the overhead can be reduced by orders of magnitude. 
Wednesday, March 6, 2019 11:27AM  11:39AM 
L27.00002: Error correcting BaconShor code with continuous measurement of noncommuting operators Juan Atalaya, Alexander N. Korotkov, Birgitta K Whaley We analyze the ninequbit BaconShor code (smallest BaconShor code capable of correcting any singlequbit error) with simultaneous continuous measurement of noncommuting gauge operators. Error syndromes are determined by crosscorrelations of three measurement signals. During continuous operation of the code, such error syndromes can be monitored via running time averages of the measurement signals with a certain weighting function. We investigate how various parameters of the latter affect the performance of the code operation. We calculate the logical error rates due to environmental decoherence, and compare the performances of the continuous and discrete implementation of the ninequbit BaconShor code. 
Wednesday, March 6, 2019 11:39AM  11:51AM 
L27.00003: Quantum Error Correction with the Semion Code Guillaume Dauphinais, Laura Ortiz, Santiago Varona, Miguel Angel MartinDelgado We present a full quantum error correcting procedure with the semion code: an offshell extension of the double semion model. We construct open strings operators that recover the quantum memory from arbitrary errors and closed string operators that implement the basic logical operations for information processing. Physically, the new open string operators provide a detailed microscopic description of the creation of semions at their end points. Remarkably, topological properties of the string operators are determined using fundamental properties of the Hamiltonian, namely the fact that it is composed of commuting local terms squaring to the identity. In all, the semion code is a topological code that, unlike previously studied topological codes, it is of nonCSS type and fits into the stabilizer formalism. This is in sharp contrast with previous attempts yielding noncommutative codes. 
Wednesday, March 6, 2019 11:51AM  12:27PM 
L27.00004: Disjointness in quantum error correction: Imposing limitations on logical gates Invited Speaker: Tomas JochymO'Connor I will introduce the notion of disjointness for stabilizer codes in quantum error correction and discuss its implications for implementing faulttolerant logical gates. The disjointness, an algebraic quantity, can be used to characterize which level of the Clifford hierarchy is naturally attainable by simple faulttolerant logical gates such as constantdepth circuits. While the results are applicable to any stabilizer code, when addressing topological code families one can reproduce known bounds on the level of the hierarchy attainable by constant depth circuits, with the addition that the circuits are unconstrained by geometric locality. For instance, symmetric 2D surface codes cannot have nonlocal constant depth circuits resulting in nonClifford logical gates. 
Wednesday, March 6, 2019 12:27PM  12:39PM 
L27.00005: Stabilizer Slicing: Coherent Error Cancellations in LDPC Codes Dripto Debroy, Muyuan Li, Michael Newman, Kenneth R Brown In this paper we propose a scheme to cancel coherent overrotation errors when implementing quantum error correcting codes. These are considered to be among the most damaging errors due to the way they grow rapidly when combined. Due to their unitary nature, these errors have inverse operations and through our technique the errors due to different parts of the circuit create pairs of errors which cancel over the logical subspace. When considering a simplified error model and allowing native 3 qubit gates we can completely eliminate error in the purely coherent case for Surface17, and show 135fold improvement at 0.99 unitarity. For an ion trap gate set and only two qubit interactions, we can observe an 89fold improvement in BaconShor13. This second implementation takes advantage of the prepared gauge of the code, and is the first implementation to our knowledge in which the state of the gauge affects the robustness of our memory. 
Wednesday, March 6, 2019 12:39PM  12:51PM 
L27.00006: Continuous Quantum Error Correction with twoqubit Annealing Hamiltonian ARMAN BABAKHANI, Herman Chan, Jeffrey Epstein, Song Zhang, Juan Atalaya, Birgitta K Whaley Adiabatic quantum computation (AQC) is expected to be robust against dephasing and control errors. Nevertheless, environmental noise on the qubits can cause transitions out of the ground state. Error suppression and correction techniques are needed to remedy this. Using an open quantum system approach, we study the effects of different noise models such as bitflip errors and more realistic thermal noise on a twoqubit system evolving under an annealing Hamiltonian. By encoding each logical qubit using stabilizer codes, we develop a continuous error correction scheme based on the weakly measured error syndromes and analyze the efficiency of this protocol with different physical parameters. We compare the effectiveness of such error correction with known error suppression techniques. 
Wednesday, March 6, 2019 12:51PM  1:03PM 
L27.00007: Neural BeliefPropagation Decoders for Quantum ErrorCorrecting Codes YeHua Liu, David Poulin Beliefpropagation (BP) decoders are responsible for the success of many modern coding schemes. While many classical coding schemes have been generalized to the quantum setting, the corresponding BP decoders are flawed by design in this setting. Inspired by an exact mapping between BP and deep neural networks, we train neural BP decoders for quantum lowdensity paritycheck codes, with a loss function tailored for the quantum setting. Training substantially improves the performance of the original BP decoders. The flexibility and adaptability of the neural BP decoders make them suitable for lowoverhead error correction in nearterm quantum devices. 
Wednesday, March 6, 2019 1:03PM  1:15PM 
L27.00008: Demonstration of ChannelOptimized Quantum Error Correction on CloudBased Quantum Computers Haimeng Zhang, Hannes Leipold, Robert Kosut, Daniel A Lidar With the introduction of several cloudbased quantum computers, for example, from IBM and Rigetti, there is a growing interest in experimenting with quantum algorithms and protocols on such platforms. We are specifically interested in testing quantum error correction protocols since noise is an important factor that limits their performance. We demonstrate the channeloptimized quantum error correction protocol [R.L. Kosut, A. Shabani, and D.A. Lidar, PRL 100, 020502 (2008)] on the IBM and Rigetti machines. Our goal is to protect quantum states from noise. The noise on the IBM machine is characterized by standard process tomography. The optimal encoding and recovery map are found numerically by solving a biconvex optimization problem which maximizes the average channel fidelity. We implement the optimal encoding and recovery maps by decomposing them into directly implementable gate operations. This error correction protocol does not require postselection and is designed specifically for the physically relevant noise to the platform. 
Wednesday, March 6, 2019 1:15PM  1:27PM 
L27.00009: Demonstration of fidelity improvement using dynamical decoupling with superconducting qubits Bibek Pokharel, Namit Anand, Benjamin Fortman, Daniel A Lidar Quantum computers must be able to function in the presence of decoherence. The simplest strategy for decoherence reduction is dynamical decoupling (DD), which requires no encoding overhead and works by converting quantum gates into decoupling pulses. Here, using the IBM and Rigetti platforms, we demonstrate that the DD method is suitable for implementation in today's relatively noisy and smallscale cloudbased quantum computers. Using DD, we achieve substantial fidelity gains relative to unprotected, free evolution of individual superconducting transmon qubits. To a lesser degree, DD is also capable of protecting entangled twoqubit states. We show that dephasing and spontaneous emission errors are dominant in these systems, and that different DD sequences are capable of mitigating both effects. Unlike previous work demonstrating the use of quantum error correcting codes on the same platforms, we make no use of postselection and hence report unconditional fidelity improvements against natural decoherence. 
Wednesday, March 6, 2019 1:27PM  1:39PM 
L27.00010: A family of subsystem codes with only weight two operators Milad Marvian We introduce a family of subsystem codes with the property that their gauge operators, single qubit logical operators and also the product of any two logical operators of the same type can all be implemented using twolocal interactions. We discuss their potential applications in error suppression techniques. 
Wednesday, March 6, 2019 1:39PM  1:51PM 
L27.00011: Higherdimensional quantum hypergraphproduct codes Weilei Zeng, Leonid Pryadko We describe a family of quantum errorcorrecting codes which 
Wednesday, March 6, 2019 1:51PM  2:03PM 
L27.00012: Continuous parity measurement and error correction William Livingston, Machiel Blok, Juan Atalaya, Jing Yang, Razieh Mohseninia, Andrew N Jordan, Justin G. Dressel, Alexander N. Korotkov, Irfan Siddiqi In a multiqubit system, performing continuous measurements of joint properties such as parity permits the study the quantum dynamics of multipartite state evolution and collapse. By performing simultaneous parity measurements in a three qubit system, we can also observe a single qubit flip in real time, providing the basis for quantum error correction. The parity of two superconducting transmons may be directly measured without qubit ancilla by coupling them to a single readout resonator, using identical dispersive couplings much larger than the resonator bandwidth. Using a chip with three qubits and connecting each pair of two to a parity readout resonator, we implement the two parity measurements needed to perform the conventional threequbit bitflip code. We control the qubits from a field programmable gate array while continuously monitoring the parity, allowing for low latency correction pulses to be applied when a qubit flip occurs. 
Wednesday, March 6, 2019 2:03PM  2:15PM 
L27.00013: Cellular Automaton Decoder for Topological Codes with Boundaries Michael Vasmer, Dan Browne, Aleksander M Kubica Topological codes are some of the most widelystudied examples of quantum errorcorrecting codes. These codes have desirable properties such as low weight stabiliser generators and high error thresholds. To correct errors using a quantum errorcorrecting code, we must use a classical algorithm (a decoder) to find a correction operator. Recently, a cellular automaton decoder was proposed for a broad family of topological codes defined on lattices without boundaries. This decoder is a local decoder and does not require multiple rounds of syndrome extraction to deal with measurement errors. In this work, we extend this cellular automaton decoder to topological codes defined on lattices with boundaries and compare its performance to other decoding algorithms. 
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