Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V46: Superconducting-Qubit Quantum Error CorrectionFocus
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Sponsoring Units: GQI Chair: Jerry Chow , IBM Room: 393 |
Thursday, March 16, 2017 2:30PM - 3:06PM |
V46.00001: Extending the lifetime of a qubit with quantum error correction Invited Speaker: Robert Schoelkopf |
Thursday, March 16, 2017 3:06PM - 3:18PM |
V46.00002: Simulating a transmon implementation of the surface code, Part I Brian Tarasinski, Thomas O'Brien, Adriaan Rol, Niels Bultink, Leo DiCarlo Current experimental efforts aim to realize Surface-17, a distance-3 surface-code logical qubit, using transmon qubits in a circuit QED architecture. Following experimental proposals for this device, and currently achieved fidelities on physical qubits, we define a detailed error model that takes experimentally relevant error sources into account, such as amplitude and phase damping, imperfect gate pulses, and coherent errors due to low-frequency flux noise. Using the GPU-accelerated software package ‘quantumsim’, we simulate the density matrix evolution of the logical qubit under this error model. Combining the simulation results with a minimum-weight matching decoder, we obtain predictions for the error rate of the resulting logical qubit when used as a quantum memory, and estimate the contribution of different error sources to the logical error budget. [Preview Abstract] |
Thursday, March 16, 2017 3:18PM - 3:30PM |
V46.00003: Simulating a transmon implementation of the surface code, Part II Thomas O'Brien, Brian Tarasinski, Adriaan Rol, Niels Bultink, Xiang Fu, Ben Criger, Leonardo DiCarlo The majority of quantum error correcting circuit simulations use Pauli error channels, as they can be efficiently calculated. This raises two questions: what is the effect of more complicated physical errors on the logical qubit error rate, and how much more efficient can decoders become when accounting for realistic noise? To answer these questions, we design a minimal weight perfect matching decoder parametrized by a physically motivated noise model and test it on the full density matrix simulation of Surface-17, a distance-3 surface code. We compare performance against other decoders, for a range of physical parameters. Particular attention is paid to realistic sources of error for transmon qubits in a circuit QED architecture, and the requirements for real-time decoding via an FPGA [Preview Abstract] |
Thursday, March 16, 2017 3:30PM - 3:42PM |
V46.00004: Extensible quantum circuit for the surface-code error correction cycle with frequency-tunable transmon qubits Richard Versluis, Stefano Poletto, Nader Khammassi, Koen Bertels, Leo DiCarlo We propose an extensible scheme for implementing the surface-code error correction cycle with fast-frequency-tuneable transmon qubits in a circuit QED architecture. This solution consists of a spatially repeating unit cell circuit with four data-carrying qubits, four ancillary qubits, four frequencies for single-qubit control, and eight frequency excursions for conditional-phase (C-Z) gates. By pipelining the interaction and ancilla measurement steps of $X$- and $Z$-type stabilizers, we engineer the frequency excursion patterns to avoid transmon-transmon interactions except the $|11\rangle-|02\rangle|$ avoided crossings exploited in C-Z gates. This scheme is amenable to planar and hole-based implementations of the surface code and to lattice surgery. Crucially, it allows the expansion of the surface-code fabric by spatial repetition and maximal exploitation of spatial and frequency multiplexing in the control architecture. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 3:54PM |
V46.00005: The surface code with a twist Theodore Yoder, Isaac H. Kim The surface code is one of the most successful approaches to topological quantum error-correction. It boasts the smallest known syndrome extraction circuits and correspondingly largest thresholds. Defect-based logical encodings of a new variety called twists have made it possible to implement the full Clifford group on surface codes without state distillation. Here we investigate a patch-based encoding involving a modified twist. The resulting codes, called triangle codes for the shape of their planar layout, have only weight-four checks and relatively simple syndrome extraction circuits, maintaining a high, near surface-code-level threshold. They also use 25\% fewer physical qubits per logical qubit than the surface code. Moreover, benefiting from the twist, we can implement all Clifford gates by lattice surgery without the need for state distillation. By a surgical transformation to the surface code, we also develop a scheme of doing the same gates on surface code patches in an atypical planar layout, though with less qubit efficiency. Finally, we remark that logical qubits encoded in triangle codes are naturally amenable to logical tomography, and the smallest triangle code can demonstrate high-pseudothreshold fault-tolerance to depolarizing noise using just 13 physical qubits. [Preview Abstract] |
Thursday, March 16, 2017 3:54PM - 4:06PM |
V46.00006: Neural network decoder for quantum error correcting codes Stefan Krastanov, Liang Jiang Artificial neural networks form a family of extremely powerful - albeit still poorly understood - tools used in anything from image and sound recognition through text generation to, in our case, decoding. We present a straightforward Recurrent Neural Network architecture capable of deducing the correcting procedure for a quantum error-correcting code from a set of repeated stabilizer measurements. We discuss the fault-tolerance of our scheme and the cost of training the neural network for a system of a realistic size. Such decoders are especially interesting when applied to codes, like the quantum LDPC codes, that lack known efficient decoding schemes. [Preview Abstract] |
Thursday, March 16, 2017 4:06PM - 4:18PM |
V46.00007: Maximum likelihood decoding threshold for oscillator quantum error correcting codes Leonid P. Pryadko, Hamed Asasi, Michael Mulligan, Alexey A. Kovalev We consider syndrome-based decoding of continuous-variable (CV) codes encoding qubits in oscillators [1], assuming independent Gaussian error model. In the channel setting, given the measured syndrome, the conditional distribution of errors is described by an integer-variable Gaussian, with the coupling matrix determined by the syndrome and the code structure. For the encoded qubits, only the parity of these variables is relevant. As a result, maximum-likelihood (ML) decoding corresponds to an analog of the Nishimori line in a random-bond Ising model, where each bond coupling depends continuously on the measured syndrome. The ML decoding threshold corresponds to the multicritical point of this model. In the presence of phenomenological measurement errors, decoding is mapped to a Villain version of compact electrodynamics with ${\mathbb{Z}}_2$ symmetry breaking. \hfill\smallskip\\[0pt] [1] D. Gottesman, A. Kitaev, and J. Preskill, Phys. Rev. A 64, 012310 (2001). [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:30PM |
V46.00008: Rotations of a logical qubit using the quantum Zeno effect extended to a manifold - Part 1 A. Grimm, S. Touzard, Z. Leghtas, S.O. Mundhada, P. Reinhold, R. Heeres, C. Axline, M. Reagor, K. Chou, J. Blumoff, K.M. Sliwa, S. Shankar, L. Frunzio, R.J. Schoelkopf, M. Mirrahimi, M.H. Devoret Encoding Quantum Information in the large Hilbert space of a harmonic oscillator has proven to have advantages over encoding in a register of physical qubits, but has also provided new challenges. While recent experiments have demonstrated quantum error correction using such an encoding based on superpositions of coherent states, these codes are still susceptible to non-corrected errors and lack controllability: compared to physical qubits it is hard to make arbitrary states and to perform operations on them. Our approach is to engineer the dynamics and the dissipation of a microwave cavity to implement a continuous dissipative measurement yielding two degenerate outcomes. This extends the quantum Zeno effect to a manifold, which in our case is spanned by two coherent states of opposite phases. In this first talk we present the concept and architecture of an experiment that performs rotations on a logical qubit encoded in this protected manifold. [Preview Abstract] |
Thursday, March 16, 2017 4:30PM - 4:42PM |
V46.00009: Rotations of a logical qubit using the quantum Zeno effect extended to a manifold S. Touzard, A. Grimm, Z. Leghtas, S.O. Mundhada, P. Reinhold, R. Heeres, C. Axline, M. Reagor, K. Chou, J. Blumoff, K.M. Sliwa, S. Shankar, L. Frunzio, R.J. Schoelkopf, M. Mirrahimi, M.H. Devoret Encoding Quantum Information in the large Hilbert space of a harmonic oscillator has proven to have advantages over encoding in a register of physical qubits, but has also provided new challenges. While recent experiments have demonstrated quantum error correction using such an encoding based on superpositions of coherent states, these codes are still susceptible to non-corrected errors and lack controllability: compared to physical qubits it is hard to make arbitrary states and to perform operations on them. Our approach is to engineer the dynamics and the dissipation of a microwave cavity to implement a continuous dissipative measurement yielding two degenerate outcomes. This extends the quantum Zeno effect to a manifold, which in our case is spanned by two coherent states of opposite phases. In this second talk we present the result and analysis of an experiment that performs rotations on a logical qubit encoded in this protected manifold. [Preview Abstract] |
Thursday, March 16, 2017 4:42PM - 4:54PM |
V46.00010: Grid state displacement sensor under noise Daniel Weigand, Kasper Duivenvoorden, Barbara Terhal Grid states have been introduced by Gottesman, Kitaev and Preskill in the context of quantum error correction, as a method to encode a qubit into an oscillator. The development of new experimental techniques in the past years provide the tools to generate and control this type of quantum state in microwave cavities. In earlier work, we developed a protocol to generate grid states without post-selection. This protocol uses a variant of phase estimation implemented by coupling a transmon ancilla qubit to a microwave oscillator. A unique property of grid states is that they can be characterized by \emph{two} squeezing parameters, one for each quadrature. We recently proposed that grid states can be used as displacement sensors that accurately resolve small displacements in both quadratures simultaneously. In this work, we investigate the performance of the generating protocol under imperfect conditions. This is done by monitoring the evolution of the two squeezing parameters during the protocol, using realistic error models from current 2D and 3D microwave-cavity experiments with transmon qubits. The error models include photon loss, amplitude damping of the ancilla qubit, measurement errors and nonlinearities in the system Hamiltonian. [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:06PM |
V46.00011: Cat codes with optimal decoherence suppression for a lossy bosonic channel Linshu Li, Chang-Ling Zou, Victor V. Albert, Sreraman Muralidharan, Steven Girvin, Liang Jiang We investigate cat codes that can correct multiple excitation losses and identify two types of logical errors: bit-flip errors due to excessive excitation loss and dephasing errors due to quantum back-action from the environment [1]. We show that selected choices of logical subspace and coherent amplitude can efficiently reduce dephasing errors. The trade-off between the two major errors enables optimized performance of cat codes in terms of minimized decoherence. With high coupling efficiency, we show that one-way quantum repeaters with cat codes feature drastically boosted secure communication rate per mode compared with conventional encoding schemes, and thus showcase the promising potential of quantum information processing with continuous variable quantum codes. [1] Li, L. et al. arXiv: 1609.06386 [Preview Abstract] |
Thursday, March 16, 2017 5:06PM - 5:18PM |
V46.00012: Logical qubit fusion Jonathan Moussa, Ciaran Ryan-Anderson The canonical modern plan for universal quantum computation is a Clifford+T gate set implemented in a topological error-correcting code. This plan has the basic disparity that logical Clifford gates are natural for codes in two spatial dimensions while logical T gates are natural in three. Recent progress has reduced this disparity by proposing logical T gates in two dimensions with doubled [arXiv:1509.03239], stacked [PRA 93, 022323 (2016)], or gauge [PRA 93, 052332 (2016)] color codes, but these proposals lack an error threshold. An alternative universal gate set is Clifford+F [QIP 16, 1113 (2016)], where a fusion (F) gate converts two logical qubits into a logical qudit. We show that logical F gates can be constructed by identifying compatible pairs of qubit and qudit codes that stabilize the same logical subspace, much like the original Bravyi-Kitaev construction of magic state distillation. The simplest example of high-distance compatible codes results in a proposal that is very similar to the stacked color code with the key improvement of retaining an error threshold. [Preview Abstract] |
Thursday, March 16, 2017 5:18PM - 5:30PM |
V46.00013: Growing Tree Clusters via Local Complementation Yaakov Weinstein Tree clusters have become the proposed building block for cluster state quantum computation and all-optical quantum repeaters. In this talk we propose novel methods of constructing tree type cluster states by exploiting local complementation to manipulate graph edges. This includes more efficient construction of so-called rake states and a protocol to deterministically construct \{2,2\} cluster trees from 9 qubit linear clusters. These clusters can be connected as needed to form larger trees. [Preview Abstract] |
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