Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W16: Superconducting Qubits: QEC and Ultrastrong Coupling |
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Sponsoring Units: DQI Chair: Christian Kraglund Andersen, ETH Zurich Room: 201 |
Friday, March 6, 2020 8:00AM - 8:12AM |
W16.00001: Ground state of a circuit QED system in the deep-strong-coupling regime coupled to an environment Tomohiro Shitara, Motoaki Bamba, Fumiki Yoshihara, Tomoko Fuse, Kouichi Semba, Kazuki Koshino We investigate theoretically how the ground state of a qubit-resonator system in the deep-strong coupling regime is modified by the coupling to an environment. We employ the qubit-dependent coherent/squeezed-coherent state as the variational state for the ground state of the enlarged qubit-resonator-environment system and observe the following points. (i) The number of virtual photons increases by the coupling to an environment, proportionally to the square root of the loss rate κ of photons in the resonator. (ii) The ground state of the qubit-resonator system becomes mixed even at the zero temperature. The decrease in purity is proportional to the loss rate κ. (iii) There exists an optimal value of the qubit-resonator coupling to maximize the nonclassicality of the qubit-resonator system, which is quantified by the metrological power. |
Friday, March 6, 2020 8:12AM - 8:24AM |
W16.00002: Time-domain measurements of an ultra-strongly coupled qubit-resonator circuit Tomoko Fuse, Fumiki Yoshihara, Sahel Ashhab, Kousuke Kakuyanagi, Shiro Saito, Kouichi Semba
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Friday, March 6, 2020 8:24AM - 8:36AM |
W16.00003: Rabi model in the dispersive regime Clemens Mueller The dispersive regime of circuit QED is the main workhorse for today’s quantum computing prototypes based on superconducting qubits. Analytic descriptions of this model typically rely on the rotating wave approximation of the interaction between qubits and resonator fields, using the Jaynes-Cummings model as starting point for the dispersive transformation. |
Friday, March 6, 2020 8:36AM - 8:48AM |
W16.00004: Conical intersections and Berry phase in deep-strongly coupled superconducting qubit-resonator system Kouichi Semba, Sahel Ashhab Conical intersections, or Dirac cones, have been observed in various modern physical systems. Measuring Berry phases and curvatures are interesting themes [1-2]. Flux-qubit resonator circuits can be well described by a simple model, the quantum Rabi-model, even in the deep strong coupling regime [3]. Numerical analysis reveals that conical intersections occur not only in the symmetric quantum Rabi-model which has clear parity symmetry, but also in the asymmetric quantum Rabi-model which has no obvious symmetry [4]. The occurrence of conical intersections in the asymmetric Rabi-model provides indirect evidence that there should be a hidden symmetry. Here, we propose an experimental scheme to observe conical intersections in the energy landscape and the Berry phase in ultra- or deep-strongly coupled qubit-resonator systems. This will also provide a clue to understand the hidden symmetry in the system. |
Friday, March 6, 2020 8:48AM - 9:00AM |
W16.00005: Quantum impurity physics meets circuit QED: observation of finite lifetime photons Roman Kuzmin, Nicholas Grabon, Nitish Jitendrakumar Mehta, Moshe Goldstein, Vladimir Manucharyan We report a new regime of quantum electrodynamics (QED) where a single photon acquires a finite lifetime due to spontaneous decay to many lower-frequency photons. This phenomenon is a hallmark of ultra-strong coupling between a sufficiently non-linear quantum system (the quantum impurity) and a continuum of 1D bosonic modes. While this situation is impossible in atomic physics, it is ubiquitously in the bosonic description of strongly-correlated 1D electronic systems. We implemented bosonic versions of two key quantum impurity models: the boundary sine-Gordon model and the Kondo model. Physically our system is a long section of a high-impedance transmission line (the bosons) connected to a single small capacitance Josephson junction (the BSG impurity) or to a fluxonium qubit (the Kondo impurity). The many-body correlation functions of these two quantum impurity problems can be extracted from the measured inelastic spectrum of microwave photons, which implements a quantum simulation of a classically difficult computational problem. |
Friday, March 6, 2020 9:00AM - 9:12AM |
W16.00006: Cavity-free circuit quantum electrodynamics: interfacing a high-coherence qubit with propagating photons Yen-Hsiang Lin, Haonan Xiong, Nathanael Pierre Cottet, Long Nguyen, Ray Mencia, Aaron Somoroff, Vladimir Manucharyan We report experiments with a fluxonium artificial atom directly connected to a 1D transmission line. Unlike conventional circuit quantum electrodynamics (cQED), here there is no extra cavity mode degree of freedom buffering the qubit from the readout environment. Thanks to the highly anharmonic fluxonium spectrum, we simultaneously achieved a strong coupling of a high-frequency “cycling” readout transition (0→3 or 1→2) to the traveling waves and a nearly complete suppression of the spontaneous emission of the low-frequency ``clocking" qubit transition (0→1). Notably, the cycling dynamics during the readout is confined to a small Hilbert space of a single quantum degree of freedom, and it can be understood within a minimal optical pumping model. Our system realizes the simplest possible interface between a highly-coherent qubit and propagating photons for constructing quantum communication networks. |
Friday, March 6, 2020 9:12AM - 9:24AM |
W16.00007: Experimental Demonstration of Quantum Error Detection with a Small Surface Code Christian Kraglund Andersen, Ants Remm, Stefania Lazar, Nathan Lacroix, Sebastian Krinner, Graham J. Norris, Mihai Gabureac, Christopher Eichler, Andreas Wallraff One of the most promising approaches for quantum error correction is the surface code, which consists of a d×d grid of data qubits combined with a set of ancilla qubits. The surface code implements a distance-d quantum error correction code that allows for the detection of d-1 errors per stabilizer-measurement cycle. The smallest non-trivial surface code that allows for complete quantum error detection of a logical qubit subspace is the d=2 surface code, consisting of 4 data qubits and 3 ancilla qubits for the stabilizer measurements.Here we present experimental results obtained on a small seven-qubit surface code device implemented in superconducting circuits. We present measurements of all three stabilizers with high fidelity and repeated detection of errors on the data qubits using the stabilizer readout. |
Friday, March 6, 2020 9:24AM - 9:36AM |
W16.00008: Evaluation of the Performance of a 7-Qubit Surface Code Ants Remm, Christian Kraglund Andersen, Stefania Lazar, Nathan Lacroix, Sebastian Krinner, Graham J. Norris, Mihai Gabureac, Christopher Eichler, Andreas Wallraff One of the promising approaches to fault tolerant quantum computation is based on the surface code. The existence of an error rate threshold, below which increasing the code size exponentially suppresses logical errors, has been shown for many common error mechanisms [R. Raussendorf and J. Harrington, Phys. Rev. Lett. 98, 190504 (2007)]. In this talk we analyse the physical error mechanisms of a seven-qubit superconducting quantum device on which we implement the elementary operations required for a surface code. By repeated stabilizer measurements we implement a distance d=2 surface code enabling the detection of any single-qubit error. We identify the error mechanisms which have the most significant effect on the logical error rate. We further discuss how to extend our scheme to larger distance surface codes given the current performance of our device. |
Friday, March 6, 2020 9:36AM - 9:48AM |
W16.00009: Real-time decoding of repeated stabilizer measurements in a bit-flip code Diego Ristè, Luke Govia, Brian Donovan, Spencer Fallek, William Kalfus, Maika Takita, Antonio D Corcoles, Markus Brink, Nicholas T Bronn, Jerry M. Chow Although qubit coherences and gate fidelities are continuously improving, logical encoding is essential to achieve fault tolerance in quantum computing. In most encoding schemes, correcting or tracking errors throughout the computation is necessary to implement a universal gate set without delaying the processor. Here we present a classical control architecture for the fast extraction of errors based on multiple rounds of stabilizer measurements, and subsequent optional correction. We demonstrate its application on a minimal bitflip code with five transmon qubits, showing that error tracking based on multiple stabilizer rounds is superior to round-by-round correction, while introducing minimal latency. This co-processing of classical and quantum information will be crucial in running a logical circuit at its full speed to outpace error accumulation. |
Friday, March 6, 2020 9:48AM - 10:00AM |
W16.00010: Real-time Quantum Error Correction for a Surface-Code Logical Qubit Miguel Moreira, Brian M Tarasinski, Jordy Gloudemans, Viacheslav P. Ostroukh, Wouter Vlothuizen, Leonardo DiCarlo Achieving fault-tolerant universal quantum computation hinges on performing multi-round quantum error correction in real time. Several technical challenges, from instruction bandwidth to timing considerations, make it challenging to engineer a control system capable of real-time error correction. We present a control architecture for surface code with a tightly-coupled real-time quantum error decoder. We demonstrate real-time decoding of error syndromes for a Surface-17 logical qubit satisfying experimental timing constraints. We present the realization of multiple error decoding strategies and compare the hardware resources required for each implementation. |
Friday, March 6, 2020 10:00AM - 10:12AM |
W16.00011: Continuous Parity Measurement and Error Correction William Livingston, Machiel S Blok, Juan Atalaya, Razieh Mohseninia, Jing Yang, Andrew N Jordan, Justin Dressel, Irfan Siddiqi In a multi-qubit system, performing continuous measurements of joint properties such as parity allows us to study the collapse dynamics of multipartite states. Simultaneous parity measurements in a three-qubit system also act as continuous stabilizer detection for a quantum error correction code, allowing us to observe a single qubit flip in real time. The parity of two superconducting transmons may be directly measured without qubit ancilla by coupling them to a single readout resonator, using identical dispersive coupling chis much larger than the resonator bandwidth kappa. Using a chip with three qubits and connecting each of two pairs to a parity readout resonator, we implement the two parity measurements needed to perform the conventional three-qubit bit-flip code. We control the qubits from a field programmable gate array board which also continuously monitors the parity, allowing for low latency correction pulses to be applied when a qubit flip occurs. |
Friday, March 6, 2020 10:12AM - 10:24AM |
W16.00012: Topological and subsystem codes on low-degree graphs with flag qubits Guanyu Zhu, Christopher Chamberland, Theodore Yoder, Jared Hertzberg, Andrew Cross In this work we introduce two code families, the heavy hexagon code and heavy square code. Both code families are implemented by assigning physical data and ancilla qubits to both vertices and edges of low degree graphs. Such a layout is particularly suitable for superconducting qubit architectures to minimize frequency collision and crosstalk. In some cases, frequency collisions can be reduced by several orders of magnitude. The heavy hexagon code is a hybrid surface/Bacon-Shor code mapped onto a (heavy) hexagonal lattice whereas the heavy square code is the surface code mapped onto a (heavy) square lattice. In both cases, the lattice includes all the ancilla qubits required for fault-tolerant error-correction. Naively, the limited qubit connectivity might be thought to limit the error-correcting capability of the code to less than its full distance. Therefore, essential to our construction is the use of flag qubits. We modify minimum weight perfect matching decoding to effciently and scalably incorporate information from measurements of the flag qubits and correct up to the full code distance while respecting the limited connectivity. Simulations show that high threshold values for both codes can be obtained using our decoding protocol. |
Friday, March 6, 2020 10:24AM - 10:36AM |
W16.00013: Detection and mitigation of leakage in simulations of a transmon implementation of the surface code, Part 1: Characterization of leakage effects in a realistic error model Francesco Battistel, Boris Varbanov, Brian M Tarasinski, Viacheslav P. Ostroukh, Thomas O'Brien, Leonardo DiCarlo, Barbara Maria Terhal Qubit leakage is present in leading quantum computing platforms, including superconducting transmon qubits. These errors fall outside the stabilizer formalism of quantum error correction (QEC), thus constituting a threatening error source for fault tolerance. While the performance of QEC codes, such as the surface code, has been investigated for simplistic leakage error models, an analysis with respect to a physically-motivated leakage error model for transmons has not been undertaken so far. In this work, we employ realistic full-trajectory simulations of the CZ gate in a transmon system (the dominant source of leakage in this system). We find novel effects within the leakage subspace of two transmons, including leakage mobility and leakage conditional phases. We use this in density-matrix simulations of the distance-3 surface code Surface-17 and study the leakage build-up, its lifetime, and how leaked qubits spread errors onto neighboring qubits. |
Friday, March 6, 2020 10:36AM - 10:48AM |
W16.00014: Detection and mitigation of leakage in simulations of a transmon implementation of the surface code, Part 2: Scalable Hidden Markov models for the identification of leakage Boris Varbanov, Francesco Battistel, Brian M Tarasinski, Viacheslav P. Ostroukh, Thomas O'Brien, Barbara Maria Terhal, Leonardo DiCarlo The detection and mitigation of leakage, a threatening error source for stabilizer quantum error correction (QEC) on leading physical hardware, is an important step towards demonstrations of fault tolerance. We explore the effects of leakage on the performance of the distance-3 surface code Surface-17 by performing density-matrix simulations, employing realistic error models for transmon qubits in a circuit QED processor. We demonstrate the indirect detection of leakage via a network of local, independent and scalable Hidden Markov models. We show that post-selecting on detection of leakage restores the logical fidelity of the encoded information. We explore the integration of leakage detection into a minimum-weight perfect-matching decoder. |
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