Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session E48: Quantum Error Correction in Superconducting QubitsFocus

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Sponsoring Units: GQI Chair: Austin Fowler, Google, Inc. Room: 349 
Tuesday, March 15, 2016 8:00AM  8:36AM 
E48.00001: Increasing error resilience in superconducting qubits based on symmetries and parametric protocols Invited Speaker: David Schuster The field of superconducting quantum computing has seen remarkable advances in the past decade, and is getting closer to realizing logical qubits partially stabilized by error correction. However, achieving full scalability necessary to build a universal quantum computer remains a significant challenge which calls for new ideas to make superconducting qubits even more resilient with respect to external noise and fabrication imperfections. Past breakthroughs in this direction include circuit QED enabled measurement and longrange interactions; reliably long coherence times in transmon qubits; and additional coherence improvements by the use of 3D cavities. Here we review previous work on topological protection in superconducting circuit networks, and report on new efforts, in experiment and theory, to increase autonomous error protection of superconducting qubits by harnessing symmetry properties of circuit Hamiltonians and employing parametric processes for robust manipulation and storage of quantum information. [Preview Abstract] 
Tuesday, March 15, 2016 8:36AM  8:48AM 
E48.00002: Engineering stabilizer measurements in circuit QED: I Kevin Chou, Jacob Blumoff, M. Reagor, C. Axline, R. Brierley, S. Nigg, P. Reinhold, R. Heeres, C. Wang, K. Sliwa, A. Narla, M. Hatridge, L. Jiang, M. H. Devoret, S. M. Girvin, R. J. Schoekopf Quantum error correction based on stabilizer codes has emerged as an attractive approach towards building a practical quantum information processor. One requirement for such a device is the ability to perform hardware efficient measurements on registers of qubits. We demonstrate a new protocol to realize such multiqubit measurements. ~A key feature of our approach is that it enables arbitrary stabilizer measurements to be selected in software, and requires a relatively small number of buses, ancillae, and control lines. This allows for a minimally complex sample realizing a simple dispersive hamiltonian while maintaining a high degree of decoupling between our fixedtuned qubits. We experimentally implement these measurements in 3D circuit QED using transmon qubits coupled to a common bus resonator. In this first of two talks, we introduce our 3D cQED system and describe the protocol for measuring nqubit parities of a three qubit register. [Preview Abstract] 
Tuesday, March 15, 2016 8:48AM  9:00AM 
E48.00003: Engineering stabilizer measurements in circuit QED: II Jacob Blumoff, Kevin Chou, M Reagor, C Axline, R Brierly, S Nigg, P Reinhold, R Heeres, C Wang, K Sliwa, A Narla, M Hatridge, L Jiang, M H Devoret, S M Girvin, R J Schoelkopf Quantum error correction based on stabilizer codes has emerged as an attractive approach towards building a practical quantum information processor. One requirement for such a device is the ability to perform hardware efficient measurements on registers of qubits. We demonstrate a new protocol to realize such multiqubit measurements. A key feature of our approach is that it enables arbitrary stabilizer measurements to be selected in software, and requires a relatively small number of buses, ancillae, and control lines. This allows for a minimally complex sample realizing a simple dispersive hamiltonian while maintaining a high degree of decoupling between our fixedtuned qubits. We experimentally implement these measurements in 3D circuit QED using transmon qubits coupled to a common bus resonator. In the second of two talks, we present a full characterization of the algorithm describing the outcome dependent projections via quantum process tomography. [Preview Abstract] 
Tuesday, March 15, 2016 9:00AM  9:12AM 
E48.00004: Weight4 Parity Checks on a Surface Code Sublattice with Superconducting Qubits Maika Takita, Antonio Corcoles, Easwar Magesan, Nicholas Bronn, Jared Hertzberg, Jay Gambetta, Matthias Steffen, Jerry Chow We present a superconducting qubit quantum processor design amenable to the surface code architecture. In such architecture, parity checks on the data qubits, performed by measuring their X and Z syndrome qubits, constitute a critical aspect. Here we show fidelities and outcomes of X and Zparity measurements done on a syndrome qubit in a full plaquette consisting of one syndrome qubit coupled via bus resonators to four code qubits. Parities are measured after four code qubits are prepared into sixteen initial states in each basis. Results show strong dependence on ZZ between qubits on the same bus resonators. [Preview Abstract] 
Tuesday, March 15, 2016 9:12AM  9:24AM 
E48.00005: Active resonator reset in the nonlinear regime of circuit QED to improve multiround quantum parity checks Cornelis Christiaan Bultink, M.A. Rol, X. Fu, B.C.S. Dikken, J.C. de Sterke, R.F.L. Vermeulen, R.N. Schouten, A. Bruno, K.L.M. Bertels, L. DiCarlo Reliable quantum parity measurements are essential for faulttolerant quantum computing. In quantum processors based on circuit QED, the fidelity and speed of multiround quantum parity checks using an ancillary qubit can be compromised by photons remaining in the readout resonator post measurement, leading to ancilla dephasing and gate errors. The challenge of quickly depleting photons is biggest when maximizing the singleshot readout fidelity involves strong pulses turning the resonators nonlinear. We experimentally demonstrate the numerical optimization of counter pulses for fast photon depletion in this nonanalytic regime. We compare two methods, one using digital feedback and another running open loop. We assess both methods by minimizing the average number of rounds to ancilla measurement error. [Preview Abstract] 
Tuesday, March 15, 2016 9:24AM  9:36AM 
E48.00006: Demonstration of quantum superiority in learning parity with noise with superconducting qubits Diego Rist\`e, Marcus da Silva, Colm Ryan, Andrew Cross, John Smolin, Jay Gambetta, Jerry Chow, Blake Johnson A problem in machine learning is to identify the function programmed in an unknown device, or oracle, having only access to its output. In particular, a parity function computes the parity of a subset of a bit register. We implement an oracle executing parity functions in a fivequbit superconducting processor and compare the performance of a classical and a quantum learner. The classical learner reads the output of multiple oracle calls and uses the results to infer the hidden function. In addition to querying the oracle, the quantum learner can apply coherent rotations on the output register before the readout. We show that, given a target success probability, the quantum approach outperforms the classical one in the number of queries needed. Moreover, this gap increases with readout noise and with the size of the qubit register. This result shows that quantum advantage can already emerge in current systems with a few, noisy qubits. [Preview Abstract] 
Tuesday, March 15, 2016 9:36AM  9:48AM 
E48.00007: A Very Small Logical Qubit Eliot Kapit Superconducting qubits are among the most promising platforms for building a quantum computer. However, individual qubit coherence times are not far past the scalability threshold for quantum error correction, meaning that millions of physical devices would be required to construct a useful quantum computer. Consequently, further increases in coherence time are very desirable. In this letter, we blueprint a simple circuit consisting of two transmon qubits and two additional lossy qubits or resonators, which is passively protected against all single qubit quantum error channels through a combination of continuous driving and engineered dissipation. Photon losses are rapidly corrected through twophoton drive fields implemented with driven SQUID couplings, and dephasing from random potential fluctuations is heavily suppressed by the drive fields used to implement the multiqubit Hamiltonian. Comparing our theoretical model to published noise estimates from recent experiments on flux and transmon qubits, we find that logical state coherence could be improved by a factor of forty or more compared to the individual qubit $T_1$ and $T_2$ using this technique. [Preview Abstract] 
Tuesday, March 15, 2016 9:48AM  10:00AM 
E48.00008: Scalable insitu qubit calibration during repetitive error detection J. Kelly, R. Barends, A. Fowler, J. Mutus, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, E. Jeffrey, E Lucero, A. Megrant, M. Neeley, C. Neill, P.J.J. O'Malley, P. Roushan, D. Sank, C. Quintana, A. Vainsencher, J. Wenner, T. White, J.M. Martinis A quantum computer protects a quantum state from the environment through the careful manipulations of thousands or millions of physical qubits. However, operating such quantities of qubits at the necessary level of precision is an open challenge, as optimal control parameters can vary between qubits and drift in time. We present a method to optimize physical qubit parameters while error detection is running using a nine qubit system performing the bitflip repetition code. We demonstrate how gate optimization can be parallelized in a largescale qubit array and show that the presented method can be used to simultaneously compensate for independent or correlated qubit parameter drifts. Our method is O(1) scalable to systems of arbitrary size, providing a path towards controlling the large numbers of qubits needed for a faulttolerant quantum computer. [Preview Abstract] 
Tuesday, March 15, 2016 10:00AM  10:12AM 
E48.00009: Tracking errors of a logical qubit comprised of superpositions of cat states in a superconducting resonator A. Petrenko, N. Ofek, R. Heeres, P. Reinhold, Y. Liu, Z. Leghtas, B. Vlastakis, L. Frunzio, Liang Jiang, M. Mirrahimi, M.H. Devoret, R.J. Schoelkopf QEC schemes involve redundantly encoding a qubit into a larger space of states that has symmetry properties that allow one to measure error syndromes. Traditional approaches involve encodings that employ large numbers of physical qubits, enhancing decay rates significantly and requiring considerable hardware overhead to realize. A hardwareefficient proposal [1,2], which we term the cat code, sheds much of this complexity by encoding a qubit in superpositions of cat states in a superconducting resonator, which has one dominant error syndrome: single photon loss. As these cat states are eigenstates of photon number parity, the loss of a photon changes the parity without corrupting the encoded information. In a superconducting cQED architecture, we demonstrate that we track these errors in realtime with repeated single shot parity measurements and map their occurrence onto applications of a unitary rotation of an arbitrary encoded state in the logical space. Our results illustrate the utility of longlived resonators in the context of a full QEC system by highlighting the advantages of employing the cat code to suppress decoherence. [1]Leghtas et.al. PRL 111 120501 2013 [2]Mirrahimi et.al. NJP 16 045014 2014 [Preview Abstract] 
Tuesday, March 15, 2016 10:12AM  10:24AM 
E48.00010: Stabilizing the phase of superpositions of cat states in a cavity using realtime feedback N. Ofek, A. Petrenko, R. Heeres, P. Reinhold, Y. Liu, Z. Leghtas, B. Vlastakis, L. Frunzio, Liang Jiang, M. Mirrahimi, M.H. Devoret, R.J. Schoelkopf In a superconducting cQED architecture, a hardware efficient quantum error correction (QEC) scheme exists, called the cat code [1,2], which maps a qubit onto superpositions of cat states in a superconducting resonator, by mapping the occurrence of errors, or single photon jumps, onto unitary rotations of the encoded state. By tracking the parity of the encoded state, we can count the number of photon jumps and are able to apply a correcting unitary transformation. However, the situation is complicated by the fact that photon jumps do not commute with the deterministic anharmonic time evolution of a resonator state, or Kerr, inherited by the resonator from its coupling to a Josephson junction. As predicted in [1], a field in the resonator will inherit an overall phase $\theta = KT$ in $IQ$ space each time a photon jumps that is proportional to the Kerr $K$ and the time $T$ at which the jump occurs. Here I will present how we can track the errors in real time, take them into account together with the time they occur and make it possible to stabilize the qubit information. [1]Leghtas et.al. PRL 111 120501 2013 [2]Mirrahimi et.al. NJP 16 045014 2014 [Preview Abstract] 
Tuesday, March 15, 2016 10:24AM  10:36AM 
E48.00011: Encoding quantum information in a stabilized manifold of a superconducting cavity S. Touzard, Z. Leghtas, S.O. Mundhada, C. Axline, M. Reagor, K. Chou, J. Blumoff, K.M. Sliwa, S. Shankar, L. Frunzio, R.J. Schoelkopf, M. Mirrahimi, M.H. Devoret In a superconducting Josephson circuit architecture, we activate a multiphoton process between two modes by applying microwave drives at specific frequencies. This creates a pairwise exchange of photons between a highQ cavity and the environment. The resulting open dynamical system develops a twodimensional quasienergy ground state manifold. Can we encode, protect and manipulate quantum information in this manifold? We experimentally investigate the convergence and escape rates in and out of this confined subspace. Finally, using quantum Zeno dynamics, we aim to perform gates which maintain the state in the protected manifold at all times. [Preview Abstract] 
Tuesday, March 15, 2016 10:36AM  10:48AM 
E48.00012: Simulation of an arbitrary quantum channel with minimal ancillary resource Chao Shen, Kyungjoo Noh, Victor V. Albert, Michel H. Devoret, Robert J. Schoelkopf, Steven M. Girvin, Liang Jiang We discuss an explicit and efficient construction of quantum circuits that can simulate an arbitrary given quantum channel acting on a dlevel quantum system, with the minimal quantum ancillary resourcea qubit and its QND readout. The elementary operations required are unitary evolutions and single qubit projective measurement. We further show that this technique opens up exciting new possibilities in the field of quantum control, quantum simulation, quantum error correction, and quantum state discrimination. Our proposal can be implemented on platforms such as a superconducting transmon qubit inside a microwave cavity. [Preview Abstract] 
Tuesday, March 15, 2016 10:48AM  11:00AM 
E48.00013: Quantum Parameter Estimation in Continuous MeasurementBased Quantum Control Luis CortezGonzalez, Andrew N. Jordan We consider continuous measurement as a quantum tracking and control method for superconducting quantum systems. In experiments with superconducting qubits information about the quantum state of the system is extracted in the form of a noisy analog voltage signal reflected from a coupled readout resonator. Every element of the measurement sequence describes a weak measurement that provides uncertain information about the quantum state of the qubit. In this talk we describe how the information contained in the continuous readout signal is used to estimate unknown parameters of the system as well as the most probable state evolution to produce the observed measurement record. We will theoretically compare a variety of estimation techniques including Maximum Likelihood methods and Bayesian hypothesis testing among others. [Preview Abstract] 
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