Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J39: Focus Session: Superconducting Qubits: Toward Fault Tolerant Quantum Computers |
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Sponsoring Units: GQI Chair: Daniel Sank, Google Inc. Room: 213AB |
Tuesday, March 3, 2015 2:30PM - 3:06PM |
J39.00001: Implementing fault tolerance in a superconducting quantum circuit Invited Speaker: Rami Barends The surface code error correction scheme is appealing for superconducting circuits as the fundamental operations have been demonstrated at the fault-tolerant threshold. Here, we present experimental results on the repetition code, a one-dimensional primitive of the surface code which can detect bit-flip errors, implemented on a device consisting of nine Xmon transmon qubits. We discuss the basic mechanics of error detection, show preservation of a Greenberger-Horne-Zeilinger state, and show suppression of environmentally-induced error. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J39.00002: Performing repetitive error detection in a superconducting quantum circuit J. Kelly, R. Barends, A. Fowler, A. Megrant, E. Jeffrey, T. White, D. Sank, J. Mutus, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, I.-C. Hoi, C. Neill, P.J.J. O'Malley, P. Roushan, C. Quintana, A. Vainsencher, J. Wenner, A.N. Cleland, J.M. Martinis Recently, there has been a large interest in the surface code error correction scheme, as gate and measurement fidelities are near the threshold. If error rates are sufficiently low, increased systems size leads to suppression of logical error. We have combined high fidelity gate and measurements in a single nine qubit device, and use it to perform up to eight rounds of repetitive bit error detection. We demonstrate suppression of environmentally-induced error as compared to a single physical qubit, as well as reduced logical error rates with increasing system size. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J39.00003: A Leakage-Resilient Approach to Fault-Tolerant Quantum Computing with Superconducting Elements Joydip Ghosh, Austin Fowler Superconducting qubits, while promising for scalability and long coherence times, contain more than two energy levels, and therefore are susceptible to errors generated by the leakage of population outside of the computational subspace. Such leakage errors are currently considered to be a prominent roadblock towards fault-tolerant quantum computing with superconducting qubits. Fault-tolerant quantum computing using topological codes is based on sequential measurements of multi-qubit stabilizer operators. In this talk, I propose a leakage-resilient scheme to perform repetitive measurements of multi-qubit stabilizer operators, and then discuss how to use this scheme as an ingredient to develop a leakage-resilient approach for surface code quantum error correction with superconducting circuits. Our protocol is based on SWAP operations between data and ancilla qubits at the end of every cycle, requiring read-out and reset operations on every physical qubit in the system, and thereby preventing persistent leakage errors from occurring. [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J39.00004: Characterization of superconducting qubits in a planar lattice Srikanth Srinivasan, Antonio Corcoles, Easwar Magesan, Nicholas Bronn, Jared Hertzberg, Jay Gambetta, Matthias Steffen, Jerry Chow The surface code is a promising implementation for quantum computing because of its relatively lenient thresholds for fault tolerance. The physical layout contains two general classes of qubits, code and syndrome, arranged in a planar lattice. In this talk we present complete characterization of a four qubit planar lattice and discuss the experimental challenges for achieving high fidelity. This includes integrating four independent readouts using parametric amplifiers, gate calibration procedures, and sample design. Careful device design is required for efficient signal delivery without deleterious microwave crosstalk. The low crosstalk is validated through measurements of simultaneous randomized benchmarking on both single and two-qubit entangling gates. This work is a step towards realizing the surface code on a planar lattice. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J39.00005: Arbitrary error detection in a planar lattice of the surface code Antonio Corcoles, Easwar Magesan, Srikanth Srinivasan, Nicholas Bronn, Jared Hertzberg, Andrew Cross, Matthias Steffen, Jay Gambetta, Jerry Chow We detect arbitrary single-qubit errors on a system of four superconducting qubits arranged in a planar lattice, amenable to the surface code. The error detection protocol is based on the stabilizer formalism and protects a codeword encoded on an entangled two-qubit state by quantum non-demolition parity measurements, ZZ and XX. These parity measurements are performed using the other two qubits acting as syndromes. We introduce a bit- or phase-flip single-qubit error applied to the codeword and show that this error can be revealed uniquely in the syndromes. The -non-trivial- geometric arrangement of the qubits is essential to the surface code algorithm and is therefore extendable throughout the two-dimensional plane, encoding progressively larger logical Hilbert spaces towards a fully scaled fault-tolerant quantum computer. [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J39.00006: Characterizing Quantum Gates with Iterated Randomized Benchmarking on Superconducting Qubits Sarah Sheldon, Lev S. Bishop, Stefan Filipp, Matthias Steffen, Jerry M. Chow, Jay M. Gambetta With coherence times exceeding 40us and single qubit gate fidelities of 0.9996, we find our current calibration schemes and DRAG pulse shaping fall short of the coherence limit. It is therefore necessary to develop new methods of finding and addressing errors in the qubit control. We present a method for characterizing small errors using a variation of interleaved randomized benchmarking to identify sources of systematic errors. Our new scheme, iterative randomized benchmarking, interleaves repetitions of gates in a randomized benchmarking sequence to determine the type of error on the target gate. The scaling of the fidelity with the number of interleaved gates reveals if the gate errors are coherent or incoherent. Experimental data indicates that our system is sensitive to an over-rotation by an angle of pi/128. We also apply this technique to identify sources of coherent errors that may be reducing our randomized benchmarking error rates. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J39.00007: Detecting and Reducing Gate Leakage in Superconducting Qubits using Randomized Benchmarking Z. Chen, J. Kelly, R. Barends, B. Campbell, Y. Chen, B. Chiaro, A. Dunsworth, A. Fowler, I.-C. Hoi, E. Jeffrey, A. Megrant, J. Mutus, C. Neill, P.J.J. O'Malley, C. Quintana, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T. White, A.N. Korotkov, A.N. Cleland, J.M. Martinis Superconducting qubits are a promising platform for building a quantum computer due to their scalability and ease of control. One potential drawback is the existence of more than two energy levels, which can allow the qubit to leak out of the computational subspace when performing operations. This leakage error is particularly detrimental in the surface code scheme, where it leads to correlated errors. I will present a method for characterizing gate leakage rates using randomized benchmarking, and present strategies based on these results for reducing leakage. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J39.00008: Efficient State Tomography for Continuous Variable Systems Chao Shen, Luyao Jiang, Stefan Krastanov, Victor V. Albert, Reinier Heeres, Brian Vlastakis, Rob Schoelkopf, Liang Jiang We propose an efficient and error robust scheme for state tomography of a continuous variable system, which is dispersively coupled to a two-level system. Our adaptive tomography protocol offers a significant speed up compared to the conventional Wigner tomography for a practically interesting class of states, such as Schrodinger cat states. In the presence of typical experimental errors, the number of measurements required is still close to the information theoretic limit. Our proposals can be readily implemented in platforms such as superconducting transmon qubit inside a microwave cavity. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J39.00009: Implementing gates in a fluxonium-like qubit with suppressed wavefunction overlap Nathan Earnest, Yao Lu, David McKay, David Czaplewski, Leonidas Ocola, David Schuster Superconducting Josephson junction qubits are a promising technology for quantum information processing, but are limited by finite lifetimes. The lifetime of the qubit, according to Fermi's golden rule, is dictated by the overall overlap of the \textbar 0\textgreater and \textbar 1\textgreater wavefunctions and so a long lived qubit may be constructed from states with well-isolated wavefunctions. A ``double-well'' fluxonium-like qubit[1] with well-separated degenerate ground states is obtained by increasing the qubit energy EJ and decreasing the charging energy EC. This qubit implementation is expected to have T2s similar to a tunable transmon qubit but, due to the isolated wavefunctions, has promise for long T1s that are insensitive to arbitrary forms of noise. This isolation, however, makes performing arbitrary quantum gates more difficult as wavefunction overlap also allows for arbitrary qubit operations. In this presentation, we will discuss methods for performing arbitrary quantum gates, the implications for decoherence due to flux noise, and discuss experimental progress. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J39.00010: High-fidelity single-shot Toffoli gate with superconducting elements via quantum control Ehsan Zahedinejad, Joydip Ghosh, Barry Sanders A single-shot Toffoli, or controlled-controlled-NOT, gate is desirable for classical and quantum information processing. The Toffoli gate alone is universal for reversible computing and, accompanied by the Hadamard gate, forms a universal gate set for quantum computing. The Toffoli gate is a key ingredient for (non-topological) quantum error correction. Currently Toffoli gates are achieved by decomposing into sequentially implemented single- and two-qubit gates, which requires much longer times and yields lower overall fidelities compared to a single-shot implementation. We develop a quantum-control procedure to directly construct single-shot Toffoli gates and devise a scheme for three nearest-neighbor-coupled superconducting transom systems that should operate with 99.9\% fidelity under realistic conditions. The gate is achieved by a non-greedy quantum control procedure using our enhanced version of the Differential Evolution algorithm. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J39.00011: The Design of Control Pulses for Heisenberg Always-On Qubit Models Rudolph Magyar One model for a universal quantum computer is a spin array with constant nearest neighbor interactions and a controlled unidirectional site-specific magnetic field to generate unitary transformations. This system can be described by a Heisenberg spin Hamiltonian and can be simulated for on the order of 50 spins. It has recently been shown that time-dependent density functional inspired methods may be used to relate various spin models of qubits to ones that may be easier to compute numerically allowing potentially the efficient simulation of greater numbers of spins. One of the challenges of such an agenda is the identification of control pulses that produce desired gate operations (CNOT and single qubit phase gates). We apply control theory to design a universal set of pulses for a Heisenberg always-on model Hamiltonian for a few qubits and compare to known pulses when available. We suggest how this approach may be useful to design control pulses in other realistic designs. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J39.00012: Demonstrated control of a Transmon using a Reciprocal Quantum Logic digital circuit - Part 1 Micah Stoutimore, James Medford, Quentin Herr, Ofer Naaman, Harold Hearne, Joel Strand, Anthony Przybysz, Aaron Pesetski, John Przybysz We report on experiments in which we used a Reciprocal Quantum Logic circuit to perform Z rotations on a Transmon qubit. Reciprocal Quantum Logic (RQL) [1] is a low-power superconducting digital technology based on pairs of single-flux quantum voltage pulses. Here we discuss the RQL hardware used in these experiments - an RQL output amplifier [2]~ whose output is non-return-to-zero (NRZ) encoded 3 mV differential signal, and is primarily intended for transmitting RQL data signals to standard room temperature CMOS hardware. We demonstrate the circuit operation at both 4 K and 20 mK with wide circuit operating margins. [1] J. Appl. Phys\textbf{ 109}, 103903 (2011) [2] Supercond. Sci. Technol.\textbf{ 23}, 022044 (2010) [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J39.00013: Demonstrated control of a Transmon using a Reciprocal Quantum Logic digital circuit - Part 2 James Medford, Micah Stoutimore, Quentin Herr, Ofer Naaman, Harold Hearne, Joel Strand, Anthony Przybysz, Aaron Pesetski, John Przybysz We demonstrate coherent manipulation of a 2D asymmetric Transmon qubit using a Reciprocal Quantum Logic (RQL) [1] distributed output amplifier mounted at 20 mK. The RQL amplifier provided active isolation and amplification for signals generated by room temperature equipment. We measured a 30{\%} suppression of the Transmon lifetime when connected to the RQL circuit, which we primarily attributed to static power dissipation associated with the on-chip 50 Ohm source termination of the amplifier. \\[4pt] [1] J. Appl. Phys 109, 103903 (2011) [Preview Abstract] |
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