Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K46: Quantum Gates in Superconducting QubitsFocus
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Sponsoring Units: GQI Chair: Alexandre Blais, University of Sherbrooke Room: 393 |
Wednesday, March 15, 2017 8:00AM - 8:36AM |
K46.00001: Fixed-Frequency Qubits Coupled via a Tunable Bus Invited Speaker: David McKay As quantum circuits increase in size there are conflicting design requirements: qubits must be highly coherent and addressable, but also interact strongly with each other “on demand”. Fixed-frequency superconducting transmon qubits coupled by bus resonators (circuit QED) excel in terms of coherence. However, it is difficult to activate interactions in this architecture because of the limited tuning parameters. To circumvent this limitation, we add tunability to the bus which results in a tunable exchange coupling between qubits. The qubits themselves become only weakly tunable, thus suppressing flux-noise limited coherence. In Ref. [1] we demonstrated a two-qubit iSWAP gate with a fidelity of 98.3\% by modulating the tunable bus at the qubit difference frequency which activates a resonant SWAP interaction. In this talk I will discuss our iSWAP gate in detail and present our work utilizing the SWAP interaction to couple four qubits, which is the prototypical surface code unit cell.\\ 1. McKay et al., “A universal gate for fixed-frequency qubits via a tunable bus”. Arxiv/1604.03076 (2016) [Preview Abstract] |
Wednesday, March 15, 2017 8:36AM - 8:48AM |
K46.00002: Direct tune-up of entangling gates generated by a cross resonance drive Sarah Sheldon, Christopher J. Wood, Easwar Magesan, David C. McKay, Jerry M. Chow, Jay M. Gambetta The cross-resonance (CR) gate, is one of the leading two-qubit gates for quantum circuits with superconducting qubits. Through continuing study of the cross resonance (CR) Hamiltonian, we have developed a better understanding of the CR gate errors. Certain unitary errors put strict limitations on how close the CR gate can be to a CNOT with single qubit $\pi$-rotations. A more accurate gate can be constructed, however, through the use of SU(2) gates on both the control and target qubits. By including single-qubit Z-rotations in the composite gate, we also eliminate the need for an echo in the CR gate, making the total gatetime shorter and increasing the gate fidelity. This talk will discuss the theoretical limits to fidelity imposed by known errors and describe the direct tune up of a CNOT gate using cross resonance. [Preview Abstract] |
Wednesday, March 15, 2017 8:48AM - 9:00AM |
K46.00003: Restless Tuneup of High-Fidelity Qubit Gates M.A. Rol, C.C. Bultink, T.E. O'Brien, S.R. de Jong, L.S. Theis, X. Fu, F. Luthi, R.F.L. Vermeulen, J.C. de Sterke, A. Bruno, D. Deurloo, R.N. Schouten, F.K. Wilhelm, L. DiCarlo We present a tuneup protocol for qubit gates with tenfold speedup over traditional methods reliant on qubit initialization by energy relax- ation. This speedup is achieved by constructing a cost function for Nelder-Mead optimization from real-time correlation of non-demolition measurements interleaving gate operations without pause. Applying the protocol on a transmon qubit achieves 0.999 average Clifford fidelity in one minute, as independently verified using randomized benchmarking and gate set tomography. The adjustable sensitivity of the cost function allows detecting fractional reductions in gate error with constant signal- to-noise ratio. The restless concept here demonstrated can be readily extended to the tuneup of two-qubit gates and measurement operations. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K46.00004: Automated Bringup of Superconducting Qubits Shane Caldwell, Michael Curtis, Anthony Polloreno, Matthew Reagor, Robert Smith, William Zeng, Chad Rigetti As the size and complexity of quantum integrated circuits increases, it is critical to maximize the rate at which qubits can be characterized for design feedback and calibrated for use in computation. Steps such as identifying and optimizing readout points, maximizing spectral purity of applied tones, and tuning a set of gates, often involve a lot of human interaction and interpretation. We present an automated qubit bring-up process from single-tone spectroscopy through randomized benchmarking. [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:24AM |
K46.00005: Tunable, Flexible and Efficient Optimization of Control Pulses for Superconducting Qubits, part I - Theory Shai Machnes, Elie Assémat, David Tannor, Frank Wilhelm Quantum computation places very stringent demands on gate fidelities, and experimental implementations require both the controls and the resultant dynamics to conform to hardware-specific ansatzes and constraints. Superconducting qubits present the additional requirement that pulses have simple parametrizations, so they can be further calibrated in the experiment, to compensate for uncertainties in system characterization. We present a novel, conceptually simple and easy-to-implement gradient-based optimal control algorithm, GOAT [1], which satisfies all the above requirements. In part II we shall demonstrate the algorithm's capabilities, by using GOAT to optimize fast high-accuracy pulses for two leading superconducting qubits architectures - Xmons and IBM's flux-tunable couplers. [1] S. Machnes et al., arXiv \underline {1507.04261} (2015) [Preview Abstract] |
Wednesday, March 15, 2017 9:24AM - 9:36AM |
K46.00006: Tunable, Flexible and Efficient Optimization of Control Pulses for Superconducting Qubits, part II - Applications Elie Assémat, Shai Machnes, David Tannor, Frank Wilhelm-Mauch In part I, we presented the theoretic foundations of the GOAT algorithm [1] for the optimal control of quantum systems. Here in part II, we focus on several applications of GOAT to superconducting qubits architecture. First, we consider a control-Z gate on Xmons [2] qubits with an Erf parametrization of the optimal pulse. We show that a fast and accurate gate can be obtained with only 16 parameters, as compared to hundreds of parameters required in other algorithms. We present numerical evidences that such parametrization should allow an efficient in-situ calibration of the pulse. Next, we consider the flux-tunable coupler by IBM [3]. We show optimization can be carried out in a more realistic model of the system than was employed in the original study, which is expected to further simplify the calibration process. Moreover, GOAT reduced the complexity of the optimal pulse to only 6 Fourier components, composed with analytic wrappers. [1] S. Machnes et al., ArXiv 1507.04261v1 (2015) [2] R. Barends et al., Phys. Rev. Lett. \textbf{100}, 080502 (2013) [3] D. C. McKay et al., ArXiv 1604.0307v2 (2016) [Preview Abstract] |
Wednesday, March 15, 2017 9:36AM - 9:48AM |
K46.00007: Error budgeting single and two qubit gates in a superconducting qubit Z. Chen, B. Chiaro, A. Dunsworth, B. Foxen, C. Neill, C. Quintana, J. Wenner, John. M. Martinis Superconducting qubits have shown promise as a platform for both error corrected quantum information processing and demonstrations of quantum supremacy. High fidelity quantum gates are crucial to achieving both of these goals, and superconducting qubits have demonstrated two qubit gates exceeding 99\% fidelity. In order to improve gate fidelity further, we must understand the remaining sources of error. In this talk, I will demonstrate techniques for quantifying the contributions of control, decoherence, and leakage to gate error, for both single and two qubit gates. I will also discuss the near term outlook for achieving quantum supremacy using a gate-based approach in superconducting qubits. [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K46.00008: Optimizing gates in the presence of leakage errors Christopher J. Wood, David C. McKay, Sarah Sheldon, Jerry M. Chow, Jay M. Gambetta As coherence times of superconducting transmon qubits improve, leakage errors become an important error source that must be taken into account for the design of high fidelity gates, and for achieving fault tolerance thresholds in surface code protocols. We discuss methods for quantifying and characterizing leakage errors in quantum gates, and show how these relate to standard measures of average gate fidelity from randomized benchmarking. We also compare methods for achieving low-leakage, high fidelity single qubit gates by using alternative techniques to standard DRAG control pulses. [Preview Abstract] |
Wednesday, March 15, 2017 10:00AM - 10:12AM |
K46.00009: Analytic approach for suppressing leakage errors in superconducting qubits Hugo Ribeiro, Alexandre Baksic, Aashish Clerk The problem of leakage errors is generic to a variety of situations in quantum information processing. The most prominent example is the problem of high-fidelity qubit gates: control sequences designed to implement a given unitary operation will in general give rise to undesirable transitions out of the logical subspace. Here, we present a new and extremely general strategy based on the Magnus expansion to suppress leakage errors~[1]. By correcting control pulses, we modify the Magnus expansion of an initially-given, imperfect unitary gate in such a way that the desired leakage-free evolution is obtained. While our method can be applied to a variety of different problems (e.g. correcting non-adiabatic errors in adiabatic evolution), in this talk we focus on demonstrating how the method can be used to correct leakage in single and two-qubit gates. \newline [1]H. Ribeiro, A. Baksic, and A. A. Clerk, ArXiv e-prints (2016), arXiv:1610:01105 [quant-ph]. [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K46.00010: Driving qubit phase gates with sech shaped pulses Junling Long, Hsiang-sheng Ku, Xian Wu, Russell Lake, Edwin Barnes, Sophia Economou, David Pappas As shown in 1932 by Rozen and Zener, the Rabi model has a unique solution whereby, for a given pulse length or amplitude, a sech(t/sigma) shaped pulse can be used to drive complete oscillations around the Bloch sphere that are independent of detuning with only a resultant detuning-dependent phase accumulation. Using this property, single qubit phase gates and two-qubit CZ gates have been proposed [S Economou, E Barnes. PRB 91 (16), 161405, 2015]. In this work we explore the effect of different drive pulse shapes, i.e. square, Gaussian, and sech, as a function of detuning for Rabi oscillations of a superconducting transmon qubit. An arbitrary, single-qubit phase gate is demonstrated with the sech(t/sigma) pulse, and full tomography is performed to extract the fidelity. This is the first step towards high fidelity, low leakage two qubit CZ gates, and illustrates the efficacy of using analytic solutions of the qubit drive prior to optimal pulse shaping. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K46.00011: Tunable inter-qubit coupling as a resource for gate based quantum computing with superconducting circuits B. Chiaro, C. Neill, Z. Chen, A. Dunsworth, B. Foxen, C. Quintana, J. Wenner, J. M. Martinis Fast, high fidelity two qubit gates are an essential requirement of a quantum processor. In this talk, we discuss how the tunable coupling of the gmon architecture provides a pathway for an improved two qubit controlled-Z gate. The maximum inter-qubit coupling strength $g_{max}$ = 60 MHz is sufficient for fast adiabatic two qubit gates to be performed as quickly as single qubit gates, reducing dephasing errors. Additionally, the ability to turn the coupling off allows all qubits to idle at low magnetic flux sensitivity, further reducing susceptibility to noise. However, the flexibility that this platform offers comes at the expense of increased control complexity. We describe our strategy for addressing the control challenges of the gmon architecture and show experimental progress toward fast, high fidelity controlled-Z gates with gmon qubits. [Preview Abstract] |
Wednesday, March 15, 2017 10:36AM - 10:48AM |
K46.00012: Implementing single qubit gates using RSFQ pulses Per Liebermann, Frank Wilhelm Rapid single flux quantum (RSFQ) pulses are a highly viable candidate for the on-chip generation of control pulses for quantum computers based on Josephson devices. With a switching time in the picoseconds range it is possible to implement fast quantum gates [1]. We show that RSFQ pulses can drive high-fidelity single-qubit rotations in leaky transmon qubits, if the sequence of these restricted digital pulses is suitably optimized compared to an evenly spaced pulse train [2]. Genetic algorithms are used to converge to gate control precision compatible with the requirements of fault tolerant quantum computing. RSFQ shift registers are essential to perform the optimized sequence, limiting the reachable set of gates in a single shot. Timing jitter of the pulses is considered as well, showing the robustness of the optimized sequence. This makes the underlying RSFQ pulse platform an attractive candidate for an integrated control layer in a quantum processor.\\ $[1]$ R. McDermott and M.G. Vavilov, Phys. Rev. Appl. 2, 014007 (2014)\\ $[2]$ P.J. Liebermann and F.K. Wilhelm, Phys. Rev. Appl. 6, 024022 (2016) [Preview Abstract] |
Wednesday, March 15, 2017 10:48AM - 11:00AM |
K46.00013: High-fidelity two qubit gate via single flux quantum pulses Maxim Vavilov, Zhenyi Qi, Robert McDermott We analyze two-qubit gates controlled by trains of single flux quantum (SFQ) pulses. The SFQ pulses are applied to one of two weakly coupled superconducting qubits. We analyze a controlled Z gate realized by a resonant transition from state $|11\rangle$ to a higher energy state outside of the qubit computational space and returning back to $|11\rangle$. We evaluate the fidelity of this two-qubit gate and find that the fidelity can be above 99\% for gate times under 100 ns. For systems with fixed coupling between qubits, the fidelity of single qubit gates can also exceed 99\%, and can be even higher if tunable coupling is used. We also discuss alternative routes for two-qubit gates utilizing single flux pulses. [Preview Abstract] |
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