Optimized driving of superconducting artificial atoms for improved single-qubit gates

Abstract

We employ pulse shaping to abate single-qubit gate errors arising from the weak anharmonicity of transmon superconducting qubits. By applying shaped pulses to both quadratures of rotation, a phase error induced by the presence of higher levels is corrected. Using a derivative of the control on the quadrature channel, we are able to remove the effect of the anharmonic levels for multiple qubits coupled to a microwave resonator. Randomized benchmarking is used to quantify the average error per gate, achieving a minimum of 0.007 ± 0.005 using 4 ns-wide pulse.PACS numbers: 03.67. Ac, 42.50.Pq, The successful realization of quantum information processing hinges upon the ability to perform high-fidelity control (gates) of quantum bits, or qubits. A value of 10 −3 error per gate (EPG) is typically quoted as the necessary threshold for fault-tolerant quantum computation [1]. For any two-level system, the optimal achievable gate performance is set by the ratio of gate time to coherence time. However, quantum information processing systems consist of multiple coupled qubits. Often these qubits are not truly two-level systems, but rather quantum objects with a rich level structure. Thus, one important challenge is to achieve optimized single-qubit gates in a large Hilbert space.Optimal control theory has been previously employed in nuclear magnetic resonance, non-linear optics, and trapped ions to combat specific errors such as alwayson qubit couplings and spatial inhomogeneities [2]. In these systems, even with optimized qubit control, limits to qubit gates are often due to systematic errors rather than decoherence. Superconducting qubit control, however, has primarily been limited by coherence times. With recent progress such as the demonstration of highfidelity single-qubit gates [3,4], two-qubit gates [5][6][7][8], entanglement [9][10][11] and simple quantum algorithms [7], the superconducting qubit architecture is growing into a more complex quantum information testbed, placing the level of qubit control under increased scrutiny. One approach towards improving single-qubit gates is to decrease the total gate time. However, in multi-level qubits such as the transmon [12] or phase qubit [3], the weak anharmonicity sets a lower limit on the gate time. Furthermore, superconducting qubit coupling schemes such as circuit quantum electrodynamics (QED), in which a transmission-line cavity couples multiple qubits [13,14], can make single-qubit control more difficult as a result of many extra levels in the Hilbert space.In this Letter, we implement simple optimal control techniques to improve single-qubit gates in a circuit QED device with two superconducting transmons. The pulseshaping protocols we investigate are guided by the recent theoretical exploration of derivative removal via adiabatic gate (DRAG) in Ref. [15]. We demonstrate the improvement of single-qubit gates on two separate qubits in the same device using the first-order correction in DRAG, by switching from rotations around a single axis induced by Gaussian-modul...

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“…With superconducting circuits, single qubit quantum gates can be carried out with fidelities approaching 99.99% [1][2][3][4], while errors in two-qubit operations are typically higher with record fidelities around 99% [3,5]. However, the realization of qubit operations with even higher fidelity is required both for reaching the error threshold for quantum computation [6][7][8][9] and for carrying out reliable quantum simulations and optimizations in large arrays of coupled qubits [10][11][12][13].…”

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