Gaussian soft control for controlled-Z gate on superconducting qubits with unilateral external driving

Soft quantum control is a valid technique for highly selective interactions recently illustrated in Haase et al (2018 Phys. Rev. Lett. 121 050402), holding efficient resonant couplings among target levels while largely suppressing unwanted off-resonant contributions. Here we present a model for implementing a controlled-Z (CZ) gate in superconducting circuit quantum electrodynamics (QED) with two qubits being coupled to a microwave cavity. An external classical field that drives only one qubit, combined with the strong single-mode quantized cavity field dressing both qubits, is employed to induce the CZ gate between two qubits, and is also further tailored as a Gaussian soft control (GSC) to improve gate performances in various aspects. By contrast, we show that, with the same gate time, the CZ gate based on GSC can hold a higher fidelity, greater resilience to parameter errors, and stronger robustness against decoherence of system than that based on a rectangular pulse.

Automated summary

Soft quantum control is a powerful technique that allows for highly selective interactions in quantum systems. In this study, a model is proposed for implementing a controlled-Z (CZ) gate in superconducting circuit quantum electrodynamics. The CZ gate is induced between two qubits by employing an external classical field and the strong single-mode quantized cavity field. The gate is further optimized using a Gaussian soft control (GSC), which improves its performance in various aspects compared to a rectangular pulse.