Two-qubit gate with a parity-violated superconducting qubit

The second-order nonlinearity has been intensively studied over the decades in nonlinear optics for applications such as squeezed light sources and optical frequency combs. Three-wave mixing, the elementary process induced by the second-order nonlinearity, is also a key concept in the latest development of quantum transducers and has also been utilized for high-fidelity quantum manipulations of trapped ions with a parametric coupling. As is well known, a parity violation is required for the second-order nonlinearity. Here we propose a parity-violated superconducting qubit, which enables us to achieve a strong parametric coupling with a neighboring qubit based on the second-order nonlinearity. The qubit consists of a capacitively-shunted SNAIL [1] circuit under a finite flux bias. We demonstrate fast two-qubit gates (CZ, SWAP, and iSWAP) combined with echo pulses to suppress the effect of a residual longitudinal coupling and evaluate the average gate fidelities using an interleaved randomized benchmarking technique.

Ref. [1] N. E. Frattini et al., Appl. Phys. Lett. 110, 222603 (2017).