Quantum Gate for a Kerr Nonlinear Parametric Oscillator Using Effective Excited States

A Kerr nonlinear parametric oscillator (KPO) can stabilize a quantum superposition of two coherent states with opposite phases, which can be used as a qubit. In a universal gate set for quantum computation with KPOs, an Rx gate, which interchanges the two coherent states, is relatively hard to perform owing to the stability of the two states. We propose a method for a high-fidelity Rx gate by exciting the KPO outside the qubit space with parity-selective transitions, which can be implemented by only adding a driving field. In this method, the utilization of higher effective excited states leads to a faster Rx gate, rather than states near the qubit space. The proposed method can realize a continuous Rx gate and thus is expected to be useful for, e.g., recently proposed variational quantum algorithms.