Implementation Of Universal Quantum Gates By Periodic Two-Step Modulation In A Weakly Nonlinear Qubit

Reducing the leakage from qubit levels to other levels is of importance for quantum computation. Here, we develop a feasible method to effectively suppress this leakage and perform universal quantum gates in a weakly nonlinear qubit. The method is to exploit periodical two-step modulation of single control parameters in a artificial large detuning regime. It works well, regardless of whether the strength of control parameter is known or not. In particular, qubit leakage can also be suppressed in a counterintuitive parameter regime, where the control field drives the transition of qubit levels with large detuning, but resonance transition with leakage level. The results demonstrate that this method is robust against parameter perturbations, and the deformation of square waves does not accumulate the error of qubit operations but affects the gate time. In addition, this method provides us with a viable approach to measure the control parameters.