One-step quantum phase gate in the ultrastrong coupling regime of circuit QED

In a previous publication (Phys Rev Lett 108: 120501, 2012), Romero et al. proposed an ultrastrong coupling circuit QED system that can implement a twoqubit quantum phase gate with four controlling pulses. Based on this architecture, we demonstrate that an ultrafast two-qubit phase gate can also be realized with only one oscillation and lower coupling strengths. In our operation scheme, two identical qubits evolve synchronously under a single pulse with a duration determined by a specific coupling strength. The phase gate can also be obtained periodically. The influences of parameter fluctuations are estimated. We demonstrate that the fidelities can be greater than 99% if the parameter fluctuations are controlled within 5%.