Tunable coupling between a superconducting resonator and an artificial atom

Coherent manipulation of a quantum system is one of the main themes in current physics researches. In this work, we design a circuit QED system with a tunable coupling between an artificial atom and a superconducting resonator while keeping the cavity frequency and the atomic frequency invariant. By controlling the time dependence of the external magnetic flux, we show that it is possible to tune the interaction from the extremely weak coupling regime to the ultrastrong coupling one. Using the quantum perturbation theory, we obtain the coupling strength as a function of the external magnetic flux. We also find that our system is remarkably insensitive to the main noises, which shows its reliability in the fields of quantum simulation and quantum computing. In particular, it is worth pointing out that the best performance of our qubit both in terms of tunable coupling and coherence is achieved when the penetrating fluxes through two SQUIDs are in a restricted range near Φ 0 /2. Graphical abstract