  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. In order to
show its reliability in the fields of quantum simulation and quantum computing,
we study its sensitivity to noises.
