  We study decoherence in a circuit QED system consisting of a charge qubit and
two superconducting transmission line resonators (TLRs). We show that in the
dispersive regime of the circuit QED system one TLR can be used as an auxiliary
subsystem to realize decoherence-free quantum dynamics of the bipartite target
system consisting of the charge qubit and the other TLR conditioned on the
auxiliary TLR initially being a proper number state. Our study gives new
insight into control and manipulation of decoherence in quantum systems.
