Flux qubits in a planar circuit quantum electrodynamics architecture: Quantum control and decoherence

We report experiments on superconducting flux qubits in a circuit quantum electrodynamics (cQED) setup. Two qubits, independently biased and controlled, are coupled to a coplanar waveguide resonator. Dispersive qubit state readout reaches a maximum contrast of 72%. We measure energy relaxation times at the symmetry point of 5 and 10 mu s, corresponding to 7 and 20 mu s when relaxation through the resonator due to Purcell effect is subtracted out, and levels of flux noise of 2.6 and 2.7 mu Phi(0)/root Hz at 1 Hz for the two qubits. We discuss the origin of decoherence in the measured devices. The strong coupling between the qubits and the cavity leads to a large, cavity-mediated, qubit-qubit coupling. This coupling, which is characterized spectroscopically, reaches 38 MHz. These results demonstrate the potential of cQED as a platform for fundamental investigations of decoherence and quantum dynamics of flux qubits.