Quantum computing and condensed matter physics with microwave photons

Superconducting Qubit with Purcell Protection and Tunable Coupling

J. M. Gambetta, A. A. Houck, and Alexandre Blais

We present a superconducting qubit for the circuit quantum electrodynamics architecture that has a tunable qubit-resonator coupling strengthg. This coupling can be tuned from zero to values that are comparable with other superconducting qubits. At g=0, the qubit is in a decoherence-free subspace with respect to spontaneous emission induced by the Purcell effect. Furthermore, we show that in this decoherence-free subspace, the state of the qubit can still be measured by either a dispersive shift on the resonance frequency of the resonator or by a cycling-type measurement.