Dispersive measurement of a metastable phase qubit using a tunable cavity

A metastable phase qubit was measured using a tunable cavity by two methods: a tunneling measurement followed by magnetometry readout by the cavity, and a non destructive dispersive measurement of the qubit by the cavity. The Purcell effect was observed as a decrease in the energy relaxation time of the qubit in the vicinity of the cavity, and could be manipulated by dynamically tuning the cavity. The observed dispersive shift of the cavity did not match the two-level system model for nonlinear qubits. Instead, a three-level model of the qubit was needed to describe the data, necessitated by the weakly nonlinear nature of the metastable phase qubit. The cavity was also used to directly observe the photons radiated by a tunneling measurement.