rf-SQUID qubit readout using a fast flux pulse

We report on development of a set-up for measuring intrawell dynamics in a Nb-based rf-SQUID qubit described by a double well potential, by rapidly tilting the potential, allowing escape to the adjacent well with high probability for an excited state but low probability for the ground state. The rapid tilt of the double well potential is accomplished via a readout flux pulse inductively coupled to the qubit from a microstrip transmission line on a separate chip suspended above the qubit chip. The readout pulse is analogous to the current bias pulse used to readout phase qubits and hysteretic dc-SQUID magnetometers. The coupling between the transmission line and the qubit is carefully controlled via a window in the ground plane between the signal conductor of the microstrip and the qubit loop. Since the high frequency transmission lines are on a separate chip, they can be independently characterized and reused for different qubit samples. Clean flux pulses as short as 5 ns with rise times of 0.5 ns have been coupled to the qubit to measure escape rates higher than 10(8) s(-1), the lifetime of the excited state, and coherent oscillations between the ground and excited states within the same well.