Rf-SQUID qubit readout using a fast flux pulse

Abstract

We report on development of a set-up for measuring intrawell dynamics in a Nb-basedrf-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 lowprobability for the ground state. The rapid tilt of the double well potential is accomplishedvia a readout flux pulse inductively coupled to the qubit from a microstrip transmissionline on a separate chip suspended above the qubit chip. The readout pulse is analogous tothe current bias pulse used to readout phase qubits and hysteretic dc-SQUIDmagnetometers.The coupling between the transmission line and the qubit is carefully controlled via a windowin the ground plane between the signal conductor of the microstrip and the qubit loop. Sincethe high frequency transmission lines are on a separate chip, they can be independentlycharacterized and reused for different qubit samples. Clean flux pulses as short as 5 ns withrise times of 0.5 ns have been coupled to the qubit to measure escape rates higher than108 s−1, the lifetime of the excited state, and coherent oscillations between the ground and excitedstates within the same well.