A tunable rf SQUID manipulated as flux and phase qubit

We report on two different manipulation procedures of a tunable rf superconducting quantum
interference device (SQUID). First, we operate this system as a flux qubit, where the coherent
evolution between the two flux states is induced by a rapid change of the energy potential,
turning it from a double well into a single well. The measured coherent Larmor-like oscillation
of the retrapping probability in one of the wells has a frequency ranging from 6 to 20 GHz,
with a theoretically expected upper limit of 40 GHz. Furthermore, here we also report a
manipulation of the same device as a phase qubit. In the phase regime, the manipulation of the
energy states is realized by applying a resonant microwave drive. In spite of the conceptual
difference between these two manipulation procedures, the measured decay times of Larmor
oscillation and microwave-driven Rabi oscillation are rather similar. Due to the higher
frequency of the Larmor oscillations, the microwave-free qubit manipulation allows for much
faster coherent operations.