Measurements of Decoherence in rf SQUID Qubits*

We report measurements of coherence times of an rf SQUID qubit using pulsed microwaves and rapid flux pulses. The modified rf SQUID has independent, in situ, controls for the relative positions of levels in different fluxoid wells and the barrier height between the wells. The decay of coherent oscillations is dominated by the lifetime of the excited state and low frequency flux noise. The low frequency flux noise is observed using microwave spectroscopy and resonant tunneling between fluxoid states in addition to the decay of coherent oscillations. These measurements are useful for evaluating the various insulating layers which are believed to be an important source of 1/f noise in many superconducting qubits.