Spectroscopic study of energy levels and transition rates in large area flux qubit

The level structure and transition rates between fluxoid states of a large area flux qubit have been studied using microwave spectroscopy and resonant tunneling spectroscopy. This qubit uses an rf-SQUID in a gradiometer configuration and has independent, \textit{in situ}, controls for the barrier height between fluxoid wells and the relative positions of levels in different wells. This design makes it well suited for the study of decoherence mechanisms that adversely affect the operation of flux as well as phase qubits based on fluxoid states. This work was supported in part by NSF and by AFOSR and ARDA through a DURINT program.