Macroscopic resonant tunneling in an rf-SQUID flux qubit

We have observed the macroscopic resonant tunneling of magnetic flux between macroscopically distinct quantum states in a superconducting flux qubit. The dependences of the macroscopic resonant tunneling on the barrier height of the potential well, the flux bias and the initial state are investigated. Detailed measurements of the tunneling rate as a function of the flux bias reveal the feature of the quantum noise in the superconducting flux qubit.