  We measured the intrawell energy relaxation time \tau_{d} between macroscopic
quantum levels in the double well potential of a Nb persistent-current qubit.
Interwell population transitions were generated by irradiating the qubit with
microwaves. Zero population in the initial well was then observed due to a
multi-level decay process in which the initial population relaxed to the lower
energy levels during transitions. The qubit's decoherence time, determined from
\tau_{d}, is longer than 20 microseconds, holding the promise of building a
quantum computer with Nb-based superconducting qubits.
