  The reduced switching current of an excited energy-band in a Bloch transistor
can be used to detect microwave-induced interband transitions. Spectroscopic
measurements of the band-gap are performed by mapping the frequency dependence
of the excitation threshold where the microwave photon energy and band-gap are
equal. This excitation threshold also provides a probe of charge-state mixing
on the island of the transistor. Any asymmetry in the junctions of the
transistor makes the coherent mixing of charge-states appear as a finite
gap-splitting at the electrostatic degeneracy point between states differing by
one excess Cooper-pair on the island. The measured gap-splitting in an
asymmetric transistor clearly demonstrates the effect.
