Observation of coherent charge-state mixing in asymmetric Bloch transistors

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.