  Bloch oscillations predicted to occur in current-biased single Josephson
junctions have eluded direct observation up to now. Here, we demonstrate
similar Bloch oscillations in a slightly richer Josephson circuit, the
quantronium. The quantronium is a Bloch transistor with two small junctions in
series, defining an island, in parallel with a larger junction. In the ground
state, the microwave impedance of the device is modulated periodically with the
charge on the gate capacitor coupled to the transistor island. When a current
flows across this capacitor, the impedance modulation occurs at the Bloch
frequency, which yields Bloch sidebands in the spectrum of a reflected
continuous microwave signal. We have measured this spectrum, and compared it to
predictions based on a simple model for the circuit. We discuss the interest of
this experiment for metrology and for mesoscopic physics.

Superconductivity;Mesoscale and Nanoscale Physics