Observation of the Bloch oscillations in an ultrasmall Josephson junction

We have studied the low-temperature behavior of lead-alloy Josephson tunnel junctions with area S{}0.01 {μ}m², isolated from their electromagnetic environment by high-resistance metallic resistors inserted into the current and voltage leads. Under irradiation with microwaves frequencies, f=3.5--10 GHz, the dc differential resistance dV/dI, as a function of the dc current I, showed peaks at I=±{}2ef. This effect, and other observations, arises due to the periodic electrical recharging of the junction by discrete Cooper pairs, and can be explained by the ``orthodox'' theory of Bloch oscillations.