Speaker: Dr. Nicolò Crescini (Institut Néel CNRS, France)
Abstract: The accuracy with which electromagnetic waves of specific frequencies can be both measured and produced have made frequency the basis for the definition of most physical units, including time, distance, and voltage. The modern primary voltage standard is based on the phenomena of Shapiro steps, where a frequency applied to a Josephson junction yields a constant voltage determined by only the frequency of the light and fundamental constants. The duality of current and voltage has long suggested the possibility of dual Shapiro steps---that a Josephson junction device could produce current steps with heights determined only on the applied frequency. To this aim, we embed an ultrasmall Josephson junction in a high impedance array of large Josephson junctions to measure dual Shapiro steps resulting directly from microwave synchronised transport of Cooper pairs. In such device we observe the formation of a microwave mode synchronous with the tone frequency. In turn, current plateaux emerge in the IV characteristics, with an amplitude depending on the frequency f of the tone as 2 e f. These two phenomena are explained by phase-locked Bloch oscillations and dual Shapiro steps, respectively. The observation of the dual AC Josephson effect opens a broad range of possibilities for future experiments, e.g. in the field of circuit quantum electrodynamics, many body quantum optics and quantum metrology.
