Since 1990, the volt has been defined using superconducting devices called Josephson junctions. The DC voltage across a Josephson junction is related only via fundamental constants to the frequency of an applied microwave signal. As existing microwave sources have frequency stability well below one part-per-billion (ppb), so is the precision of the Josephson voltage standard.
Although these standards have been used for over two decades in metrology, suprisingly there is no application of Josephson junctions as an ultra low-noise voltage supply for physics experiments. One limitation is technical: implementation of Josephson standards requires expertise in superconductivity and cryogenics. The other is functional: typical Josephson standards are fixed and one needs tunable sources for experiments.
Driven by a practical need, we propose a design for a tunable (0.01-1 mV), ppb-noise Josephson voltage supply. Continuous tunability is achieved in a novel way: the microwave frequency and amplitude are simultaneously changed in order to maintain phase-locking of the Josephson junction to the microwave source.
The postdoc will implement the design, test the device, and develop a stand-alone unit for applications in mesoscopic physics. The candidate should have a background in experimental condensed matter physics and have experience with superconductivity and microwave electronics.
