Using a Josephson junction as an effective on-chip temperature sensor

Temperature sensing and control are essential in experiments running in cryocooled systems, as with the case of liquid helium-free superconducting devices. A Josephson junction as on-chip temperature sensor operated in ac provides the highest sensitivity and minimal power loading to the cryogenic environment, thanks to the noise rejection of lock-in detection. To demonstrate the advantages of on-chip sensing, we tested it with a Josephson voltage standard array in cryocooler and compared with the conventional case of a sensor on the cold surface of the refrigerator, showing that the power dissipated within the chip may further increment the device temperature up to some tenths of kelvin. An ac Josephson junction sensor is proven to be capable of directly stabilizing the temperature of the superconductive circuit from fluctuations of dissipated power during operation. Reliability issues related to flux trapping are discussed and solutions are proposed suited to different applications. Overall, on-chip control with ac Josephson temperature sensing has the advantage of avoiding the complexities in minimization of cryogenic thermal links, virtually reducing to zero the contact resistance and keeping the operating temperature of the superconductor constant, independently of instantaneous operating power.