As a second basic research line, we implement microwave circuits with Josephson junctions and superconducting quantum interference devices (SQUIDs). Josephson elements constitute a nonlinear inductance and therefore the resulting microwave circuit with an integrated Josephson junction will also be nonlinear and in addition have a current-tunable or magnetic-flux tunable (in the case of an integrated SQUID) resonance frequency. On one hand, such Josephson or quantum interference circuits can be used to investigate the fundamental properties of the integrated Josephson junction, such as its current-phase-relation. On the other hand, these circuits are highly sensitive detectors for small currents, magnetic flux or mechanical displacement. Finally, they can be tuned in and out of resonance with other fixed frequency devices by changing the magnetic flux through the SQUID. Our focus here is on the development and investigation of high-quality quantum interference circuits with novel types of Josephson junctions, with high-magnetic-field compatibility and with operability at a wide temperature range from 10 mK to several 10 K.

Funded by the German Research Foundation (DFG) via grant no. BO 6068/1-1 (Daniel Bothner) and grant no. BO 6068/2-1 (Daniel Bothner).

Key publications:
K. Uhl, D. Hackenbeck, C. Füger, R. Kleiner, D. Koelle, and D. Bothner, Applied Physics Letters 122, 182603 (2023)