The Nonlocal Superconducting Quantum Interference Device

Superconducting quantum interference devices (SQUIDs) are the basis for some of the most sensitive detectors of magnetic fields. SQUIDs are typically made with 2 parallel superconductor/insulator/superconductor (SIS) Josephson junctions and operated with voltage bias. The voltage bias causes the SQUID to emit microwave radiation, which can affect the sample being measured, due to the ac Josephson effect. Here we describe a SQUID with superconductor/normal-metal/superconductor (SNS) Josephson junctions. SNS SQUIDs can be operated with a voltage bias with expected performance comparable to conventional SIS SQUIDs. However, SNS SQUIDS can be operated in a novel mode due to the unusual interplay between quasiparticle currents and supercurrents in the junction. This new method allows measurement of the flux dependent critical current without a voltage bias. Simulations based on the quasi-classical theory of superconductivity elucidate the operation of this device. Estimates of the flux sensitivity in this mode show that it should be comparable to conventional SIS SQUIDS.