  Superconducting quantum interference devices (SQUIDs) that incorporate two
superconductor/insulator/superconductor (SIS) Josephson junctions in a closed
loop form the core of some of the most sensitive detectors of magnetic and
electric fields currently available. SQUIDs in these applications are typically
operated with a finite voltage which generates microwave radiation through the
ac Josephson effect. This radiation may impact the system being measured. We
describe here a SQUID in which the Josephson junctions are formed from strips
of normal metal (N) in good electrical contact with the superconductor (S).
Such SNS SQUIDs can be operated under a finite voltage bias with performance
comparable or potentially better than conventional SIS SQUIDs. However, they
also permit a novel mode of operation that is based on the unusual interplay of
quasiparticle currents and supercurrents in the normal metal of the Josephson
junction. The new method allows measurements of the flux dependence of the
critical current of the SNS SQUID without applying a finite voltage bias across
the SNS junction, enabling sensitive flux detection without generating
microwave radiation.
