  Superconducting nanowire single photon detectors (SNSPDs) offer
high-quantum-efficiency and low-dark-count-rate single photon detection. In a
growing number of cases, large magnetic fields are being incorporated into
quantum microscopes, nanophotonic devices, and sensors for nuclear and
high-energy physics that rely on SNSPDs, but superconducting devices generally
operate poorly in large magnetic fields. Here, we demonstrate robust
performance of amorphous SNSPDs in magnetic fields of up to $\pm 6$ T with a
negligible dark count rate and unchanged quantum efficiency at typical bias
currents. Critically, we also show that in the electrothermal oscillation
regime, the SNSPD can be used as a magnetometer with sensitivity of better than
100 $\mathrm{\mu T/\sqrt{Hz}}$ and as a thermometer with sensitivity of 20
$\mathrm{\mu K/\sqrt{Hz}}$ at 1 K. Thus, a single photon detector integrated
into a quantum device can be used as a multifunctional quantum sensor capable
of describing the temperature and magnetic field on-chip simply by varying the
bias current to change the operating modality from single photon detection to
thermometry or magnetometry.

Quantum Physics