  We experimentally investigate the effect of a magnetic field on photon
detection in superconducting single-photon detectors. At low fields, the effect
of a magnetic field is through the direct modification of the quasiparticle
density of states of the superconductor, and magnetic field and bias current
are interchangable, as is expected for homogeneous dirty-limit superconductors.
At the field where a first vortex enters the detector, the effect of the
magnetic field is reduced, up until the point where the critical current of the
detector starts to be determined by flux flow. From this field on, increasing
the magnetic field does not alter the detection of photons anymore, whereas it
does still change the rate of dark counts. This result points at an intrinsic
difference in dark and light counts, and also shows that no enhancement of the
intrinsic detection efficiency of a straight SSPD wire is achievable in a
magnetic field.
