  The microscopic mechanism of photon detection in superconducting nanowire
single-photon detectors is still under debate. We present a simple, but
powerful theoretical model that allows us to identify essential differences
between competing detection mechanisms. The model is based on quasi-particle
multiplication and diffusion after the absorption of a photon. We then use the
calculated spatial and temporal evolution of this quasi-particle cloud to
determine detection criteria of three distinct detection mechanisms, based on
the formation of a normal conducting spot, the reduction of the effective
depairing critical current below the bias current and a vortex-crossing
scenario, respectively. All our calculations as well as a comparison to
experimental data strongly support the vortex-crossing detection mechanism by
which vortices and antivortices enter the superconducting strip from the edges
and subsequently traverse it thereby triggering the detectable normal
conducting domain. These results may therefore help to reveal the microscopic
mechanism responsible for the detection of photons in superconducting
nanowires.
