  We study dynamics of the order parameter in a superconducting film with
transport current after absorption of a single photon. The system from the
time-dependent Ginszburg-Landau equation, Poisson's equation for an electrical
potential and the heat diffusion equation was solved numerically. For each
photon energy in the absence of fluctuations, there exists a corresponding
threshold current below which the superconducting state is stable and no
voltage appears between the ends of the film. At larger currents the
superconducting state collapses starting from the appearance of a
vortex-antivortex pair in the center of the region with suppressed
superconducting order parameter which has been created by the absorbed photon.
Lorentz force causes motion of these vortices that heats the film locally and
gives rise to a normal domain. When biased with the fixed current, the film
latches in the normal state. In the regime when current via superconductor may
change, which is more relevant for experiments, the normal domain exists only
for a short time resulting in the voltage pulse with the duration controlled by
the kinetic inductance of the superconducting film.
