  Single-photon detectors have typically consisted of macroscopic materials
where both the photon absorption and transduction to an electrical signal
happen. Newly proposed designs suggest that large arrays of nanoscale detectors
could provide improved performance in addition to decoupling the absorption and
transduction processes. Here we study the properties of such a detector
consisting of a nanoscale superconducting (SC) transport channel functionalized
by a photon absorber. We explore two detection mechanisms based on
photo-induced electrostatic gating and magnetic effects. To this end we model
the narrow channel as a one-dimensional atomic chain and use a self-consistent
Keldysh-Nambu Green's function formalism to describe non-equilibrium effects
and SC phenomena. We consider cases where the photon creates electrostatic and
magnetic changes in the absorber, as well as devices with strong and weak
coupling to the metal leads. Our results indicate that the most promising case
is when the SC channel is weakly coupled to the leads and in the presence of a
background magnetic field, where photo-excitation of a magnetic molecule can
trigger a SC-to-normal transition in the channel that leads to a change in the
device current several times larger than in the case of a normal-phase channel
device.
