  Single photon detectors have historically consisted of macroscopic-sized
materials but recent experimental and theoretical progress suggests new
approaches based on nanoscale and molecular electronics. Here we present a
theoretical study of photodetection in a system composed of a quantum
electronic transport channel functionalized by a photon absorber. Notably, the
photon field, absorption process, transduction mechanism, and measurement
process are all treated as part of one fully-coupled quantum system, with
explicit interactions. Using non-equilibrium, time-dependent quantum transport
simulations, we reveal the unique temporal signatures of the single photon
detection process, and show that the system can be described using optical
Bloch equations, with a new non-linearity as a consequence of time-dependent
detuning caused by the backaction from the transport channel via the dynamical
Stark effect. We compute the photodetector signal-to-noise ratio and
demonstrate that single photon detection at high count rate is possible for
realistic parameters by exploiting a novel non-equilibrium control of
backaction.
