  We consider the free carrier dispersion effect in a semiconductor nanocavity
in the limit of discrete photoexcited electron-hole pairs. This analysis
reveals the possibility of ultrafast, incoherent transduction and gain from a
single photon signal to a strong coherent probe field. Homodyne detection of
the displaced probe field enables a new method for room temperature,
photon-number-resolving single photon detection. In particular, we estimate
that a single photon absorbed within a silicon nanocavity can, within tens of
picoseconds, be detected with $\sim 99\%$ efficiency and a dark count rate on
the order of kHz assuming a mode volume $V_\text{eff}\sim
10^{-2}(\lambda/n_\text{Si})^3$ for a 4.5 micron probe wavelength and a loaded
quality factor $Q$ on the order of $10^4$.
