  We report a procedure to detect mid-infrared single photons at 4.65 um via a
two-stage scheme based on Sum Frequency Generation, using a Periodically Poled
Lithium Niobate (PPLN) nonlinear crystal and a Silicon Avalanche Photodiode. An
experimental investigation shows that, in addition to a high timing resolution,
this technique yields a detection sensitivity of 1.24 pW with 63mW of net pump
power.
