Mid-infrared single-photon counting

We report a procedure to detect mid-infrared single photons at 4.65 μm by means of a two-stage scheme based on sum-frequency generation, by using a periodically poled lithium niobate 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.