Experimental demonstration of single photon rangefinding using parametric downconversion

We exploit the coincidence properties of photon pairs created in spontaneous parametric fluorescence to perform a low light level rangefinding experiment. One photon of each pair is sent to the target, and the small fraction returned is delayed by the return trip time. This delay is measured using a multichannel coincidence counter with gate time down to 10 ns. With a few picowatts output power and a target return of 10(-4)we obtain 1-m resolution range estimates in a few seconds with low levels of background light. We discuss the potential of the technique for millimeter accuracy ranging (or surveying) in high ambient light levels.