Single-photon source

A single-photon source is a light source that emits light as single particles or photons. Single-photon sources are distinct from coherent light sources (lasers) and thermal light sources such as incandescent light bulbs. The Heisenberg uncertainty principle dictates that a state with an exact number of photons of a single frequency cannot be created. However, Fock states (or number states) can be studied for a system where the electric field amplitude is distributed over a narrow bandwidth. In this context, a single-photon source gives rise to an effectively one-photon number state.

Photons from an ideal single-photon source exhibit quantum mechanical characteristics. These characteristics include photon antibunching, so that the time between two successive photons is never less than some minimum value. This behaviour can be experimentally demonstrated by using a beam splitter and single photon detectors, such as avalanche photodiodes, which monitor the output of the beam splitter. A detection from one detector is used to provide a 'counter start' signal, to a fast electronic timer, and the other, is used to provide a 'counter stop' signal. In the case of a stream of single photons to this measurement apparatus there will not be any coincidental detection events. By repeatedly measuring the times between 'start' and 'stop' signals, one can form a histogram of time delays between two consecutive photons. If a true single photon source is observed, the photons are timely separated and a clear notch around zero delay is visible.