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Concept
Single-photon source
Summary
Single-photon sources are light sources that emit light as single particles or photons. These 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 is normally demonstrated by using a beam splitter to direct about half of the incident photons toward one avalanche photodiode, and half toward a second. Pulses from one detector are used to provide a ‘counter start’ signal, to a fast electronic timer, and the other, delayed by a known number of nanoseconds, is used to provide a ‘counter stop’ signal. By repeatedly measuring the times between ‘start’ and ‘stop’ signals, one can form a histogram of time delay between two photons and the coincidence count- if bunching is not occurring, and photons are indeed well spaced, a clear notch around zero delay is visible.
Although the concept of a single photon was proposed by Planck as early as 1900, a true single-photon source was not created in isolation until 1974. This was achieved by utilising a cascade transition within mercury atoms. Individual atoms emit two photons at different frequencies in the cascade transition and by spectrally filtering the light the observation of one photon can be used to 'herald' the other. The observation of these single photons was characterised by its anticorrelation on the two output ports of a beamsplitter in a similar manner to the famous Hanbury Brown and Twiss experiment of 1956.
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