FWIW...
Although the concept of a single photon was proposed by Planck as early as 1900,[1] a true single-photon source was not created in isolation until 1974.
This was achieved by utilising a cascade transition within calcium atoms.[2] 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.[3]
Another single-photon source came in 1977 which utilised the fluorescence from an attenuated beam of sodium atoms.[4] A beam of sodium atoms was attenuated so that no more than one or two atoms contributed to the observed fluorescence radiation at any one time.
In this way, only single emitters were producing light and the observed fluorescence showed the characteristic antibunching. The isolation of individual atoms continued with ion traps in the mid-1980s.
A single ion could be held in a radio frequency Paul trap for an extended period of time (10 min) thus acting as a single emitter of multiple single photons as in the experiments of Diedrich and Walther.[5]
At the same time the nonlinear process of parametric down conversion began to be utilised and from then until the present day it has become the workhorse of experiments requiring single photons.
Advances in microscopy led to the isolation of single molecules in the end of the 1980s.[6] Subsequently, single pentacene molecules were detected in p-terphenyl crystals.[7] The single molecules have begun to be utilised as single-photon sources.[8]
Within the 21st century defect centres in various solid state materials have emerged,[9] most notably diamond, silicon carbide [10] and boron nitride.[11] the most studied defect is the nitrogen vacancy (NV) centers in diamond that was utilised as a source of single photons.[12]
These sources along with molecules can use the strong confinement of light (mirrors, microresonators, optical fibres, waveguides, etc.) to enhance the emission of the NV centres.
As well as NV centres and molecules, quantum dots (QDs) can emit single photons and can be constructed from the same semiconductor materials as the light-confining structures.[13]
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