The Heisenberg uncertainty principle dictates that a state with an exact number of photons of a single frequency cannot be created.
Although the concept of a single photon was proposed by Planck as early as 1900,[2] a true single-photon source was not created in isolation until 1974.
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.
[6] 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.
[9] Within the 21st century defect centres in various solid state materials have emerged,[10] most notably diamond, silicon carbide[11][12] and boron nitride.
[13] the most studied defect is the nitrogen vacancy (NV) centers in diamond that was utilised as a source of single photons.
[14] These sources along with molecules can use the strong confinement of light (mirrors, microresonators, optical fibres, waveguides, etc.)
As well as NV centres and molecules, quantum dots (QDs),[15] quantum dots trapped in optical antenna,[16] functionalized carbon nanotubes,[17][18] and two-dimensional materials[19][20][21][22][23][24][25] can also emit single photons and can be constructed from the same semiconductor materials as the light-confining structures.
It is noted that the single photon sources at telecom wavelength of 1,550 nm are very important in fiber-optic communication and they are mostly indium arsenide QDs.
However, in quantum optics, single-photon states also refer to mathematical superpositions of single-frequency (monochromatic) radiation modes.
[31] This definition is general enough to include photon wave-packets, i.e., states of radiation that are localized to some extent in space and time.
[33] Currently, there are many active nanomaterials engineered into single quantum emitters where their spontaneous emission could be tuned by changing the local density of optical states in dielectric nanostructures.
The dielectric nanostructures are usually designed within the heterostructures to enhance the light-matter interaction, and thus further improve the efficiency of these single photon sources.
Nowadays the most common sources of single photons[citation needed] are single molecules, Rydberg atoms,[37][dubious – discuss] diamond colour centres and quantum dots, with the last being widely studied[citation needed]} with efforts from many research groups to realize quantum dots that fluoresce single photons at room temperature with photons in the low loss window of fiber-optic communication.
One of the first and easiest sources was created by attenuating a conventional laser beam to reduce its intensity and thereby the mean photon number per pulse.
The two photons need not generally be the same wavelength, but the total energy and resulting polarisation are defined by the generation process.