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Single-photon sources are light sources that emit light as single particles or photons. They are distinct from coherent light sources (lasers) and thermal light sources (such as incandescent light bulbs and mercury-vapor lamps). 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 anti-bunching, so that the time between two successive photons is never less than some minimum value. This 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 no. 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 calcium 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.
Another single-photon source came in 1977 which utilised the fluorescence from an attenuated beam of sodium atoms. 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 emitter 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. 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. Subsequently, single pentacene molecules were detected in p-terphenyl crystals. The single molecules have begun to be utilised as single-photon sources.
Within the 21st century defect centres in various solid state materials have emerged, most notably diamond, silicon carbide and boron nitride . the most studied defect is the nitrogen vacancy (NV) centers in diamond that was utilised as a source of single photons. 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.
The generation of a single photon occurs when a source creates only one photon within its fluorescence lifetime after being optically or electrically excited. An ideal single-photon source has yet to be created. The requirements are that it should be on-demand, efficient, robust and easy to implement. Given that the main applications for a high-quality single-photon source are quantum key distribution, quantum repeaters and quantum information science, the photons generated should also have a wavelength that would give low loss and attenuation when travelling through an optical fiber. Nowadays the most common sources of single photons are single molecules, diamond colour centres and quantum dots, with the last being widely studied 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. For many purposes single photons need to be anti-bunched, and this can be verified
Heralded single photons
Pairs of single photons can be generated in highly correlated states from using a single a high energy photon to create two lower energy ones. One photon from the resulting pair may be detected to 'herald' the other (so it's state is pretty well known prior to detection). The two photons need not generally be the same wavelength, but the total energy and resulting polarisation are defined by the generation process. One area of keen interest for such pairs of photons is QKD.
The heralded single photon sources are also used to examine the fundamental physics laws in quantum mechanics. There are two commonly used types of heralded single photon sources, Spontaneous parametric down-conversion and Spontaneous four-wave mixing. The first source has line-width around THz and the second one has line-width around MHz or narrower. The heralded single photon has been used to demonstrate photonics storage and loading to the optical cavity.
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