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.
History 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. Another single-photon source came in 1977 which used 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 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. 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), quantum dots trapped in optical antenna, functionalized carbon nanotubes, and two-dimensional materials 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, by creating downconversion quantum interface from visible single photon sources, one still can create single photon at 1,550 nm with preserved antibunching. Exciting atoms and excitons to highly interacting Rydberg levels prevents more than one excitation over the so-called blockade volume. Hence Rydberg excitation in a small atomic ensembles or crystals could act as a single photon emitters.
Definition In quantum theory, photons describe quantized electromagnetic radiation. Specifically, a photon is an elementary excitation of a normal mode of the electromagnetic field. Thus a single-photon state is the quantum state of a radiation mode that contains a single excitation. This type of state is completely delocalized. Single radiation modes are labelled by, among other quantities, the frequency of the electromagnetic radiation that they describe. However, in quantum optics, single-photon states also refer to mathematical superpositions of single-frequency (monochromatic) radiation modes. 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. Single-photon sources generate single-photon states as described above. In other words, ideal single-photon sources generate radiation with a photon-number distribution that has a mean one and variance zero.
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