The nature of light has always attracted the interest of human kind. While a great number of theories were formulated in the past, a huge step towards its comprehension followed Plank’s theory of the black body spectrum in the first years of the 20th century. The quantization of the electromagnetic field in a single harmonic oscillator opened up the doors for the formulation of quantum mechanics and a deeper understanding of light since the times of Newton. Now, light may be described uniquely either as a wave or as a particle, but is composed of wavepackets of energy, that possess the particle-wave duality of quantum mechanics, called photons.
By considering the photon emission statistic of common light sources, i.e. the number of photons that can be detected in a definite time interval, it is interesting to notice that is quite hard to detect single photon emission in nature, in fact such emitters are exclusively a trait of quantum systems.
The reason is that thermal light sources, such as common lamps and the sun, are an ensemble of independent atoms that emit photons at random times, with huge fluctuation in the overall intensity. Due to the bosonic nature of photons, the emission statistic for each mode follows the Bose-Einstein distribution, and the most populated state on average is that of 0 photons, so that no detection for small time interval occurs. Even coherent light, like lasers, that has reduced fluctuation in intensity, shows a Poissonian emission statistic. Hence, no classical system can reproduce the sub-Poissonian emission statistics of a single photon sources, which would ideally always give a single value peaked on 1 photon, as shown in the image below.
As stated before, only a two-level quantum system can exhibit a single photon emission. This is due to the fact that, in order to have the emission of a photon from an atom, an electron needs to acquire some energy to jump from the ground state of the system to an excited state; the consequent decay back to the lower energy state is accompanied by the emission of a photon, with energy equal to the difference of the two states. Now that the electron is back again on his fundamental state, we need to excite it again before the subsequent photon emission can be detected. This will induce a measurable dead time between subsequent emissions, proving the single photon emission.
The reason why a lot of interest is given to these emitters is that the possibility of harnessing a single photon state, creating states of light with a huge reduction in noise and a higher degree of coherence, would bring advances in many fields. Due to the fact that information can be encoded in the physical properties of the photon, quantum information processing could rely of these systems like a classical computer relies on transistor to process information. Moreover, advances in secure communication through Quantum Key Distribution protocols, along with an increase in sensing and measurements sensibility, could be achieved.
Interestingly, every atom around us can be seen as a two-level quantum mechanical system, to achieve the control of a single photon emission from one of them we would simply need to isolate that atom from any kind of disturbance and also being able to control their emission properties. While the idea of manipulating such a tiny piece of matter would seem impossible to achieve, the first realisation of single photon states was obtained already in the late 1970s using of attenuated laser beams. Only after the year 2000, new and reliable emitters were developed in solid state systems, such as self-assembled quantum dots (QDs), that acquire an atomic electronic structure due to the three-dimensional confinement, and defects in large band gap crystals .
Since then a multitude of researchers have undertaken studies on various systems in order to pursue a pure single photon emitter and many members of our Spin-NANO community are actively working on solid state emitters like QDs and NV vacancies in diamonds. Some of them, including me, are researching newly discovered solid state single photon emitters in two-dimensional materials, like Transition Metal Dichalcogenides and hexagonal Boron Nitride. Although the first reports of these emitters happened only in the last two years, and they still lack a solid understanding of their nature, the appealing properties of two-dimensional materials combined with the desired qualities of single photon sources hold much promise for future technologies.
By Luca Sortino, PhD Student of the Low Dimensional Structures and Devices Group at Univeristy of Sheffield, UK.
 Aharonovich, I., Englund, D. & Toth, M. Solid-state single-photon emitters. Nat. Photonics 10, 631–641 (2016) doi:10.1038/nphoton.2016.186