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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. 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 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 number 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.

History

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 mercury 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]^[11] and boron nitride.^[12] the most studied defect is the nitrogen vacancy (NV) centers in diamond that was utilised as a source of single photons.^[13] 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),^[14] functionalized carbon nanotubes,^[15]^[16] and two-dimensional materials^[17]^[18]^[19]^[20]^[21]^[22]^[23] 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 at telecom wavelength of 1,550 nm is very important in fiber telecommunication applications and they are hitherto not so many.^[24] ^[25]

Exciting atoms and excitons to highly interacting Rydberg levels prevents more than one excitation over the so called blockade volume. Making small ensembles and crystals could act as a single photon emitters.^[26]^[27]

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.

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.^[28] 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.^[29]

Characteristics

An ideal-single photon source produces single-photon states with 100% probability and optical vacuum or multi-photon states with 0% probability. Desirable properties of real-world single-photon sources include efficiency, robustness, ease of implementation and on-demand nature, i.e., generating single-photons at arbitrarily chosen times. Single-photon sources including single emitters such as single atoms, ions and molecules, and including solid-state emitters such as quantum dots, color centers and carbon nanotubes are on-demand.^[30] 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.^[31]^[32] Another type of source comprises non-deterministic sources, i.e., not on demand, and these include examples such as weak lasers, atomic cascades and parametric down-conversion.

The single-photon nature of a source can be quantized using the second-order correlation function $g^{(2)}(\tau )$ . Ideal single-photon sources show $g^{(2)}(0)=0$ and good single-photon sources have small $g^{(2)}(0)$ . The second-order correlation function can be measured using the Hanbury-Brown–Twiss effect.

Types

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. Given that the main applications for a high-quality single-photon source are quantum key distribution, quantum repeaters^[33] 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, Rydberg atoms,^[34]^[35] 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.

Faint laser

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.^[36] Since the photon statistics follow a Poisson distribution one can achieve sources with a well defined probability ratio for the emission of one versus two or more photons. For example a mean value of μ = 0.1 leads to a probability of 90% for zero photons, 9% for one photon and 1% for more than one photon.^[37]

Although such a source can be used for certain applications, it has a second-order intensity correlation function equal to one (no antibunching). For many applications however, antibunching is required, for instance in quantum cryptography.

Heralded single photons

Pairs of single photons can be generated in highly correlated states from using a single high-energy photon to create two lower-energy ones. One photon from the resulting pair may be detected to 'herald' the other (so its 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.

References

^ Planck, M. (1900). "Über eine Verbesserung der Wienschen Spektralgleichung". Verhandlungen der Deutschen Physikalischen Gesellschaft. 2: 202–204. {{cite journal}}: Invalid |ref=harv (help)
^ Clauser, John F. (1974). "Experimental distinction between the quantum and classical field-theoretic predictions for the photoelectric effect". Phys. Rev. D. 9 (4): 853–860. Bibcode:1974PhRvD...9..853C. doi:10.1103/physrevd.9.853. {{cite journal}}: Invalid |ref=harv (help)
^ Hanbury Brown, R.; Twiss, R. Q. (1956). "A test of a new type of stellar interferometer on sirius". Nature. 175 (4541): 1046–1048. Bibcode:1956Natur.178.1046H. doi:10.1038/1781046a0.
^ Kimble, H. J.; Dagenais, M.; Mandel, L. (1977). "Photon Antibunching in Resonance Fluorescence" (PDF). Phys. Rev. Lett. 39 (11): 691–695. Bibcode:1977PhRvL..39..691K. doi:10.1103/physrevlett.39.691.
^ Diedrich, Frank; Walther, Herbert (1987). "Nonclassical Radiation of a Single Stored Ion". Phys. Rev. Lett. 58 (3): 203–206. Bibcode:1987PhRvL..58..203D. doi:10.1103/physrevlett.58.203. PMID 10034869.
^ Moerner, W. E.; Kador, L. (22 May 1989). "Optical detection and spectroscopy of single molecules in a solid". Physical Review Letters. 62 (21): 2535–2538. Bibcode:1989PhRvL..62.2535M. doi:10.1103/PhysRevLett.62.2535. PMID 10040013.
^ Orrit, M.; Bernard, J. (1990). "Single Pentacene Molecules Detected by Fluorescence Excitation in a p-Terphenyl Crystal". Phys. Rev. Lett. 65 (21): 2716–2719. Bibcode:1990PhRvL..65.2716O. doi:10.1103/physrevlett.65.2716. PMID 10042674.
^ Basché, T.; Moerner, W.E.; Orrit, M.; Talon, H. (1992). "Photon antibunching in the fluorescence of a single dye molecule trapped in a solid". Phys. Rev. Lett. 69 (10): 1516–1519. Bibcode:1992PhRvL..69.1516B. doi:10.1103/PhysRevLett.69.1516. PMID 10046242.
^ Aharonovich, Igor; Englund, Dirk; Toth, Milos (2016). "Solid-state single-photon emitters". Nature Photonics. 10 (10): 631–641. Bibcode:2016NaPho..10..631A. doi:10.1038/nphoton.2016.186.
^ Castelletto, S.; Johnson, B. C.; Ivády, V.; Stavrias, N.; Umeda, T.; Gali, A.; Ohshima, T. (February 2014). "A silicon carbide room-temperature single-photon source". Nature Materials. 13 (2): 151–156. doi:10.1038/nmat3806. ISSN 1476-1122. PMID 24240243.
^ Lohrmann, A.; Castelletto, S.; Klein, J. R.; Ohshima, T.; Bosi, M.; Negri, M.; Lau, D. W. M.; Gibson, B. C.; Prawer, S.; McCallum, J. C.; Johnson, B. C. (2016). "Activation and control of visible single defects in 4H-, 6H-, and 3C-SiC by oxidation". Applied Physics Letters. 108 (2): 021107. Bibcode:2016ApPhL.108b1107L. doi:10.1063/1.4939906.
^ Tran, Toan Trong; Bray, Kerem; Ford, Michael J.; Toth, Milos; Aharonovich, Igor (2016). "Quantum emission from hexagonal boron nitride monolayers". Nature Nanotechnology. 11 (1): 37–41. arXiv:1504.06521. Bibcode:2016NatNa..11...37T. doi:10.1038/nnano.2015.242.
^ Kurtsiefer, Christian; Mayer, Sonja; Zarda, Patrick; Weinfurter, Harald (2000). "Stable Solid-State Source of Single Photons". Phys. Rev. Lett. 85 (2): 290–293. Bibcode:2000PhRvL..85..290K. doi:10.1103/physrevlett.85.290.
^ Michler, P.; Kiraz, A.; Becher, C.; Schoenfeld, W. V.; Petroff, P. M.; Zhang, Lidong; Imamoglu, A. (200). "A Quantum Dot Single-Photon Turnstile Device". Science. 290 (5500): 2282–2285. doi:10.1126/science.290.5500.2282. PMID 11125136.
^ Htoon, Han; Doorn, Stephen K.; Baldwin, Jon K. S.; Hartmann, Nicolai F.; Ma, Xuedan (August 2015). "Room-temperature single-photon generation from solitary dopants of carbon nanotubes". Nature Nanotechnology. 10 (8): 671–675. Bibcode:2015NatNa..10..671M. doi:10.1038/nnano.2015.136. ISSN 1748-3395. PMID 26167766.
^ He, Xiaowei; Hartmann, Nicolai F.; Ma, Xuedan; Kim, Younghee; Ihly, Rachelle; Blackburn, Jeffrey L.; Gao, Weilu; Kono, Junichiro; Yomogida, Yohei (September 2017). "Tunable room-temperature single-photon emission at telecom wavelengths from sp3 defects in carbon nanotubes". Nature Photonics. 11 (9): 577–582. doi:10.1038/nphoton.2017.119. ISSN 1749-4885.
^ Tonndorf, Philipp; Schmidt, Robert; Schneider, Robert; Kern, Johannes; Buscema, Michele; Steele, Gary A.; Castellanos-Gomez, Andres; van der Zant, Herre S. J.; Michaelis de Vasconcellos, Steffen (2015-04-20). "Single-photon emission from localized excitons in an atomically thin semiconductor". Optica. 2 (4): 347. doi:10.1364/OPTICA.2.000347. ISSN 2334-2536.
^ Chakraborty, Chitraleema; Kinnischtzke, Laura; Goodfellow, Kenneth M.; Beams, Ryan; Vamivakas, A. Nick (June 2015). "Voltage-controlled quantum light from an atomically thin semiconductor". Nature Nanotechnology. 10 (6): 507–511. Bibcode:2015NatNa..10..507C. doi:10.1038/nnano.2015.79. ISSN 1748-3387. PMID 25938569.
^ Palacios-Berraquero, Carmen; Barbone, Matteo; Kara, Dhiren M.; Chen, Xiaolong; Goykhman, Ilya; Yoon, Duhee; Ott, Anna K.; Beitner, Jan; Watanabe, Kenji (December 2016). "Atomically thin quantum light-emitting diodes". Nature Communications. 7 (1): 12978. arXiv:1603.08795. Bibcode:2016NatCo...712978P. doi:10.1038/ncomms12978. ISSN 2041-1723. PMC 5052681. PMID 27667022.
^ Palacios-Berraquero, Carmen; Kara, Dhiren M.; Montblanch, Alejandro R.-P.; Barbone, Matteo; Latawiec, Pawel; Yoon, Duhee; Ott, Anna K.; Loncar, Marko; Ferrari, Andrea C. (August 2017). "Large-scale quantum-emitter arrays in atomically thin semiconductors". Nature Communications. 8 (1): 15093. arXiv:1609.04244. Bibcode:2017NatCo...815093P. doi:10.1038/ncomms15093. ISSN 2041-1723. PMC 5458119. PMID 28530249.
^ Branny, Artur; Kumar, Santosh; Proux, Raphaël; Gerardot, Brian D (August 2017). "Deterministic strain-induced arrays of quantum emitters in a two-dimensional semiconductor". Nature Communications. 8 (1): 15053. arXiv:1610.01406. Bibcode:2017NatCo...815053B. doi:10.1038/ncomms15053. ISSN 2041-1723. PMC 5458118. PMID 28530219.
^ Wu, Wei; Dass, Chandriker K.; Hendrickson, Joshua R.; Montaño, Raul D.; Fischer, Robert E.; Zhang, Xiaotian; Choudhury, Tanushree H.; Redwing, Joan M.; Wang, Yongqiang (2019-05-27). "Locally defined quantum emission from epitaxial few-layer tungsten diselenide". Applied Physics Letters. 114 (21): 213102. Bibcode:2019ApPhL.114u3102W. doi:10.1063/1.5091779. ISSN 0003-6951.
^ He, Yu-Ming; Clark, Genevieve; Schaibley, John R.; He, Yu; Chen, Ming-Cheng; Wei, Yu-Jia; Ding, Xing; Zhang, Qiang; Yao, Wang (June 2015). "Single quantum emitters in monolayer semiconductors". Nature Nanotechnology. 10 (6): 497–502. arXiv:1411.2449. Bibcode:2015NatNa..10..497H. doi:10.1038/nnano.2015.75. ISSN 1748-3387. PMID 25938571.
^ Birowosuto, M. D.; Sumikura, H.; Matsuo, S.; Taniyama, H.; Veldhoven, P.J.; Notzel, R.; Notomi, M. (2012). "Fast Purcell-enhanced single photon source in 1,550-nm telecom band from a resonant quantum dot-cavity coupling". Sci. Rep. 2: 321. arXiv:1203.6171. doi:10.1038/srep00321. PMID 22432053.
^ Muller, T.; Skiba-Szymanska, J.; Krysa, A.B.; Huwer, J.; Felle, M.; Anderson, M.; Stevenson, R.M.; Heffernan, J.; Ritchie, A.J.; Shields (2018). "A quantum light-emitting diode for the standard telecom window around 1,550 nm". Nat. Comm. 9: 862. doi:10.1038/s41467-018-03251-7. {{cite journal}}: Unknown parameter |arXiv= ignored (|arxiv= suggested) (help); Unknown parameter |pmcid= ignored (|pmc= suggested) (help)
^ Dudin, Y. O.; Kuzmich, A. (2012-05-18). "Strongly Interacting Rydberg Excitations of a Cold Atomic Gas". Science. 336 (6083): 887–889. doi:10.1126/science.1217901. ISSN 0036-8075. PMID 22517325.
^ Ripka, Fabian; Kübler, Harald; Löw, Robert; Pfau, Tilman (2018-10-26). "A room-temperature single-photon source based on strongly interacting Rydberg atoms". Science. 362 (6413): 446–449. arXiv:1806.02120. doi:10.1126/science.aau1949. ISSN 0036-8075. PMID 30361371.
^ Scully, Marlan O. (1997). Quantum optics. Zubairy, Muhammad Suhail, 1952-. Cambridge: Cambridge University Press. ISBN 9780521435956. OCLC 817937365.
^ Eisaman, M. D.; Fan, J.; Migdall, A.; Polyakov, S. V. (2011-07-01). "Invited Review Article: Single-photon sources and detectors". Review of Scientific Instruments. 82 (7): 071101–071101–25. Bibcode:2011RScI...82g1101E. doi:10.1063/1.3610677. ISSN 0034-6748. PMID 21806165.
^ Eisaman, M. D.; Fan, J.; Migdall, A.; Polyakov, S. V. (2011-07-01). "Invited Review Article: Single-photon sources and detectors". Review of Scientific Instruments. 82 (7): 071101–071101–25. Bibcode:2011RScI...82g1101E. doi:10.1063/1.3610677. ISSN 0034-6748. PMID 21806165.
^ Birowosuto, M.; et al. (2014). "Movable high-Q nanoresonators realized by semiconductor nanowires on a Si photonic crystal platform". Nature Materials. 13 (3): 279–285. arXiv:1403.4237. Bibcode:2014NatMa..13..279B. doi:10.1038/nmat3873. PMID 24553654.
^ Diguna, L., Birowosuto, M; et al. (2018). "Light–matter interaction of single quantum emitters with dielectric nanostructures". Photonics. 5 (2): 14. doi:10.3390/photonics5020014.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)
^ Meter, R.V.; Touch, J. (2013). "Designing quantum repeater networks". IEEE Communications Magazine. 51 (8): 64–71. doi:10.1109/mcom.2013.6576340.
^ Dudin, Y. O.; Kuzmich, A. (2012-04-19). "Strongly Interacting Rydberg Excitations of a Cold Atomic Gas". Science. 336 (6083): 887–889. doi:10.1126/science.1217901. ISSN 0036-8075. PMID 22517325.
^ Ripka, Fabian; Kübler, Harald; Löw, Robert; Pfau, Tilman (2018-10-25). "A room-temperature single-photon source based on strongly interacting Rydberg atoms". Science. 362 (6413): 446–449. arXiv:1806.02120. doi:10.1126/science.aau1949. ISSN 0036-8075. PMID 30361371.
^ Eisaman, M. D.; Fan, J.; Migdall, A.; Polyakov, S. V. (2011-07-01). "Invited Review Article: Single-photon sources and detectors". Review of Scientific Instruments. 82 (7): 071101–071101–25. Bibcode:2011RScI...82g1101E. doi:10.1063/1.3610677. ISSN 0034-6748. PMID 21806165.
^ Al-Kathiri, S.; Al-Khateeb, W.; Hafizulfika, M.; Wahiddin, M. R.; Saharudin, S. (May 2008). "Characterization of mean photon number for key distribution system using faint laser". 2008 International Conference on Computer and Communication Engineering: 1237–1242. doi:10.1109/ICCCE.2008.4580803. ISBN 978-1-4244-1691-2.

Bibliography

R. Loudon, The Quantum Theory of Light,:Oxford University Press, 3rd edition (2000).
Planck, M. (1900). "Über eine Verbesserung der Wienschen Spektralgleichung". Verhandlungen der Deutschen Physikalischen Gesellschaft. 2: 202–204. {{cite journal}}: Invalid |ref=harv (help) Translated in ter Haar, D. (1967). "On an Improvement of Wien's Equation for the Spectrum" (PDF). The Old Quantum Theory. Pergamon Press. pp. 79–81. LCCN 66029628.

[Planck00-1] Planck, M. (1900). "Über eine Verbesserung der Wienschen Spektralgleichung". Verhandlungen der Deutschen Physikalischen Gesellschaft. 2: 202–204. {{cite journal}}: Invalid |ref=harv (help)

[Clauser74-2] Clauser, John F. (1974). "Experimental distinction between the quantum and classical field-theoretic predictions for the photoelectric effect". Phys. Rev. D. 9 (4): 853–860. Bibcode:1974PhRvD...9..853C. doi:10.1103/physrevd.9.853. {{cite journal}}: Invalid |ref=harv (help)

[HanburyBrown56-3] Hanbury Brown, R.; Twiss, R. Q. (1956). "A test of a new type of stellar interferometer on sirius". Nature. 175 (4541): 1046–1048. Bibcode:1956Natur.178.1046H. doi:10.1038/1781046a0.

[Kimble77-4] Kimble, H. J.; Dagenais, M.; Mandel, L. (1977). "Photon Antibunching in Resonance Fluorescence" (PDF). Phys. Rev. Lett. 39 (11): 691–695. Bibcode:1977PhRvL..39..691K. doi:10.1103/physrevlett.39.691.

[Deidrich87-5] Diedrich, Frank; Walther, Herbert (1987). "Nonclassical Radiation of a Single Stored Ion". Phys. Rev. Lett. 58 (3): 203–206. Bibcode:1987PhRvL..58..203D. doi:10.1103/physrevlett.58.203. PMID 10034869.

[6] Moerner, W. E.; Kador, L. (22 May 1989). "Optical detection and spectroscopy of single molecules in a solid". Physical Review Letters. 62 (21): 2535–2538. Bibcode:1989PhRvL..62.2535M. doi:10.1103/PhysRevLett.62.2535. PMID 10040013.

[Orrit90-7] Orrit, M.; Bernard, J. (1990). "Single Pentacene Molecules Detected by Fluorescence Excitation in a p-Terphenyl Crystal". Phys. Rev. Lett. 65 (21): 2716–2719. Bibcode:1990PhRvL..65.2716O. doi:10.1103/physrevlett.65.2716. PMID 10042674.

[Basche-8] Basché, T.; Moerner, W.E.; Orrit, M.; Talon, H. (1992). "Photon antibunching in the fluorescence of a single dye molecule trapped in a solid". Phys. Rev. Lett. 69 (10): 1516–1519. Bibcode:1992PhRvL..69.1516B. doi:10.1103/PhysRevLett.69.1516. PMID 10046242.

[9] Aharonovich, Igor; Englund, Dirk; Toth, Milos (2016). "Solid-state single-photon emitters". Nature Photonics. 10 (10): 631–641. Bibcode:2016NaPho..10..631A. doi:10.1038/nphoton.2016.186.

[10] Castelletto, S.; Johnson, B. C.; Ivády, V.; Stavrias, N.; Umeda, T.; Gali, A.; Ohshima, T. (February 2014). "A silicon carbide room-temperature single-photon source". Nature Materials. 13 (2): 151–156. doi:10.1038/nmat3806. ISSN 1476-1122. PMID 24240243.

[11] Lohrmann, A.; Castelletto, S.; Klein, J. R.; Ohshima, T.; Bosi, M.; Negri, M.; Lau, D. W. M.; Gibson, B. C.; Prawer, S.; McCallum, J. C.; Johnson, B. C. (2016). "Activation and control of visible single defects in 4H-, 6H-, and 3C-SiC by oxidation". Applied Physics Letters. 108 (2): 021107. Bibcode:2016ApPhL.108b1107L. doi:10.1063/1.4939906.

[12] Tran, Toan Trong; Bray, Kerem; Ford, Michael J.; Toth, Milos; Aharonovich, Igor (2016). "Quantum emission from hexagonal boron nitride monolayers". Nature Nanotechnology. 11 (1): 37–41. arXiv:1504.06521. Bibcode:2016NatNa..11...37T. doi:10.1038/nnano.2015.242.

[Kurtsiefer00-13] Kurtsiefer, Christian; Mayer, Sonja; Zarda, Patrick; Weinfurter, Harald (2000). "Stable Solid-State Source of Single Photons". Phys. Rev. Lett. 85 (2): 290–293. Bibcode:2000PhRvL..85..290K. doi:10.1103/physrevlett.85.290.

[Michler00-14] Michler, P.; Kiraz, A.; Becher, C.; Schoenfeld, W. V.; Petroff, P. M.; Zhang, Lidong; Imamoglu, A. (200). "A Quantum Dot Single-Photon Turnstile Device". Science. 290 (5500): 2282–2285. doi:10.1126/science.290.5500.2282. PMID 11125136.

[15] Htoon, Han; Doorn, Stephen K.; Baldwin, Jon K. S.; Hartmann, Nicolai F.; Ma, Xuedan (August 2015). "Room-temperature single-photon generation from solitary dopants of carbon nanotubes". Nature Nanotechnology. 10 (8): 671–675. Bibcode:2015NatNa..10..671M. doi:10.1038/nnano.2015.136. ISSN 1748-3395. PMID 26167766.

[16] He, Xiaowei; Hartmann, Nicolai F.; Ma, Xuedan; Kim, Younghee; Ihly, Rachelle; Blackburn, Jeffrey L.; Gao, Weilu; Kono, Junichiro; Yomogida, Yohei (September 2017). "Tunable room-temperature single-photon emission at telecom wavelengths from sp3 defects in carbon nanotubes". Nature Photonics. 11 (9): 577–582. doi:10.1038/nphoton.2017.119. ISSN 1749-4885.

[17] Tonndorf, Philipp; Schmidt, Robert; Schneider, Robert; Kern, Johannes; Buscema, Michele; Steele, Gary A.; Castellanos-Gomez, Andres; van der Zant, Herre S. J.; Michaelis de Vasconcellos, Steffen (2015-04-20). "Single-photon emission from localized excitons in an atomically thin semiconductor". Optica. 2 (4): 347. doi:10.1364/OPTICA.2.000347. ISSN 2334-2536.

[18] Chakraborty, Chitraleema; Kinnischtzke, Laura; Goodfellow, Kenneth M.; Beams, Ryan; Vamivakas, A. Nick (June 2015). "Voltage-controlled quantum light from an atomically thin semiconductor". Nature Nanotechnology. 10 (6): 507–511. Bibcode:2015NatNa..10..507C. doi:10.1038/nnano.2015.79. ISSN 1748-3387. PMID 25938569.

[19] Palacios-Berraquero, Carmen; Barbone, Matteo; Kara, Dhiren M.; Chen, Xiaolong; Goykhman, Ilya; Yoon, Duhee; Ott, Anna K.; Beitner, Jan; Watanabe, Kenji (December 2016). "Atomically thin quantum light-emitting diodes". Nature Communications. 7 (1): 12978. arXiv:1603.08795. Bibcode:2016NatCo...712978P. doi:10.1038/ncomms12978. ISSN 2041-1723. PMC 5052681. PMID 27667022.

[20] Palacios-Berraquero, Carmen; Kara, Dhiren M.; Montblanch, Alejandro R.-P.; Barbone, Matteo; Latawiec, Pawel; Yoon, Duhee; Ott, Anna K.; Loncar, Marko; Ferrari, Andrea C. (August 2017). "Large-scale quantum-emitter arrays in atomically thin semiconductors". Nature Communications. 8 (1): 15093. arXiv:1609.04244. Bibcode:2017NatCo...815093P. doi:10.1038/ncomms15093. ISSN 2041-1723. PMC 5458119. PMID 28530249.

[21] Branny, Artur; Kumar, Santosh; Proux, Raphaël; Gerardot, Brian D (August 2017). "Deterministic strain-induced arrays of quantum emitters in a two-dimensional semiconductor". Nature Communications. 8 (1): 15053. arXiv:1610.01406. Bibcode:2017NatCo...815053B. doi:10.1038/ncomms15053. ISSN 2041-1723. PMC 5458118. PMID 28530219.

[22] Wu, Wei; Dass, Chandriker K.; Hendrickson, Joshua R.; Montaño, Raul D.; Fischer, Robert E.; Zhang, Xiaotian; Choudhury, Tanushree H.; Redwing, Joan M.; Wang, Yongqiang (2019-05-27). "Locally defined quantum emission from epitaxial few-layer tungsten diselenide". Applied Physics Letters. 114 (21): 213102. Bibcode:2019ApPhL.114u3102W. doi:10.1063/1.5091779. ISSN 0003-6951.

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@@ Line 9: / Line 9: @@
 Advances in microscopy led to the isolation of single molecules in the end of the 1980s.<ref>{{cite journal|last1=Moerner|first1=W. E.|last2=Kador|first2=L.|title=Optical detection and spectroscopy of single molecules in a solid|journal=Physical Review Letters|date=22 May 1989|volume=62|issue=21|pages=2535–2538|doi=10.1103/PhysRevLett.62.2535|pmid=10040013|bibcode=1989PhRvL..62.2535M}}</ref> Subsequently, single [[pentacene]] molecules were detected in ''p''-[[terphenyl]] crystals.<ref name=Orrit90>{{cite journal |last1=Orrit |first1=M. |last2=Bernard |first2=J. |year=1990 |title=Single Pentacene Molecules Detected by Fluorescence Excitation in a ''p''-Terphenyl Crystal |journal=Phys. Rev. Lett. |volume=65 |number=21 |pages=2716–2719 |doi=10.1103/physrevlett.65.2716|pmid=10042674 |bibcode=1990PhRvL..65.2716O }}</ref> The single molecules have begun to be utilised as single-photon sources.<ref name=Basche>{{cite journal | last1=Basché | first1=T. | last2=Moerner | first2=W.E. | last3=Orrit | first3=M. | last4=Talon | first4=H. | year=1992 | title=Photon antibunching in the fluorescence of a single dye molecule trapped in a solid |journal=Phys. Rev. Lett. |volume= 69 | number=10 | pages=1516–1519 | doi=10.1103/PhysRevLett.69.1516 | pmid=10046242| url=http://www.dtic.mil/get-tr-doc/pdf?AD=ADA252261 | bibcode=1992PhRvL..69.1516B }}</ref>
-Within the 21st century defect centres in various solid state materials have emerged,<ref>{{Cite journal |doi = 10.1038/nphoton.2016.186|title = Solid-state single-photon emitters|journal = Nature Photonics|volume = 10|issue = 10|pages = 631–641|year = 2016|last1 = Aharonovich|first1 = Igor|last2 = Englund|first2 = Dirk|last3 = Toth|first3 = Milos|bibcode = 2016NaPho..10..631A}}</ref> most notably diamond, silicon carbide<ref>{{Cite journal|last=Castelletto|first=S.|last2=Johnson|first2=B. C.|last3=Ivády|first3=V.|last4=Stavrias|first4=N.|last5=Umeda|first5=T.|last6=Gali|first6=A.|last7=Ohshima|first7=T.|date=February 2014|title=A silicon carbide room-temperature single-photon source|journal=Nature Materials|volume=13|issue=2|pages=151–156|doi=10.1038/nmat3806|pmid=24240243|issn=1476-1122}}</ref><ref>{{Cite journal |doi = 10.1063/1.4939906|title = Activation and control of visible single defects in 4H-, 6H-, and 3C-SiC by oxidation|journal = Applied Physics Letters|volume = 108|issue = 2|pages = 021107|year = 2016|last1 = Lohrmann|first1 = A.|last2 = Castelletto|first2 = S.|last3 = Klein|first3 = J. R.|last4 = Ohshima|first4 = T.|last5 = Bosi|first5 = M.|last6 = Negri|first6 = M.|last7 = Lau|first7 = D. W. M.|last8 = Gibson|first8 = B. C.|last9 = Prawer|first9 = S.|last10 = McCallum|first10 = J. C.|last11 = Johnson|first11 = B. C.|bibcode = 2016ApPhL.108b1107L}}</ref> and boron nitride.<ref>{{Cite journal | doi=10.1038/nnano.2015.242| title=Quantum emission from hexagonal boron nitride monolayers| journal=Nature Nanotechnology| volume=11| issue=1| pages=37–41| year=2016| last1=Tran| first1=Toan Trong| last2=Bray| first2=Kerem| last3=Ford| first3=Michael J.| last4=Toth| first4=Milos| last5=Aharonovich| first5=Igor| bibcode=2016NatNa..11...37T| arxiv=1504.06521}}</ref> the most studied defect is the nitrogen vacancy (NV) centers in diamond that was utilised as a source of single photons.<ref name="Kurtsiefer00">{{cite journal |last1=Kurtsiefer |first1=Christian |last2=Mayer |first2=Sonja |last3=Zarda |first3=Patrick |last4=Weinfurter |first4=Harald |year=2000 |title=Stable Solid-State Source of Single Photons |journal=Phys. Rev. Lett. |volume=85 |number=2| pages=290–293 |doi=10.1103/physrevlett.85.290|bibcode=2000PhRvL..85..290K }}</ref> 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),<ref name="Michler00">{{cite journal|last1=Michler|first1=P.|last2=Kiraz|first2=A.|last3=Becher|first3=C.|last4=Schoenfeld|first4=W. V.|last5=Petroff|first5=P. M.|last6=Zhang|first6=Lidong|last7=Imamoglu|first7=A.|year=200|title=A Quantum Dot Single-Photon Turnstile Device|journal=Science|volume=290|pages=2282–2285|doi=10.1126/science.290.5500.2282|pmid=11125136|number=5500}}</ref> functionalized carbon nanotubes,<ref>{{Cite journal|last=Htoon|first=Han|last2=Doorn|first2=Stephen K.|last3=Baldwin|first3=Jon K. S.|last4=Hartmann|first4=Nicolai F.|last5=Ma|first5=Xuedan|date=August 2015|title=Room-temperature single-photon generation from solitary dopants of carbon nanotubes|journal=Nature Nanotechnology|volume=10|issue=8|pages=671–675|doi=10.1038/nnano.2015.136|pmid=26167766|issn=1748-3395|bibcode=2015NatNa..10..671M}}</ref><ref>{{Cite journal|last=He|first=Xiaowei|last2=Hartmann|first2=Nicolai F.|last3=Ma|first3=Xuedan|last4=Kim|first4=Younghee|last5=Ihly|first5=Rachelle|last6=Blackburn|first6=Jeffrey L.|last7=Gao|first7=Weilu|last8=Kono|first8=Junichiro|last9=Yomogida|first9=Yohei|date=September 2017|title=Tunable room-temperature single-photon emission at telecom wavelengths from sp3 defects in carbon nanotubes|journal=Nature Photonics|volume=11|issue=9|pages=577–582|doi=10.1038/nphoton.2017.119|issn=1749-4885}}</ref> and two-dimensional materials<ref>{{Cite journal|last=Tonndorf|first=Philipp|last2=Schmidt|first2=Robert|last3=Schneider|first3=Robert|last4=Kern|first4=Johannes|last5=Buscema|first5=Michele|last6=Steele|first6=Gary A.|last7=Castellanos-Gomez|first7=Andres|last8=van der Zant|first8=Herre S. J.|last9=Michaelis de Vasconcellos|first9=Steffen|date=2015-04-20|title=Single-photon emission from localized excitons in an atomically thin semiconductor|journal=Optica|volume=2|issue=4|pages=347|doi=10.1364/OPTICA.2.000347|issn=2334-2536}}</ref><ref>{{Cite journal|last=Chakraborty|first=Chitraleema|last2=Kinnischtzke|first2=Laura|last3=Goodfellow|first3=Kenneth M.|last4=Beams|first4=Ryan|last5=Vamivakas|first5=A. Nick|date=June 2015|title=Voltage-controlled quantum light from an atomically thin semiconductor|journal=Nature Nanotechnology|volume=10|issue=6|pages=507–511|doi=10.1038/nnano.2015.79|pmid=25938569|issn=1748-3387|bibcode=2015NatNa..10..507C}}</ref><ref>{{Cite journal|last=Palacios-Berraquero|first=Carmen|last2=Barbone|first2=Matteo|last3=Kara|first3=Dhiren M.|last4=Chen|first4=Xiaolong|last5=Goykhman|first5=Ilya|last6=Yoon|first6=Duhee|last7=Ott|first7=Anna K.|last8=Beitner|first8=Jan|last9=Watanabe|first9=Kenji|date=December 2016|title=Atomically thin quantum light-emitting diodes|journal=Nature Communications|volume=7|issue=1|pages=12978|doi=10.1038/ncomms12978|pmid=27667022|pmc=5052681|issn=2041-1723|bibcode=2016NatCo...712978P|arxiv=1603.08795}}</ref><ref>{{Cite journal|last=Palacios-Berraquero|first=Carmen|last2=Kara|first2=Dhiren M.|last3=Montblanch|first3=Alejandro R.-P.|last4=Barbone|first4=Matteo|last5=Latawiec|first5=Pawel|last6=Yoon|first6=Duhee|last7=Ott|first7=Anna K.|last8=Loncar|first8=Marko|last9=Ferrari|first9=Andrea C.|date=August 2017|title=Large-scale quantum-emitter arrays in atomically thin semiconductors|journal=Nature Communications|volume=8|issue=1|pages=15093|doi=10.1038/ncomms15093|issn=2041-1723|pmc=5458119|pmid=28530249|bibcode=2017NatCo...815093P|arxiv=1609.04244}}</ref><ref>{{Cite journal|last=Branny|first=Artur|last2=Kumar|first2=Santosh|last3=Proux|first3=Raphaël|last4=Gerardot|first4=Brian D|date=August 2017|title=Deterministic strain-induced arrays of quantum emitters in a two-dimensional semiconductor|journal=Nature Communications|volume=8|issue=1|pages=15053|doi=10.1038/ncomms15053|issn=2041-1723|pmc=5458118|pmid=28530219|bibcode=2017NatCo...815053B|arxiv=1610.01406}}</ref><ref>{{Cite journal|last=Wu|first=Wei|last2=Dass|first2=Chandriker K.|last3=Hendrickson|first3=Joshua R.|last4=Montaño|first4=Raul D.|last5=Fischer|first5=Robert E.|last6=Zhang|first6=Xiaotian|last7=Choudhury|first7=Tanushree H.|last8=Redwing|first8=Joan M.|last9=Wang|first9=Yongqiang|date=2019-05-27|title=Locally defined quantum emission from epitaxial few-layer tungsten diselenide|journal=Applied Physics Letters|volume=114|issue=21|pages=213102|doi=10.1063/1.5091779|issn=0003-6951|bibcode=2019ApPhL.114u3102W}}</ref><ref>{{Cite journal|last=He|first=Yu-Ming|last2=Clark|first2=Genevieve|last3=Schaibley|first3=John R.|last4=He|first4=Yu|last5=Chen|first5=Ming-Cheng|last6=Wei|first6=Yu-Jia|last7=Ding|first7=Xing|last8=Zhang|first8=Qiang|last9=Yao|first9=Wang|date=June 2015|title=Single quantum emitters in monolayer semiconductors|journal=Nature Nanotechnology|volume=10|issue=6|pages=497–502|doi=10.1038/nnano.2015.75|pmid=25938571|issn=1748-3387|bibcode=2015NatNa..10..497H|arxiv=1411.2449}}</ref> can also emit single photons and can be constructed from the same semiconductor materials as the light-confining structures.
+Within the 21st century defect centres in various solid state materials have emerged,<ref>{{Cite journal |doi = 10.1038/nphoton.2016.186|title = Solid-state single-photon emitters|journal = Nature Photonics|volume = 10|issue = 10|pages = 631–641|year = 2016|last1 = Aharonovich|first1 = Igor|last2 = Englund|first2 = Dirk|last3 = Toth|first3 = Milos|bibcode = 2016NaPho..10..631A}}</ref> most notably diamond, silicon carbide<ref>{{Cite journal|last=Castelletto|first=S.|last2=Johnson|first2=B. C.|last3=Ivády|first3=V.|last4=Stavrias|first4=N.|last5=Umeda|first5=T.|last6=Gali|first6=A.|last7=Ohshima|first7=T.|date=February 2014|title=A silicon carbide room-temperature single-photon source|journal=Nature Materials|volume=13|issue=2|pages=151–156|doi=10.1038/nmat3806|pmid=24240243|issn=1476-1122}}</ref><ref>{{Cite journal |doi = 10.1063/1.4939906|title = Activation and control of visible single defects in 4H-, 6H-, and 3C-SiC by oxidation|journal = Applied Physics Letters|volume = 108|issue = 2|pages = 021107|year = 2016|last1 = Lohrmann|first1 = A.|last2 = Castelletto|first2 = S.|last3 = Klein|first3 = J. R.|last4 = Ohshima|first4 = T.|last5 = Bosi|first5 = M.|last6 = Negri|first6 = M.|last7 = Lau|first7 = D. W. M.|last8 = Gibson|first8 = B. C.|last9 = Prawer|first9 = S.|last10 = McCallum|first10 = J. C.|last11 = Johnson|first11 = B. C.|bibcode = 2016ApPhL.108b1107L}}</ref> and boron nitride.<ref>{{Cite journal | doi=10.1038/nnano.2015.242| title=Quantum emission from hexagonal boron nitride monolayers| journal=Nature Nanotechnology| volume=11| issue=1| pages=37–41| year=2016| last1=Tran| first1=Toan Trong| last2=Bray| first2=Kerem| last3=Ford| first3=Michael J.| last4=Toth| first4=Milos| last5=Aharonovich| first5=Igor| bibcode=2016NatNa..11...37T| arxiv=1504.06521}}</ref> the most studied defect is the nitrogen vacancy (NV) centers in diamond that was utilised as a source of single photons.<ref name="Kurtsiefer00">{{cite journal |last1=Kurtsiefer |first1=Christian |last2=Mayer |first2=Sonja |last3=Zarda |first3=Patrick |last4=Weinfurter |first4=Harald |year=2000 |title=Stable Solid-State Source of Single Photons |journal=Phys. Rev. Lett. |volume=85 |number=2| pages=290–293 |doi=10.1103/physrevlett.85.290|bibcode=2000PhRvL..85..290K }}</ref> 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),<ref name="Michler00">{{cite journal|last1=Michler|first1=P.|last2=Kiraz|first2=A.|last3=Becher|first3=C.|last4=Schoenfeld|first4=W. V.|last5=Petroff|first5=P. M.|last6=Zhang|first6=Lidong|last7=Imamoglu|first7=A.|year=200|title=A Quantum Dot Single-Photon Turnstile Device|journal=Science|volume=290|pages=2282–2285|doi=10.1126/science.290.5500.2282|pmid=11125136|number=5500}}</ref> functionalized carbon nanotubes,<ref>{{Cite journal|last=Htoon|first=Han|last2=Doorn|first2=Stephen K.|last3=Baldwin|first3=Jon K. S.|last4=Hartmann|first4=Nicolai F.|last5=Ma|first5=Xuedan|date=August 2015|title=Room-temperature single-photon generation from solitary dopants of carbon nanotubes|journal=Nature Nanotechnology|volume=10|issue=8|pages=671–675|doi=10.1038/nnano.2015.136|pmid=26167766|issn=1748-3395|bibcode=2015NatNa..10..671M}}</ref><ref>{{Cite journal|last=He|first=Xiaowei|last2=Hartmann|first2=Nicolai F.|last3=Ma|first3=Xuedan|last4=Kim|first4=Younghee|last5=Ihly|first5=Rachelle|last6=Blackburn|first6=Jeffrey L.|last7=Gao|first7=Weilu|last8=Kono|first8=Junichiro|last9=Yomogida|first9=Yohei|date=September 2017|title=Tunable room-temperature single-photon emission at telecom wavelengths from sp3 defects in carbon nanotubes|journal=Nature Photonics|volume=11|issue=9|pages=577–582|doi=10.1038/nphoton.2017.119|issn=1749-4885}}</ref> and two-dimensional materials<ref>{{Cite journal|last=Tonndorf|first=Philipp|last2=Schmidt|first2=Robert|last3=Schneider|first3=Robert|last4=Kern|first4=Johannes|last5=Buscema|first5=Michele|last6=Steele|first6=Gary A.|last7=Castellanos-Gomez|first7=Andres|last8=van der Zant|first8=Herre S. J.|last9=Michaelis de Vasconcellos|first9=Steffen|date=2015-04-20|title=Single-photon emission from localized excitons in an atomically thin semiconductor|journal=Optica|volume=2|issue=4|pages=347|doi=10.1364/OPTICA.2.000347|issn=2334-2536}}</ref><ref>{{Cite journal|last=Chakraborty|first=Chitraleema|last2=Kinnischtzke|first2=Laura|last3=Goodfellow|first3=Kenneth M.|last4=Beams|first4=Ryan|last5=Vamivakas|first5=A. Nick|date=June 2015|title=Voltage-controlled quantum light from an atomically thin semiconductor|journal=Nature Nanotechnology|volume=10|issue=6|pages=507–511|doi=10.1038/nnano.2015.79|pmid=25938569|issn=1748-3387|bibcode=2015NatNa..10..507C}}</ref><ref>{{Cite journal|last=Palacios-Berraquero|first=Carmen|last2=Barbone|first2=Matteo|last3=Kara|first3=Dhiren M.|last4=Chen|first4=Xiaolong|last5=Goykhman|first5=Ilya|last6=Yoon|first6=Duhee|last7=Ott|first7=Anna K.|last8=Beitner|first8=Jan|last9=Watanabe|first9=Kenji|date=December 2016|title=Atomically thin quantum light-emitting diodes|journal=Nature Communications|volume=7|issue=1|pages=12978|doi=10.1038/ncomms12978|pmid=27667022|pmc=5052681|issn=2041-1723|bibcode=2016NatCo...712978P|arxiv=1603.08795}}</ref><ref>{{Cite journal|last=Palacios-Berraquero|first=Carmen|last2=Kara|first2=Dhiren M.|last3=Montblanch|first3=Alejandro R.-P.|last4=Barbone|first4=Matteo|last5=Latawiec|first5=Pawel|last6=Yoon|first6=Duhee|last7=Ott|first7=Anna K.|last8=Loncar|first8=Marko|last9=Ferrari|first9=Andrea C.|date=August 2017|title=Large-scale quantum-emitter arrays in atomically thin semiconductors|journal=Nature Communications|volume=8|issue=1|pages=15093|doi=10.1038/ncomms15093|issn=2041-1723|pmc=5458119|pmid=28530249|bibcode=2017NatCo...815093P|arxiv=1609.04244}}</ref><ref>{{Cite journal|last=Branny|first=Artur|last2=Kumar|first2=Santosh|last3=Proux|first3=Raphaël|last4=Gerardot|first4=Brian D|date=August 2017|title=Deterministic strain-induced arrays of quantum emitters in a two-dimensional semiconductor|journal=Nature Communications|volume=8|issue=1|pages=15053|doi=10.1038/ncomms15053|issn=2041-1723|pmc=5458118|pmid=28530219|bibcode=2017NatCo...815053B|arxiv=1610.01406}}</ref><ref>{{Cite journal|last=Wu|first=Wei|last2=Dass|first2=Chandriker K.|last3=Hendrickson|first3=Joshua R.|last4=Montaño|first4=Raul D.|last5=Fischer|first5=Robert E.|last6=Zhang|first6=Xiaotian|last7=Choudhury|first7=Tanushree H.|last8=Redwing|first8=Joan M.|last9=Wang|first9=Yongqiang|date=2019-05-27|title=Locally defined quantum emission from epitaxial few-layer tungsten diselenide|journal=Applied Physics Letters|volume=114|issue=21|pages=213102|doi=10.1063/1.5091779|issn=0003-6951|bibcode=2019ApPhL.114u3102W}}</ref><ref>{{Cite journal|last=He|first=Yu-Ming|last2=Clark|first2=Genevieve|last3=Schaibley|first3=John R.|last4=He|first4=Yu|last5=Chen|first5=Ming-Cheng|last6=Wei|first6=Yu-Jia|last7=Ding|first7=Xing|last8=Zhang|first8=Qiang|last9=Yao|first9=Wang|date=June 2015|title=Single quantum emitters in monolayer semiconductors|journal=Nature Nanotechnology|volume=10|issue=6|pages=497–502|doi=10.1038/nnano.2015.75|pmid=25938571|issn=1748-3387|bibcode=2015NatNa..10..497H|arxiv=1411.2449}}</ref> can also emit single photons and can be constructed from the same semiconductor materials as the light-confining structures. 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 Exciting atoms and excitons to highly interacting Rydberg levels prevents more than one excitation over the so called blockade volume. Making small ensembles and crystals could act as a single photon emitters.<ref>{{Cite journal|last=Dudin|first=Y. O.|last2=Kuzmich|first2=A.|date=2012-05-18|title=Strongly Interacting Rydberg Excitations of a Cold Atomic Gas|journal=Science|volume=336|issue=6083|pages=887–889|doi=10.1126/science.1217901|pmid=22517325|issn=0036-8075}}</ref><ref>{{Cite journal|last=Ripka|first=Fabian|last2=Kübler|first2=Harald|last3=Löw|first3=Robert|last4=Pfau|first4=Tilman|date=2018-10-26|title=A room-temperature single-photon source based on strongly interacting Rydberg atoms|journal=Science|volume=362|issue=6413|pages=446–449|doi=10.1126/science.aau1949|pmid=30361371|issn=0036-8075|arxiv=1806.02120}}</ref>