Editing Photon (section)

== Technological applications ==
Photons have many applications in technology. These examples are chosen to illustrate applications of photons ''per se'', rather than general optical devices such as lenses, etc. that could operate under a classical theory of light. The laser is an important application and is discussed above under [[stimulated emission]].

Individual photons can be detected by several methods. The classic [[photomultiplier]] tube exploits the [[photoelectric effect]]: a photon of sufficient energy strikes a metal plate and knocks free an electron, initiating an ever-amplifying avalanche of electrons. [[Semiconductor]] [[charge-coupled device]] chips use a similar effect: an incident photon generates a charge on a microscopic [[capacitor]] that can be detected. Other detectors such as [[Geiger counter]]s use the ability of photons to [[ionize]] gas molecules contained in the device, causing a detectable change of [[Electrical conductivity|conductivity]] of the gas.<ref>Photomultiplier section 1.1.10, CCDs section 1.1.8, Geiger counters section 1.3.2.1 in {{cite book |last=Kitchin |first=C. R. |title=Astrophysical Techniques |publisher=CRC Press |year=2008 |isbn=978-1-4200-8243-2 |location=Boca Raton, Florida |language=en-us}}</ref>

Planck's energy formula <math>E=h\nu</math> is often used by engineers and chemists in design, both to compute the change in energy resulting from a photon absorption and to determine the frequency of the light emitted from a given photon emission. For example, the [[emission spectrum]] of a [[gas-discharge lamp]] can be altered by filling it with (mixtures of) gases with different electronic [[energy level]] configurations.<ref>{{cite book |last=Waymouth |first=John |url=https://archive.org/details/electricdischarg00waym |title=Electric Discharge Lamps |date=1971 |publisher=The M.I.T. Press |isbn=978-0-262-23048-3 |location=Cambridge, Massachusetts |language=en-us |url-access=registration}}</ref>

Under some conditions, an energy transition can be excited by "two" photons that individually would be insufficient. This allows for higher resolution microscopy, because the sample absorbs energy only in the spectrum where two beams of different colors overlap significantly, which can be made much smaller than the excitation volume of a single beam (see [[two-photon excitation microscopy]]). Moreover, these photons cause less damage to the sample, since they are of lower energy.<ref>{{cite journal|author1=Denk, W. |author1-link=Winfried Denk|author2-link=Karel Svoboda (scientist)|author2=Svoboda, K. |title=Photon upmanship: Why multiphoton imaging is more than a gimmick|journal=[[Neuron (journal)|Neuron]]|volume=18|issue=3|pages=351–357|year=1997|pmid=9115730|doi=10.1016/S0896-6273(00)81237-4|s2cid=2414593 |doi-access=free}}</ref>

In some cases, two energy transitions can be coupled so that, as one system absorbs a photon, another nearby system "steals" its energy and re-emits a photon of a different frequency. This is the basis of [[fluorescence resonance energy transfer]], a technique that is used in [[molecular biology]] to study the interaction of suitable [[protein]]s.<ref>{{Cite book |last=Lakowicz |first=J. R. |url={{google books |plainurl=y |id=-PSybuLNxcAC|page=529}} |title=Principles of Fluorescence Spectroscopy |publisher=Springer |year=2006 |isbn=978-0-387-31278-1 |pages=529 ff |language=en}}</ref>

Several different kinds of [[hardware random number generator]]s involve the detection of single photons. In one example, for each bit in the random sequence that is to be produced, a photon is sent to a [[beam-splitter]]. In such a situation, there are two possible outcomes of equal probability. The actual outcome is used to determine whether the next bit in the sequence is "0" or "1".<ref>{{Cite journal|first1=T.|last1=Jennewein|first2=U.|last2=Achleitner|first3=G.|last3=Weihs|first4=H.|last4=Weinfurter|first5=A.|last5=Zeilinger|title=A fast and compact quantum random number generator|doi=10.1063/1.1150518|journal=[[Review of Scientific Instruments]]|volume=71|pages=1675–1680|year=2000|arxiv=quant-ph/9912118|bibcode=2000RScI...71.1675J|issue=4 |s2cid=13118587}}</ref><ref>{{Cite journal|first1=A.|last1=Stefanov|first2=N.|last2=Gisin|first3=O.|last3=Guinnard|first4=L.|last4=Guinnard|first5=H.|last5=Zbiden|title=Optical quantum random number generator|journal=[[Journal of Modern Optics]]|volume=47|pages=595–598|year=2000|doi=10.1080/095003400147908|issue=4}}</ref>