Editing Laser cooling (section)

== Methods ==

The first realization of laser cooling and the most ubiquitous method for cooling atoms and molecules (so much so that it is often referred to simply as 'laser cooling'), is [[Doppler cooling]]. 

=== Doppler cooling ===

{{main|Doppler cooling}}
Doppler cooling is by far the most common method of laser cooling. It is used to cool low density gases down to the [[Doppler cooling limit]], which for [[rubidium]] (a popular choice in the field of atomic physics) is around 150&nbsp;[[wiktionary:microkelvin|microkelvin]].  It is often often combined with a magnetic field gradient to realize a [[magneto-optical trap]].

In Doppler cooling, initially, the frequency of light is tuned slightly below an [[electronic transition]] in the [[atom]]. Because the light is [[Laser detuning|detuned]] to the "red" (i.e.,&nbsp;at lower frequency) of the transition, the atoms will absorb more [[photon]]s if they move towards the light source, due to the [[Doppler effect]]. Thus if one applies light from two opposite directions, the atoms will always scatter more photons from the laser beam pointing opposite to their direction of motion. In each scattering event the atom loses a [[momentum]] equal to the momentum of the photon. If the atom, which is now in the excited state, then emits a photon spontaneously, it will be kicked by the same amount of momentum, but in a random direction. Since the initial momentum change is a pure loss (opposing the direction of motion), while the subsequent change is random, the probable result of the absorption and emission process is to reduce the momentum of the atom, and therefore its [[speed]]&mdash;provided its initial speed was larger than the recoil speed from scattering a single photon. If the absorption and emission are repeated many times, the average speed, and therefore the [[kinetic energy]] of the atom, will be reduced. Since the [[temperature]] of a group of atoms is a measure of the average random internal kinetic energy, this is equivalent to cooling the atoms.

When atoms are Doppler cooled in three dimensions, traditionally by 6 counter-propagating red-detuned laser beams, this is called [[optical molasses]] because the atoms move slowly, as if they are moving through molasses.  
=== Sub-Doppler cooling ===
After Doppler cooling it is often helpful to cool atoms (or molecules) below their Doppler limit.  This is accomplished with a variety sub-Doppler cooling techniques.  Different atomic structures are amenable to different sub-Doppler cooling techniques.  For example, [[gray molasses]] is used with lithium and potassium because they have unresolved hyperfine structure in their excited states where [[polarization gradient cooling]] would not work.  

Sub-Doppler cooling methods include:

* [[Sisyphus cooling]]<ref>[https://www.nobelprize.org/uploads/2018/06/phillips-lecture.pdf Laser cooling and trapping of neutral atoms] Nobel Lecture by [[William Daniel Phillips|William D. Phillips]], Dec 8, 1997: {{cite journal | doi = 10.1103/RevModPhys.70.721 | bibcode=1998RvMP...70..721P | volume=70 | title=Nobel Lecture: Laser cooling and trapping of neutral atoms | year=1998 | journal=Reviews of Modern Physics | pages=721–741 | last1 = Phillips | first1 = William D.| issue=3 | doi-access=free }}</ref>
* [[Polarization gradient cooling]]
* [[Resolved sideband cooling]]
* [[Raman cooling|Raman sideband cooling]]
* Velocity selective coherent population trapping (VSCPT)<ref>{{cite journal |author1=A. Aspect |author2=E. Arimondo |author3=R. Kaiser |author4=N. Vansteenkiste |author5=C. Cohen-Tannoudji | title=Laser Cooling below the One-Photon Recoil Energy by Velocity-Selective Coherent Population Trapping | journal=Physical Review Letters | year=1988 | volume=61 |issue=7 | pages=826–829 | doi=10.1103/PhysRevLett.61.826 |pmid=10039440 | bibcode=1988PhRvL..61..826A|doi-access=free }}</ref>
* [[Gray molasses]]
* [[Electromagnetically induced transparency|Electromagnetically induced transparency (EIT)]] cooling<ref>{{Cite journal|title = Single-atom imaging of fermions in a quantum-gas microscope|journal = Nature Physics|pages = 738–742|volume = 11|issue = 9|doi = 10.1038/nphys3403|first1 = Elmar|last1 = Haller|first2 = James|last2 = Hudson|first3 = Andrew|last3 = Kelly|first4 = Dylan A.|last4 = Cotta|first5 = Bruno|last5 = Peaudecerf|first6 = Graham D.|last6 = Bruce|first7 = Stefan|last7 = Kuhr|arxiv = 1503.02005 |bibcode = 2015NatPh..11..738H |year = 2015|s2cid = 51991496}}</ref>

=== Other methods ===
Other laser cooling methods include:

* Cavity-mediated cooling<ref>{{cite journal |author1=Peter Horak |author2=Gerald Hechenblaikner |author3=Klaus M. Gheri |author4=Herwig Stecher |author5=Helmut Ritsch | title=Cavity-Induced Atom Cooling in the Strong Coupling Regime | journal=Physical Review Letters | year=1988 | volume=79 |issue=25 | pages=4974–4977 | doi=10.1103/PhysRevLett.79.4974 | bibcode=1997PhRvL..79.4974H}}</ref>
* [[Anti-Stokes cooling]] in solids
* Photonic cooling – It is under development as a spot cooling system that can target areas of hundreds of microns in diameter. The laser(s) cool a plate less than one millimeter thick, made largely of [[gallium arsenide]].<ref name=":3">{{Cite web |last=Ghoshal |first=Abhimanyu |date=2025-04-14 |title=Laser cooling breakthrough could make data centers much greener |url=https://newatlas.com/physics/laser-cooling-data-centers-photonic/ |access-date=2025-04-15 |website=New Atlas |language=en-US}}</ref>