Editing Laser cooling (section)

== History ==

=== Radiation pressure ===
[[Radiation pressure]] is the force that electromagnetic radiation exerts on matter.  In 1873 Maxwell published his treatise on [[electromagnetism]] in which he predicted radiation pressure.<ref name="Maxwell1873">
{{cite book| title=A Treatise on Electricity and Magnetism, II|edition= 1st| author-link=James Clerk Maxwell| first=J.C.| last=Maxwell| year=1873| location=Oxford| page=391}}
</ref> The force was experimentally demonstrated for the first time by [[Pyotr Lebedev|Lebedev]] and reported at a conference in Paris in 1900,<ref name="Lebedev1900">
{{cite conference| url=https://www.europhysicsnews.org/articles/epn/pdf/2019/04/epn2019504p15.pdf| title=Les forces de Maxwell-Bartoli dues à la pression de la lumière| first=Pyotr| last=Lebedew| year=1900| conference=Rapports présentés au Congrès International de Physique | volume=2| location=Paris| page=133 |language=fr|author-link= Pyotr Lebedev}}
</ref> and later published in more detail in 1901.<ref name="Lebedew1901">
{{cite journal
 |last=Lebedew |first=P. |author-link= Pyotr Lebedev
 |year=1901
 |title=Untersuchungen über die Druckkräfte des Lichtes
 |journal=[[Annalen der Physik]]
 |language=de
 |volume=311 |issue=11 |pages=433–458
 |bibcode=1901AnP...311..433L
 |doi=10.1002/andp.19013111102
 |url=https://zenodo.org/record/1424005
}}</ref> Following Lebedev's measurements [[Ernest Fox Nichols|Nichols]] and [[Gordon Ferrie Hull|Hull]] also demonstrated the force of radiation pressure in 1901,<ref name="Nichols1901">
{{cite journal|last1=Nichols |first1=E.F. |last2=Hull |first2=G.F. |author1-link=Ernest Fox Nichols |author2-link=Gordon Ferrie Hull|title=A Preliminary Communication on the Pressure of Heat and Light Radiation|url=https://link.aps.org/doi/10.1103/PhysRevSeriesI.13.307|journal=Physical Review |series=Series I |doi=10.1103/PhysRevSeriesI.13.307 |year=1901|volume=13 |issue=5 |pages=307–320 |bibcode=1901PhRvI..13..307N }}
</ref> with a refined measurement reported in 1903.<ref name="Nichols1">
{{cite journal|last1=Nichols |first1=E.F. |last2=Hull |first2=G.F. |author1-link=Ernest Fox Nichols |author2-link=Gordon Ferrie Hull|title=The Pressure Due To Radiation . (Second Paper.) <!--DUPLICATE|url=http://massey.dur.ac.uk/articles/newoptics.pdf--> | year=1903 | pages=26–50| volume=17 | url=https://journals.aps.org/pri/abstract/10.1103/PhysRevSeriesI.17.26| journal=Physical Review|issue=1 |doi=10.1103/PhysRevSeriesI.17.26 |bibcode=1903PhRvI..17...26N }}
</ref><ref name="Nichols2">
{{cite journal|last1=Nichols |first1=E.F. |last2=Hull |first2=G.F. |author1-link=Ernest Fox Nichols |author2-link=Gordon Ferrie Hull|title=The Pressure Due To Radiation. (Second Paper.) <!--DUPLICATE|url=http://massey.dur.ac.uk/articles/newoptics.pdf--> | year=1903 | pages=91–104| volume=17 | url=https://journals.aps.org/pri/abstract/10.1103/PhysRevSeriesI.17.91| journal=Physical Review|issue=2 |doi=10.1103/PhysRevSeriesI.17.91 |bibcode=1903PhRvI..17...91N }}
</ref> 

Atoms and molecules have bound states and transitions can occur between these states in the presence of light that is near the transition frequency.  Sodium is historically notable because it has a strong transition at 589 nm, a wavelength which is close to the peak sensitivity of the human eye.  This made it relatively easy to see the interaction of light with sodium atoms.  In 1933, [[Otto Frisch]] deflected an atomic beam of sodium atoms with light.<ref name="Frisch">
{{cite journal|last=Frisch|first=R. |title= Experimenteller Nachweis des Einsteinschen Strahlungsrückstoßes |url=https://link.springer.com/article/10.1007/BF01340182| year=1933 | pages=42–48| volume=86 | language=de|journal=Zeitschrift für Physik|issue=1–2 |doi=10.1007/BF01340182 |bibcode=1933ZPhy...86...42F |s2cid=123038196 }}
</ref> 
This was the first realization of radiation pressure acting on an atom or molecule.

=== Laser cooling proposals ===
The introduction of [[laser]]s in atomic physics experiments was the precursor to the laser cooling proposals in the mid 1970s. Laser cooling was proposed separately in 1975 by two different research groups: [[Theodor W. Hänsch|Hänsch]] and [[Arthur Leonard Schawlow|Schawlow]],<ref name="Hänsch1975">
{{cite journal |last1=Hänsch |first1=T. W. |last2=Schawlow |first2=A. L. |author1-link= Theodor W. Hänsch|author2-link=Arthur Leonard Schawlow|title=Cooling of gases by laser radiation |journal=Optics Communications |date=January 1975 |volume=13 |issue=1 |pages=68–69 |doi=10.1016/0030-4018(75)90159-5|doi-access=free |bibcode=1975OptCo..13...68H }}</ref> and [[David Wineland|Wineland]] and [[Hans Georg Dehmelt|Dehmelt]].<ref>{{cite journal |last1=Wineland |first1=David |last2=Dehmelt |first2=Hans |author1-link=David Wineland|author2-link=Hans Georg Dehmelt|title=Proposed 10<sup>14</sup> {{nowrap|∆''ν'' < ''ν''}} laser fluorescence spectroscopy on T1<sup>+</sup> mono-ion oscillator III  |journal=Bulletin of the American Physical Society |date=January 1, 1975 |volume=20 |issue=4 |page=637}}</ref> Both proposals outlined the simplest laser cooling process, known as [[Doppler cooling]], where laser light tuned below an atom's resonant frequency is preferentially absorbed by atoms moving towards the laser and after absorption a photon is emitted in a random direction.  This process is repeated many times and in a configuration with counterpropagating laser cooling light the velocity distribution of the atoms is reduced.<ref name=":0">
{{cite journal|last=Phillips|first=William D.|title=Nobel Lecture: Laser cooling and trapping of neutral atoms|journal=Reviews of Modern Physics|volume=70|issue=3|pages=721–741|doi=10.1103/revmodphys.70.721|bibcode=1998RvMP...70..721P|year=1998|doi-access=free}}</ref> 

In 1977 [[Arthur Ashkin|Ashkin]] submitted a paper which describes how Doppler cooling could be used to provide the necessary damping to load atoms into an optical trap.<ref>{{cite journal |last1=Ashkin |first1=A. |author-link=Arthur Ashkin|title=Trapping of Atoms by Resonance Radiation Pressure |journal=Physical Review Letters |date=20 March 1978 |volume=40 |issue=12 |pages=729–732 |doi=10.1103/PhysRevLett.40.729|bibcode=1978PhRvL..40..729A }}</ref>  In this work he emphasized how this could allow for long [[Spectroscopy|spectroscopic]] measurements which would increase precision because the atoms would be held in place.  He also discussed overlapping [[Optical tweezers|optical traps]] to study interactions between different atoms.

=== Initial realizations ===
Following the laser cooling proposals, in 1978 two research groups that Wineland, Drullinger and Walls of NIST, and Neuhauser, Hohenstatt, Toscheck and Dehmelt of the University of Washington succeeded in laser cooling atoms. The NIST group wanted to reduce the effect of Doppler broadening on spectroscopy. They cooled magnesium ions in a Penning trap to below 40&nbsp;K.  The Washington group cooled barium ions.  

Influenced by the Wineland's work on laser cooling ions, [[William Daniel Phillips|William Phillips]] applied the same principles to laser cool neutral atoms. In 1982, he published the first paper where neutral atoms were laser cooled.<ref>{{cite journal |last1=Phillips |first1=William|author-link =William Daniel Phillips|title=Laser Deceleration of an Atomic Beam |journal=Physical Review Letters |date=1 March 1982 |volume=48 |issue=9 |pages=596–599 |doi=10.1103/PhysRevLett.48.596 |bibcode=1982PhRvL..48..596P |doi-access=free }}</ref> The process used is now known as the [[Zeeman slower]] and is a standard technique for slowing an atomic beam.

The 1997 [[Nobel Prize in Physics]] was awarded to [[Claude Cohen-Tannoudji]], [[Steven Chu]], and [[William Daniel Phillips]] "for development of methods to cool and trap atoms with laser light".<ref>{{cite web | title = The Nobel Prize in Physics 1997 | publisher = Nobel Foundation | url = http://nobelprize.org/nobel_prizes/physics/laureates/1997/index.html | access-date = 9 October 2008 | archive-url = https://web.archive.org/web/20081007171154/http://nobelprize.org/nobel_prizes/physics/laureates/1997/index.html | archive-date = 7 October 2008 | url-status = live }}</ref>

=== Modern advances ===

==== Atoms ====
[[File:First Laser Cooling 1008.png|thumb|The cumulative number of unique atomic systems, including different ionization states, (red) and unique isotopes (blue) that have been laser cooled vs. year.]]

The Doppler cooling limit for electric dipole transitions is typically in the hundreds of microkelvins. In the 1980s this limit was seen as the lowest achievable temperature. It was a surprise then when sodium atoms were cooled to 43&nbsp;microkelvins when their Doppler cooling limit is 240&nbsp;microkelvins,<ref>{{cite journal |author1=Paul D. Lett |author2= Richard N. Watts |author3=Christoph I. Westbrook |author4= William D. Phillips |author5=A. Winnicki |author6= Phillip L. Gould |author7= Harold J. Metcalf | title=Observation of Atoms Laser Cooled below the Doppler Limit | journal= Physical Review Letters| year=1988 |volume= 61 |issue= 2 |pages= 169–172 | doi=10.1103/PhysRevLett.61.169|pmid= 10039050 |bibcode= 1988PhRvL..61..169L |s2cid= 8479501 |doi-access= free }}</ref> this unforeseen low temperature was explained by considering the interaction of polarized laser light with more atomic states and transitions. Previous conceptions of laser cooling were decided to have been too simplistic.<ref name=":1">{{Cite journal|last=Bardi|first=Jason Socrates|date=2008-04-02|title=Focus: Landmarks: Laser Cooling of Atoms|url=https://physics.aps.org/story/v21/st11|journal=Physics|language=en-US|volume=21|page=11 |doi=10.1103/physrevfocus.21.11}}</ref> The major laser cooling breakthroughs in the 70s and 80s led to several improvements to preexisting technology and new discoveries with temperatures just above [[absolute zero]]. The cooling processes were utilized to make [[atomic clock]]s more accurate and to improve spectroscopic measurements, and led to the observation of a new [[state of matter]] at ultracold temperatures.<ref name=":2">{{Cite journal|last1=Adams |last2= Riis|first1=Charles S. |first2=Erling|title=Laser Cooling and Manipulation of Neutral Particles|url=http://massey.dur.ac.uk/articles/newoptics.pdf|journal=New Optics|access-date=2017-05-06|archive-date=2017-11-15|archive-url=https://web.archive.org/web/20171115193505/http://massey.dur.ac.uk/articles/newoptics.pdf|url-status=dead}}</ref><ref name=":1" /> The new state of matter, the [[Bose–Einstein condensate]], was observed in 1995 by [[Eric Allin Cornell|Eric Cornell]], [[Carl Wieman]], and [[Wolfgang Ketterle]].<ref>{{cite journal |last1=Chin |first1=Cheng |title=Ultracold atomic gases going strong |journal=National Science Review |date=1 June 2016 |volume=3 |issue=2 |pages=168–170 |doi=10.1093/nsr/nwv073 |doi-access=free }}</ref>

==== Exotic Atoms ====
Most laser cooling experiments bring the atoms close to at rest in the laboratory frame, but cooling of relativistic atoms has also been achieved, where the effect of cooling manifests as a narrowing of the velocity distribution. In 1990, a group at [[Johannes Gutenberg-Universität Mainz|JGU]] successfully laser-cooled a beam of <sup>7</sup>Li<sup>+</sup> at {{val|13.3|u=MeV}} in a storage ring <ref>{{cite journal |last1=Schröder |first1=S. |last2=Klein |first2=R. |last3=Boos |first3=N. |last4=Gerhard |first4=M. |last5=Grieser |first5=R. |last6=Huber |first6=G. |last7=Karafillidis |first7=A. |last8=Krieg |first8=M. |last9=Schmidt |first9=N. |last10=Kühl |first10=T. |last11=Neumann |first11=R. |last12=Balykin |first12=V. |last13=Grieser |first13=M. |last14=Habs |first14=D. |last15=Jaeschke |first15=E. |last16=Krämer |first16=D. |last17=Kristensen |first17=M. |last18=Music |first18=M. |last19=Petrich |first19=W. |last20=Schwalm |first20=D. |last21=Sigray |first21=P. |last22=Steck |first22=M. |last23=Wanner |first23=B. |last24=Wolf |first24=A. |title=First laser cooling of relativistic ions in a storage ring |journal=Physical Review Letters |date=11 June 1990 |volume=64 |issue=24 |pages=2901–2904 |doi=10.1103/PhysRevLett.64.2901|pmid=10041842 |bibcode=1990PhRvL..64.2901S }}</ref> from {{val|260|u=K}} to lower than {{val|2.9|u=K}}, using two counter-propagating lasers addressing the same transition, but at {{val|514.5|u=nm}} and {{val|584.8|u=nm}}, respectively, to compensate for the large [[Doppler effect|Doppler shift]]. 

Laser cooling of antimatter has also been demonstrated, first in 2021 by the [[ALPHA experiment|ALPHA]] collaboration on antihydrogen atoms.<ref>{{cite journal |last1=Baker |first1=C. J. |last2=Bertsche |first2=W. |last3=Capra |first3=A. |last4=Carruth |first4=C. |last5=Cesar |first5=C. L. |last6=Charlton |first6=M. |last7=Christensen |first7=A. |last8=Collister |first8=R. |last9=Mathad |first9=A. Cridland |last10=Eriksson |first10=S. |last11=Evans |first11=A. |last12=Evetts |first12=N. |last13=Fajans |first13=J. |last14=Friesen |first14=T. |last15=Fujiwara |first15=M. C. |last16=Gill |first16=D. R. |last17=Grandemange |first17=P. |last18=Granum |first18=P. |last19=Hangst |first19=J. S. |last20=Hardy |first20=W. N. |last21=Hayden |first21=M. E. |last22=Hodgkinson |first22=D. |last23=Hunter |first23=E. |last24=Isaac |first24=C. A. |last25=Johnson |first25=M. A. |last26=Jones |first26=J. M. |last27=Jones |first27=S. A. |last28=Jonsell |first28=S. |last29=Khramov |first29=A. |last30=Knapp |first30=P. |last31=Kurchaninov |first31=L. |last32=Madsen |first32=N. |last33=Maxwell |first33=D. |last34=McKenna |first34=J. T. K. |last35=Menary |first35=S. |last36=Michan |first36=J. M. |last37=Momose |first37=T. |last38=Mullan |first38=P. S. |last39=Munich |first39=J. J. |last40=Olchanski |first40=K. |last41=Olin |first41=A. |last42=Peszka |first42=J. |last43=Powell |first43=A. |last44=Pusa |first44=P. |last45=Rasmussen |first45=C. Ø |last46=Robicheaux |first46=F. |last47=Sacramento |first47=R. L. |last48=Sameed |first48=M. |last49=Sarid |first49=E. |last50=Silveira |first50=D. M. |last51=Starko |first51=D. M. |last52=So |first52=C. |last53=Stutter |first53=G. |last54=Tharp |first54=T. D. |last55=Thibeault |first55=A. |last56=Thompson |first56=R. I. |last57=van der Werf |first57=D. P. |last58=Wurtele |first58=J. S. |title=Laser cooling of antihydrogen atoms |journal=Nature |date=April 2021 |volume=592 |issue=7852 |pages=35–42 |doi=10.1038/s41586-021-03289-6|pmid=33790445 |pmc=8012212 |bibcode=2021Natur.592...35B }}</ref> In 2024, [[Positronium|positronium]], made up of an electron and a positron, was laser cooled to about 1K.<ref>{{cite journal |last1=Shu |first1=K. |last2=Tajima |first2=Y. |last3=Uozumi |first3=R. |last4=Miyamoto |first4=N. |last5=Shiraishi |first5=S. |last6=Kobayashi |first6=T. |last7=Ishida |first7=A. |last8=Yamada |first8=K. |last9=Gladen |first9=R. W. |last10=Namba |first10=T. |last11=Asai |first11=S. |last12=Wada |first12=K. |last13=Mochizuki |first13=I. |last14=Hyodo |first14=T. |last15=Ito |first15=K. |last16=Michishio |first16=K. |last17=O’Rourke |first17=B. E. |last18=Oshima |first18=N. |last19=Yoshioka |first19=K. |title=Cooling positronium to ultralow velocities with a chirped laser pulse train |journal=Nature |date=September 2024 |volume=633 |issue=8031 |pages=793–797 |doi=10.1038/s41586-024-07912-0|pmid=39261730 |arxiv=2310.08761 |bibcode=2024Natur.633..793S }}</ref>

==== Molecules ====
[[File:Laser cooled molecules.svg|thumb|alt=Graph showing the growing number of laser-cooled molecules as a function of a year.|Directly laser cooled molecules.]]
Molecules are significantly more challenging to laser cool than atoms because molecules have vibrational and rotational degrees of freedom.  These extra degrees of freedom result in more energy levels that can be populated from excited state decays, requiring more lasers compared to atoms to address the more complex level structure.  Vibrational decays are particularly challenging because there are no symmetry rules that restrict the vibrational states that can be populated.

In 2010, at team at Yale led by [[David DeMille|Dave DeMille]] successfully laser-cooled a [[diatomic molecule]].<ref>{{cite journal |author1=E. S. Shuman |author2=J. F. Barry |author3=D. DeMille | title=Laser cooling of a diatomic molecule | journal=Nature | year=2010 | volume=467 |issue=7317 | pages=820–823 | doi=10.1038/nature09443 | pmid=20852614|arxiv = 1103.6004 |bibcode = 2010Natur.467..820S |s2cid=4430586 }}</ref> In 2016, a group at [[Max-Planck-Institut für Quantenoptik|MPQ]] successfully cooled [[formaldehyde]] to {{val|420|u=μK}} via optoelectric Sisyphus cooling.<ref>{{cite journal |last1=Prehn |first1=Alexander |last2=Ibrügger |first2=Martin |last3=Glöckner |first3=Rosa |last4=Rempe |first4=Gerhard |last5=Zeppenfeld |first5=Martin |title=Optoelectrical Cooling of Polar Molecules to Submillikelvin Temperatures |journal=Physical Review Letters |date=10 February 2016 |volume=116 |issue=6 |pages=063005 |doi=10.1103/PhysRevLett.116.063005 |pmid=26918988 |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.063005 |access-date=10 January 2024|arxiv=1511.09427 |bibcode=2016PhRvL.116f3005P }}</ref> In 2022, a group at Harvard successfully laser cooled and trapped CaOH to {{val|720|(40)|u=μK}} in a [[magneto-optical trap]].<ref>{{cite journal |author1=N. B. Vilas |author2= C. Hallas |author3=L. Anderegg |author4=P. Robichaud |author5=A. Winnicki |author6=D. Mitra |author7=J. M. Doyle | title=Magneto-optical trapping and sub-Doppler cooling of a polyatomic molecule | journal=Nature | year=2022 |volume= 606 |issue= 7912 |pages= 70–74 | doi=10.1038/s41586-022-04620-5 |pmid= 35650357 | arxiv = 2112.08349v1 |bibcode= 2022Natur.606...70V |s2cid= 245144894 }}</ref>

==== Mechanical systems ====
Starting in the 2000s, laser cooling was applied to [[Cavity optomechanics|small mechanical systems]], ranging from small cantilevers to the mirrors used in the [[LIGO]] observatory.  These devices are connected to a larger substrate, such as a mechanical membrane attached to a frame, or they are held in optical traps, in both cases the mechanical system is a harmonic oscillator.  Laser cooling reduces the random vibrations of the mechanical oscillator, removing thermal phonons from the system.

In 2007, an MIT team successfully laser-cooled a macro-scale (1&nbsp;gram) object to 0.8&nbsp;K.<ref>{{cite news |title=Laser-cooling Brings Large Object Near Absolute Zero |url=https://www.sciencedaily.com/releases/2007/04/070406171036.htm |work=ScienceDaily }}</ref> In 2011, a team from the California Institute of Technology and the University of Vienna became the first to laser-cool a (10&nbsp;μm × 1&nbsp;μm) mechanical object to its quantum ground state.<ref>{{cite web |url=https://www.caltech.edu/about/news/caltech-team-uses-laser-light-cool-object-quantum-ground-state-1726|title =Caltech Team Uses Laser Light to Cool Object to Quantum Ground State|website =Caltech.edu|access-date= June 27, 2013|date= 5 October 2011}}</ref>