Editing Laser cooling (section)

=== 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>