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An '''optical clock''' is a clock that uses light to track time. It differs from an [[atomic clock]] in that it uses [[visible light]], rather than [[microwaves]].<ref>{{Cite web |date=2005-05-04 |title=Optical clocks |url=https://physicsworld.com/a/optical-clocks/ |access-date=2024-10-11 |website=Physics World |language=en-GB}}</ref><ref>{{Cite web |title=Optical clocks {{!}} Menlo Systems |url=https://www.menlosystems.com/applications/all/metrology/optical-clocks/ |access-date=2024-10-11 |website=www.menlosystems.com}}</ref> Several chemical elements have been studied for possible use in optical clocks. These include [[aluminum]], [[Mercury (element)|mercury]], [[strontium]], [[indium]], [[magnesium]], [[calcium]], [[ytterbium]], and [[thorium]]. The concept of an optical clock originated with [[John L. Hall]] and [[Theodor W. Hansch]], who together won the 2005 Nobel Prize in Physics.

== Overview ==
The development of [[femtosecond]] [[frequency comb]]s, [[optical lattice]]s has led to a new generation of atomic clocks. These clocks are based on atomic transitions that emit visible [[light]] instead of [[microwaves]]. A major obstacle to developing an optical clock is the difficulty of directly measuring optical frequencies. This problem has been solved with the development of self-referenced mode-locked lasers, commonly referred to as femtosecond frequency combs.<ref>{{Cite web |date=18 December 2009 |title=Femtosecond-Laser Frequency Combs for Optical Clocks |url=https://www.nist.gov/programs-projects/femtosecond-laser-frequency-combs-optical-clocks |access-date=21 September 2016 |website=NIST |language=en}}</ref> Before the demonstration of the frequency comb in 2000, [[Terahertz radiation|terahertz]] techniques were needed to bridge the gap between radio and optical frequencies, and the systems for doing so were cumbersome and complicated. With the refinement of the frequency comb, these measurements have become much more accessible and numerous optical clock systems are now being developed around the world.<ref name=":3">{{Cite journal |last1=Fortier |first1=Tara |last2=Baumann |first2=Esther |date=6 December 2019 |title=20 years of developments in optical frequency comb technology and applications |url=https://www.nature.com/articles/s42005-019-0249-y |journal=Communications Physics |language=en |volume=2 |issue=1 |arxiv=1909.05384 |bibcode=2019CmPhy...2..153F |doi=10.1038/s42005-019-0249-y |issn=2399-3650 |s2cid=202565677 |article-number=153}}</ref>

=== Spectroscopy ===
As in the radio range, absorption spectroscopy is used to stabilize an oscillator—in this case, a laser. When the optical frequency is divided down into a countable radio frequency using a [[femtosecond comb]], the [[bandwidth (signal processing)|bandwidth]] of the [[phase noise]] is also divided by that factor. Although the bandwidth of laser phase noise is generally greater than stable microwave sources, after division it is less.<ref name=":3" />

=== Systems used ===
The primary systems under consideration for use in optical frequency standards are:
* single ions isolated in an ion trap;<ref>{{Cite book |last1=Zuo |first1=Yani |title=2021 IEEE 6th Optoelectronics Global Conference (OGC) |last2=Dai |first2=Shaoyao |last3=Chen |first3=Shiying |publisher=IEEE |year=2021 |isbn=978-1-6654-3194-1 |pages=92–95 |language=en |chapter=Towards a High-Performance Optical Clock Based on Single 171-Yb Ion |doi=10.1109/OGC52961.2021.9654373 |chapter-url=https://ieeexplore.ieee.org/document/9654373 |s2cid=245520666}}</ref>
* neutral atoms trapped in an optical lattice and<ref name="saoc">{{Cite journal |author=Oskay |first=W. H. |display-authors=etal |year=2006 |title=Single-atom optical clock with high accuracy |url=http://www.boulder.nist.gov/timefreq/general/pdf/2096.pdf |url-status=dead |journal=[[Physical Review Letters]] |volume=97 |issue=2 |bibcode=2006PhRvL..97b0801O |doi=10.1103/PhysRevLett.97.020801 |pmid=16907426 |archive-url=https://wayback.archive-it.org/all/20070417220053/http://www.boulder.nist.gov/timefreq/general/pdf/2096.pdf |archive-date=17 April 2007 |article-number=020801}}</ref><ref>{{cite web |author=Riehle |first=Fritz |title=On Secondary Representations of the Second |url=http://www.ursi.org/Proceedings/ProcGA08/papers/A01p2.pdf |url-status=dead |archive-url=https://web.archive.org/web/20150623002643/http://www.ursi.org/Proceedings/ProcGA08/papers/A01p2.pdf |archive-date=23 June 2015 |access-date=22 June 2015 |website=Physikalisch-Technische Bundesanstalt, Division Optics}}</ref>
* atoms packed in a three-dimensional quantum gas optical lattice.<ref name="wired.co.uk">{{Cite magazine |title=The most accurate clock ever made runs on quantum gas |url=https://www.wired.co.uk/article/quantum-gas-atomic-clocks-measure-time |access-date=11 February 2022 |magazine=Wired UK |language=en-GB |issn=1357-0978}}</ref>

These techniques allow the atoms or ions to be highly isolated from external perturbations, thus producing an extremely stable frequency reference.<ref name="wired.co.uk" /><ref>{{Cite arXiv |eprint=2004.09987 |class=physics.atom-ph |first=Bonnie L. |last=Schmittberger |title=A Review of Contemporary Atomic Frequency Standards |date=21 April 2020 |pages=13}}</ref> [[Lasers]] and [[magneto-optical trap]]s are used to cool the atoms for improved precision.<ref>{{Cite journal |last1=Golovizin |first1=A. |last2=Tregubov |first2=D. |last3=Mishin |first3=D. |last4=Provorchenko |first4=D. |last5=Kolachevsky |first5=N. |last6=Kolachevsky |first6=N. |date=25 October 2021 |title=Compact magneto-optical trap of thulium atoms for a transportable optical clock |url=https://opg.optica.org/oe/abstract.cfm?uri=oe-29-22-36734 |journal=Optics Express |language=EN |volume=29 |issue=22 |pages=36734–36744 |bibcode=2021OExpr..2936734G |doi=10.1364/OE.435105 |issn=1094-4087 |pmid=34809077 |s2cid=239652525 |doi-access=free}}</ref>

=== Atoms used ===
[[File:Ytterbium Lattice Atomic Clock (10444764266).jpg|thumb|upright=1.2|One of [[NIST]]'s 2013 pair of ytterbium optical lattice atomic clocks]]

Atomic systems under consideration include [[Aluminium|Al]]<sup>+</sup>, Hg<sup>+/2+</sup>,<ref name="saoc" /> [[Mercury (element)|Hg]], [[Strontium|Sr]], Sr<sup>+/2+</sup>, [[Indium|In]]<sup>+/3+</sup>, [[Magnesium|Mg]], [[Calcium|Ca]], Ca<sup>+</sup>, Yb<sup>+/2+/3+</sup>, [[Ytterbium|Yb]] and [[Thorium|Th]]<sup>+/3+</sup>.<ref>{{Cite web |title=<sup>171</sup>Ytterbium BIPM document |url=http://www.bipm.org/utils/common/pdf/mep/171Yb_518THz_2013.pdf |url-status=live |archive-url=https://web.archive.org/web/20150627191522/http://www.bipm.org/utils/common/pdf/mep/171Yb_518THz_2013.pdf |archive-date=27 June 2015 |access-date=26 June 2015}}</ref><ref>{{Cite web |title=PTB Time and Frequency Department 4.4 |url=https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44.html |url-status=live |archive-url=https://web.archive.org/web/20171107030323/https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44.html |archive-date=7 November 2017 |access-date=3 November 2017}}</ref><ref>{{Cite web |title=PTB Optical nuclear spectroscopy of <sup>229</sup>Th |url=https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-443/optical-nuclear-spectroscopy-of-229th.html |url-status=live |archive-url=https://web.archive.org/web/20171107012256/https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-443/optical-nuclear-spectroscopy-of-229th.html |archive-date=7 November 2017 |access-date=3 November 2017}}</ref> The color of a clock's [[electromagnetic radiation]] depends on the element that is stimulated. For example, calcium optical clocks resonate when red light is produced, and ytterbium clocks resonate in the presence of violet light.<ref>{{Cite magazine |last=Norton |first=Quinn |title=How Super-Precise Atomic Clocks Will Change the World in a Decade |url=https://www.wired.com/2007/12/time-nist/ |access-date=15 February 2022 |magazine=Wired |language=en-US |issn=1059-1028}}</ref>

The rare-earth element ytterbium (Yb) is valued not so much for its mechanical properties but for its complement of internal energy levels. "A particular transition in Yb atoms, at a wavelength of 578 nm, currently provides one of the world's most accurate optical atomic frequency standards," said Marianna Safronova.<ref name="Marianna Safronova">{{cite web |title=Blackbody Radiation Shift: Quantum Thermodynamics Will Redefine Clocks |url=http://www.science20.com/news_articles/blackbody_radiation_shift_quantum_thermodynamics_will_redefine_clocks-98064 |url-status=live |archive-url=https://web.archive.org/web/20121218160616/http://www.science20.com/news_articles/blackbody_radiation_shift_quantum_thermodynamics_will_redefine_clocks-98064 |archive-date=18 December 2012 |access-date=5 December 2012}}</ref> The estimated uncertainty achieved corresponds to about one second over the lifetime of the universe so far, 15 billion years, according to scientists at the Joint Quantum Institute (JQI) and the [[University of Delaware]] in December 2012.<ref>{{Cite web |date=5 December 2012 |title=Ytterbium in quantum gases and atomic clocks: van der Waals interactions and blackbody shifts |url=https://jqi.umd.edu/pfc/news/reports/ytterbium-quantum-gases-and-atomic-clocks-van-der-waals-interactions-and-blackbody |access-date=11 February 2022 |website=Joint Quantum Institute |language=en}}</ref>

== History ==

=== 20th century ===
[[File:JILA's strontium optical atomic clock.jpg|thumb|May 2009– [[JILA]]'s strontium optical atomic clock is based on neutral atoms. Shining a blue laser onto ultracold strontium atoms in an optical trap tests how efficiently a previous burst of light from a red laser has boosted the atoms to an excited state. Only those atoms that remain in the lower energy state respond to the blue laser, causing the fluorescence seen here.<ref name="opticalatomic1">{{cite web |author=Lindley |first=D. |date=20 May 2009 |title=Coping With Unusual Atomic Collisions Makes an Atomic Clock More Accurate |url=https://www.nsf.gov/discoveries/disc_summ.jsp?org=DMR&cntn_id=114850&preview=false |url-status=live |archive-url=https://web.archive.org/web/20110605190933/http://www.nsf.gov/discoveries/disc_summ.jsp?org=DMR&cntn_id=114850&preview=false |archive-date=5 June 2011 |access-date=10 July 2009 |publisher=[[National Science Foundation]]}}</ref>]]

The idea of trapping atoms in an optical lattice using [[laser]]s was proposed by Russian physicist [[Vladilen Letokhov]] in the 1960s.<ref>{{Cite journal|last=sarah.henderson@nist.gov|date=29 September 2020|title=Optical Lattices: Webs of Light|url=https://www.nist.gov/topics/physics/what-are-optical-lattices|access-date=14 February 2022|journal=NIST|language=en}}</ref> 

=== 2000s ===
The theoretical move from microwaves as the atomic "escapement" for clocks to light in the optical range, harder to measure but offering better performance, earned [[John L. Hall]] and [[Theodor W. Hänsch]] the [[Nobel Prize in Physics]] in 2005. One of 2012's Physics Nobelists, [[David J. Wineland]], is a pioneer in exploiting the properties of a single ion held in a trap to develop clocks of the highest stability.<ref>{{Cite journal|date=3 March 2017|title=The Prize's Legacy: Dave Wineland|url=https://www.nist.gov/nist-and-nobel/dave-wineland/prizes-legacy-dave-wineland|access-date=11 February 2022|journal=NIST}}</ref> The development of the first optical clock was started at the [[National Institute of Standards and Technology]] in 2000 and finished in 2006.<ref>{{Cite journal|date=29 September 2020|title=Optical Lattices: Webs of Light|url=https://www.nist.gov/topics/physics/what-are-optical-lattices|access-date=16 February 2022|journal=NIST|language=en}}</ref> 

=== 2010s ===
In 2013 optical lattice clocks (OLCs) were shown to be as good as or better than caesium fountain clocks. Two optical lattice clocks containing about {{val|10000|u=atoms}} of [[strontium-87]] were able to stay in synchrony with each other at a precision of at least {{val|1.5|e=-16}}, which is as accurate as the experiment could measure.<ref>{{cite journal|last=Ost|first=Laura|date=22 January 2014|title=JILA Strontium Atomic Clock Sets New Records in Both Precision and Stability|url=https://www.nist.gov/pml/div689/20140122_strontium.cfm|url-status=live|archive-url=https://web.archive.org/web/20141208224048/http://www.nist.gov/pml/div689/20140122_strontium.cfm|archive-date=8 December 2014|access-date=5 December 2014|journal=NIST|publisher=National Institute of Standards and Technology}}</ref> These clocks have been shown to keep pace with all three of the caesium fountain clocks at the [[Paris Observatory]]. There are two reasons for the possibly better precision. Firstly, the frequency is measured using light, which has a much higher frequency than microwaves, and secondly, by using many atoms, any errors are averaged.<ref name="strontclk">{{cite journal|last1=Ball|first1=Philip|date=9 July 2013|title=Precise atomic clock may redefine time|url=http://www.nature.com/news/precise-atomic-clock-may-redefine-time-1.13363|url-status=live|journal=Nature|doi=10.1038/nature.2013.13363|archive-url=https://web.archive.org/web/20130825114343/http://www.nature.com/news/precise-atomic-clock-may-redefine-time-1.13363|archive-date=25 August 2013|access-date=24 August 2013|s2cid=124850552}}</ref>

Using [[ytterbium-171]] atoms, a new record for stability with a precision of {{val|1.6|e=-18}}<!-- stable to within less than two parts in 10^-18 --> over a 7-hour period was published on 22 August 2013. At this stability, the two optical lattice clocks working independently from each other used by the [[National Institute of Standards and Technology|NIST]] research team would differ less than a second over the [[age of the universe]] ({{val|13.8|e=9|u=years}}); this was {{nowrap|10 times}} better than previous experiments. The clocks rely on {{nowrap|10 000 ytterbium}} atoms cooled to {{nowrap|10 microkelvin}} and trapped in an optical lattice. A laser at {{nowrap|578 nm}} excites the atoms between two of their energy levels.<ref name="ytterclk">{{cite journal|date=22 August 2013|title=NIST Ytterbium Atomic Clocks Set Record for Stability|journal=NIST |url=https://www.nist.gov/pml/div688/clock-082213.cfm|url-status=live|archive-url=https://web.archive.org/web/20130823012832/http://www.nist.gov/pml/div688/clock-082213.cfm|archive-date=23 August 2013|access-date=24 August 2013}}</ref> Having established the stability of the clocks, the researchers are studying external influences and evaluating the remaining systematic uncertainties, in the hope that they can bring the clock's accuracy down to the level of its stability.<ref name="ytterclk2">{{cite web|date=27 August 2013|title=New atomic clock sets the record for stability|url=http://physicsworld.com/cws/article/news/2013/aug/27/new-atomic-clock-sets-the-record-for-stability|url-status=live|archive-url=https://web.archive.org/web/20140202092612/http://physicsworld.com/cws/article/news/2013/aug/27/new-atomic-clock-sets-the-record-for-stability|archive-date=2 February 2014|access-date=19 January 2014}}</ref> An improved optical lattice clock was described in a 2014 Nature paper.<ref>{{Cite journal|last1=Bloom|first1=B. J.|last2=Nicholson|first2=T. L.|last3=Williams|first3=J. R.|last4=Campbell|first4=S. L.|last5=Bishof|first5=M.|last6=Zhang|first6=X.|last7=Zhang|first7=W.|last8=Bromley|first8=S. L.|last9=Ye|first9=J.|date=22 January 2014|title=An optical lattice clock with accuracy and stability at the 10<sup>−18</sup> level|url=https://jila.colorado.edu/yelabs/sites/default/files/uploads/Sr%20best%20clock_Bloom_Nature.pdf|url-status=live|journal=Nature|volume=506|issue=7486|pages=71–5|arxiv=1309.1137|bibcode=2014Natur.506...71B|doi=10.1038/nature12941|pmid=24463513|archive-url=https://web.archive.org/web/20160917074152/https://jila.colorado.edu/yelabs/sites/default/files/uploads/Sr%20best%20clock_Bloom_Nature.pdf|archive-date=17 September 2016|access-date=5 September 2016|s2cid=4461081}}</ref>

In 2015, [[JILA]] evaluated the absolute frequency uncertainty of a strontium-87 optical lattice clock at {{val|2.1|e=-18}}, which corresponds to a measurable [[gravitational time dilation]] for an elevation change of {{convert|2|cm|abbr=on}} on planet Earth that according to JILA/NIST Fellow [[Jun Ye]] is "getting really close to being useful for relativistic [[geodesy]]".<ref>{{cite journal |last1=Nicholson |first1=T. L. |last2=Campbell |first2=S. L. |last3=Hutson |first3=R. B. |last4=Marti |first4=G. E. |last5=Bloom |first5=B. J. |last6=McNally |first6=R. L. |last7=Zhang |first7=W. |last8=Barrett |first8=M. D. |last9=Safronova |first9=M. S. |last10=Strouse |first10=G. F. |last11=Tew |first11=W. L. |last12=Ye |first12=Jun |date=21 April 2015 |title=Systematic evaluation of an atomic clock at {{val|2|e=-18}} total uncertainty |journal=Nature Communications |volume=6 |issue=6896 |pages=6896 |arxiv=1412.8261 |bibcode=2015NatCo...6.6896N |doi=10.1038/ncomms7896 |pmc=4411304 |pmid=25898253}}</ref><ref>{{cite web|author=JILA Scientific Communications|date=21 April 2015|title=About Time|url=http://jila.colorado.edu/news-highlights/about-time|url-status=dead|archive-url=https://web.archive.org/web/20150919105141/https://jila.colorado.edu/news-highlights/about-time|archive-date=19 September 2015|access-date=27 June 2015}}</ref><ref>{{cite journal |author=Ost |first=Laura |date=21 April 2015 |title=Getting Better All the Time: JILA Strontium Atomic Clock Sets New Record |url=https://www.nist.gov/pml/div689/20150421_strontium_clock.cfm |url-status=live |journal=NIST |archive-url=https://web.archive.org/web/20151009082345/http://www.nist.gov/pml/div689/20150421_strontium_clock.cfm |archive-date=9 October 2015 |access-date=17 October 2015}}</ref> At this frequency uncertainty, this JILA optical lattice clock is expected to neither gain nor lose a second in more than 15 billion years.<ref>{{cite web |author=Vincent |first=James |date=22 April 2015 |title=The most accurate clock ever built only loses one second every 15 billion years |url=https://www.theverge.com/2015/4/22/8466681/most-accurate-atomic-clock-optical-lattice-strontium |url-status=live |archive-url=https://web.archive.org/web/20180127084115/https://www.theverge.com/2015/4/22/8466681/most-accurate-atomic-clock-optical-lattice-strontium |archive-date=27 January 2018 |access-date=26 June 2015 |website=The Verge}}</ref><ref>{{cite journal |last1=Huntemann |first1=N. |last2=Sanner |first2=C. |last3=Lipphardt |first3=B. |last4=Tamm |first4=Chr. |last5=Peik |first5=E. |date=8 February 2016 |title=Single-Ion Atomic Clock with {{val|3|e=-18}} Systematic Uncertainty |journal=Physical Review Letters |volume=116 |issue=6 |arxiv=1602.03908 |bibcode=2016PhRvL.116f3001H |doi=10.1103/PhysRevLett.116.063001 |pmid=26918984 |s2cid=19870627 |article-number=063001}}</ref>

[[File:17jila003 3d strontium atomic clock.jpg|thumb|JILA's 2017 three-dimensional (3-D) quantum gas atomic clock consists of a grid of light formed by three pairs of laser beams. A stack of two tables is used to configure optical components around a vacuum chamber. Shown here is the upper table, where lenses and other optics are mounted. A blue laser beam excites a cube-shaped cloud of strontium atoms located behind the round window in the middle of the table. Strontium atoms fluoresce strongly when excited with blue light.]]

In 2017 JILA reported an experimental 3D [[quantum gas]] strontium optical lattice clock in which strontium-87 atoms are packed into a tiny three-dimensional (3-D) cube at 1,000 times the density of previous one-dimensional (1-D) clocks, such as the 2015 JILA clock. A comparison between two regions of the same 3D lattice yielded a residual precision of {{val|5|e=-19}} in 1 hour of averaging time.<ref>{{cite journal |last1=Campbell |first1=S. L. |last2=Hutson |first2=R. B. |last3=Marti |first3=G. E. |last4=Goban |first4=A. |last5=Oppong |first5=N. Darkwah |last6=McNally |first6=R. L. |last7=Sonderhouse |first7=L. |last8=Zhang |first8=W. |last9=Bloom |first9=B. J. |last10=Ye |first10=J. |year=2017 |title=A Fermi-degenerate three-dimensional optical lattice clock |url=https://jila.colorado.edu/yelabs/sites/default/files/uploads/Fermi_degenerate_3d_clock_Science%202017.pdf |url-status=dead |journal=[[Science (journal)|Science]] |volume=358 |issue=6359 |pages=90–94 |arxiv=1702.01210 |bibcode=2017Sci...358...90C |doi=10.1126/science.aam5538 |pmid=28983047 |s2cid=206656201 |archive-url=https://web.archive.org/web/20191215091444/https://jila.colorado.edu/yelabs/sites/default/files/uploads/Fermi_degenerate_3d_clock_Science%202017.pdf |archive-date=15 December 2019 |access-date=29 March 2017}}</ref> This precision value does not represent the absolute accuracy or precision of the clock, which remain above {{val|1|e=-18}} and {{val|1|e=-17}} respectively. The 3D quantum gas strontium optical lattice clock's centerpiece is an unusual state of matter called a [[degenerate matter|degenerate]] [[Fermi gas]] (a quantum gas for Fermi particles). The experimental data shows the 3D quantum gas clock achieved a residual precision of {{val|3.5|e=-19}} in about two hours. According to Jun Ye, "this represents a significant improvement over any previous demonstrations". Ye further commented "the most important potential of the 3D quantum gas clock is the ability to scale up the atom numbers, which will lead to a huge gain in stability" and "the ability to scale up both the atom number and [[coherence time]] will make this new-generation clock qualitatively different from the previous generation".<ref>{{cite magazine|last1=Beall|first1=Abigail|date=5 October 2017|title=A Fermi-degenerate three-dimensional optical lattice clock|url=https://www.wired.co.uk/article/quantum-gas-atomic-clocks-measure-time|url-status=live|magazine=[[Wired UK]]|archive-url=https://web.archive.org/web/20171006001050/http://www.wired.co.uk/article/quantum-gas-atomic-clocks-measure-time|archive-date=6 October 2017|access-date=29 March 2017}}</ref><ref>{{cite press release|publisher=NIST|date=5 October 2017|title=JILA's 3-D Quantum Gas Atomic Clock Offers New Dimensions in Measurement|url=https://www.nist.gov/news-events/news/2017/10/jilas-3-d-quantum-gas-atomic-clock-offers-new-dimensions-measurement|url-status=live |archive-url=https://web.archive.org/web/20171005224415/https://www.nist.gov/news-events/news/2017/10/jilas-3-d-quantum-gas-atomic-clock-offers-new-dimensions-measurement|archive-date=5 October 2017|access-date=29 March 2017}}</ref><ref>{{cite journal|last1=Phillips|first1=Julie|date=10 October 2017|title=The Clock that Changed the World|url=http://jilawww.colorado.edu/yelabs/news/clock-changed-world|url-status=live|journal=JILA|archive-url=https://web.archive.org/web/20171214074440/http://jilawww.colorado.edu/yelabs/news/clock-changed-world|archive-date=14 December 2017|access-date=30 March 2017}}</ref>

In 2018, JILA reported the 3D quantum gas clock reached a residual frequency precision of {{val|2.5|e=-19}} over 6 hours.<ref>{{cite journal |last1=Marti |first1=G. Edward |last2=Hutson |first2=Ross B. |last3=Goban |first3=Akihisa |last4=Campbell |first4=Sara L. |last5=Poli |first5=Nicola |last6=Ye |first6=Jun |year=2018 |title=Imaging Optical Frequencies with 100&nbsp;μHz Precision and 1.1&nbsp;μm Resolution |url=https://jila.colorado.edu/yelabs/sites/default/files/uploads/PRL.120.103201.ClockImaging.pdf |url-status=live |journal=[[Physical Review Letters]] |volume=120 |issue=10 |pages=103201 |arxiv=1711.08540 |bibcode=2018PhRvL.120j3201M |doi=10.1103/PhysRevLett.120.103201 |pmid=29570334 |s2cid=3763878 |archive-url=https://web.archive.org/web/20200602004751/https://jila.colorado.edu/yelabs/sites/default/files/uploads/PRL.120.103201.ClockImaging.pdf |archive-date=2 June 2020 |access-date=30 March 2017}}</ref><ref>{{cite journal|last1=Ost|first1=Laura|date=5 March 2018|title=JILA Team Invents New Way to 'See' the Quantum World|url=https://www.nist.gov/news-events/news/2018/03/jila-team-invents-new-way-see-quantum-world|url-status=live|journal=JILA|archive-url=https://web.archive.org/web/20190517092816/https://www.nist.gov/news-events/news/2018/03/jila-team-invents-new-way-see-quantum-world|archive-date=17 May 2019|access-date=30 March 2017}}</ref> Recently it has been proved that the [[quantum entanglement]] can help to further enhance the clock stability.<ref>{{cite journal|last1=Pedrozo-Peñafiel|first1=Edwin|last2=Colombo|first2=Simone|last3=Shu|first3=Chi|last4=Adiyatullin|first4=Albert F.|last5=Li|first5=Zeyang|last6=Mendez|first6=Enrique|last7=Braverman|first7=Boris|last8=Kawasaki|first8=Akio|last9=Akamatsu|first9=Daisuke|last10=Xiao|first10=Yanhong|last11=Vuletić|first11=Vladan|date=16 December 2020|title=Entanglement on an optical atomic-clock transition|url=https://www.nature.com/articles/s41586-020-3006-1|url-status=live|journal=Nature|volume=588|issue=7838|pages=414–418|arxiv=2006.07501|bibcode=2020Natur.588..414P|doi=10.1038/s41586-020-3006-1|pmid=33328668|archive-url=https://web.archive.org/web/20210204145358/https://www.nature.com/articles/s41586-020-3006-1|archive-date=4 February 2021|access-date=16 February 2021|s2cid=229300882}}</ref>

=== 2020s ===
In 2020 optical clocks were researched for space applications like future generations of [[global navigation satellite systems]] (GNSSs) as replacements for microwave based clocks.<ref>{{cite journal|last1=Schuldt|first1=Thilo|last2=Gohlke|first2=Martin|last3=Oswald|first3=Markus|last4=Wüst|first4=Jan|last5=Blomberg|first5=Tim|last6=Döringshoff|first6=Klaus|last7=Bawamia|first7=Ahmad|last8=Wicht|first8=Andreas|last9=Lezius|first9=Matthias|last10=Voss|first10=Kai|last11=Krutzik|first11=Markus|date=July 2021|title=Optical clock technologies for global navigation satellite systems|url=https://elib.dlr.de/141236/1/Schuldt2021_Article_OpticalClockTechnologiesForGlo.pdf|journal=GPS Solutions|volume=25|issue=3|pages=83|doi=10.1007/s10291-021-01113-2|first15=Claus|last15=Braxmaier|first14=Achim|last14=Peters|first13=Evgeny|last13=Kovalchuk|first12=Sven|last12=Herrmann|bibcode=2021GPSS...25...83S |s2cid=233030680}}</ref> Ye's strontium-87 clock has not surpassed the aluminum-27<ref name="journals.aps.org">{{Cite journal|last1=Brewer|first1=S. M.|last2=Chen|first2=J.-S.|last3=Hankin|first3=A. M.|last4=Clements|first4=E. R.|last5=Chou|first5=C. W.|last6=Wineland|first6=D. J.|last7=Hume|first7=D. B.|last8=Leibrandt|first8=D. R.|date=15 July 2019|title=Al+27 Quantum-Logic Clock with a Systematic Uncertainty below {{val|e=-18}}|url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.033201|journal=Physical Review Letters|volume=123|issue=3|article-number=033201|doi=10.1103/physrevlett.123.033201|pmid=31386450|arxiv=1902.07694 |s2cid=119075546|issn=0031-9007}}</ref> or ytterbium-171<ref>{{Cite journal|last1=McGrew|first1=W. F.|last2=Zhang|first2=X.|last3=Fasano|first3=R. J.|last4=Schaffer|first4=S. A.|last5=Beloy|first5=K.|last6=Nicolodi|first6=D.|last7=Brown|first7=R. C.|last8=Hinkley|first8=N.|last9=Milani|first9=G.|last10=Schioppo|first10=M.|last11=Yoon|first11=T. H.|last12=Ludlow|first12=A. D.|date=6 December 2018|title=Atomic clock performance enabling geodesy below the centimetre level|url=https://www.nature.com/articles/s41586-018-0738-2|journal=Nature|volume=564|issue=7734 |pages=87–90 |doi=10.1038/s41586-018-0738-2|pmid=30487601 |arxiv=1807.11282|bibcode=2018Natur.564...87M }}</ref> optical clocks in terms of frequency accuracy.

See <ref>{{Cite journal |last1=Diddams |first1=Scott A. |last2=Vahala |first2=Kerry |last3=Udem |first3=Thomas |date=2020-07-17 |title=Optical frequency combs: Coherently uniting the electromagnetic spectrum |url=https://www.science.org/doi/10.1126/science.aay3676 |journal=Science |language=en |volume=369 |issue=6501 |page=367 |bibcode=2020Sci...369..367D |doi=10.1126/science.aay3676 |issn=0036-8075 |pmid=32675346}}</ref> for a review up to 2020.

In February 2022, scientists at the [[University of Wisconsin-Madison]] reported a "[[multiplexed]]" optical atomic clock, where individual clocks deviated from each other with an accuracy equivalent to losing a second in 300 billion years. The reported minor deviation is explainable as the concerned clock oscillators are in slightly different environments. These are causing differing reactions to gravity, magnetic fields, or other conditions. This miniaturized clock network approach is novel in that it uses an optical lattice of strontium atoms and a configuration of six clocks that can be used to demonstrate relative stability, fractional uncertainty between clocks and methods for ultra-high-precision comparisons between optical atomic clock ensembles that are located close together in a metrology facility.<ref name="auto">{{cite web|last=University of Wisconsin-Madison|title=Ultraprecise atomic clock poised for new physics discoveries|url=https://www.eurekalert.org/news-releases/943211}}</ref><ref>{{Cite journal |last1=Zheng |first1=Xin |last2=Dolde |first2=Jonathan |last3=Lochab |first3=Varun |last4=Merriman |first4=Brett N. |last5=Li |first5=Haoran |last6=Kolkowitz |first6=Shimon |year=2022 |title=Differential clock comparisons with a multiplexed optical lattice clock |url=https://www.nature.com/articles/s41586-021-04344-y |journal=Nature |volume=602 |issue=7897 |pages=425–430 |doi=10.1038/s41586-021-04344-y|pmid=35173344 |arxiv=2109.12237 |bibcode=2022Natur.602..425Z |s2cid=237940240 }}</ref>

As of 2022, optical clocks are primarily research projects, and less mature than rubidium and caesium microwave standards, which regularly deliver time to the [[International Bureau of Weights and Measures]] (BIPM) for establishing [[International Atomic Time|International Atomic Time (TAI)]].<ref>{{cite web|title=BIPM Time Coordinated Universal Time (UTC)|url=http://www.bipm.org/en/scientific/tai/|url-status=live|archive-url=https://web.archive.org/web/20131104023703/http://www.bipm.org/en/scientific/tai/|archive-date=4 November 2013|access-date=29 December 2013|publisher=BIPM}}</ref> As the optical experimental clocks move beyond their microwave counterparts in terms of accuracy and stability performance, this puts them in a position to replace the current standard for time, the caesium fountain clock.<ref name="saoc" /><ref>{{cite journal |last1=Poli |first1=N. |last2=Oates |first2=C. W. |last3=Gill |first3=P. |last4=Tino |first4=G. M. |date=13 January 2014 |title=Optical atomic clocks |journal=Rivista del Nuovo Cimento |volume=36 |issue=12 |pages=555–624 |arxiv=1401.2378 |bibcode=2013NCimR..36..555P |doi=10.1393/ncr/i2013-10095-x |s2cid=118430700}}</ref> In the future this might lead to redefining the caesium microwave-based SI second, and other new dissemination techniques at the highest level of accuracy to transfer clock signals will be required that can be used in both shorter-range and longer-range (frequency) comparisons between better clocks and to explore their fundamental limitations without significantly compromising their performance.<ref name="saoc" /><ref>{{cite web|title=BIPM work programme: Time|url=http://www.bipm.org/en/bipm/tai/|url-status=live|archive-url=https://web.archive.org/web/20150626102802/http://www.bipm.org/en/bipm/tai/|archive-date=26 June 2015|access-date=25 June 2015|publisher=BIPM}}</ref><ref>{{cite journal|author=Margolis|first=Helen|author-link=Helen Margolis|date=12 January 2014|title=Timekeepers of the future|journal=Nature Physics|volume=10|issue=2|pages=82–83|bibcode=2014NatPh..10...82M|doi=10.1038/nphys2834|s2cid=119938546 }}</ref><ref>{{cite journal|last1=Grebing|first1=Christian|last2=Al-Masoudi|first2=Ali|last3=Dörscher|first3=Sören|last4=Häfner|first4=Sebastian|last5=Gerginov|first5=Vladislav|last6=Weyers|first6=Stefan|last7=Lipphardt|first7=Burghard|last8=Riehle|first8=Fritz|last9=Sterr|first9=Uwe|last10=Lisdat|first10=Christian|year=2016|title=Realization of a timescale with an accurate optical lattice clock|journal=Optica|volume=3|issue=6|pages=563–569|arxiv=1511.03888|bibcode=2016Optic...3..563G|doi=10.1364/OPTICA.3.000563|s2cid=119112716}}</ref><ref>{{cite journal|last1=Ludlow|first1=Andrew D|last2=Boyd|first2=Martin M|last3=Ye|first3=Jun|last4=Peik|first4=Ekkehard|last5=Schmidt|first5=Piet O|year=2015|title=Optical atomic clocks|journal=Reviews of Modern Physics|volume=87|issue=2|pages=673|arxiv=1407.3493|bibcode=2015RvMP...87..637L|doi=10.1103/RevModPhys.87.637|s2cid=119116973}}</ref> The BIPM reported in December 2021 based on the progress of optical standards contributing to TAI the Consultative Committee for Time and Frequency (CCTF) initiated work towards a redefinition of the second expected during the 2030s.<ref>{{cite web|title=BIPM work programme: Time|url=https://www.bipm.org/en/-/2021-12-21-record-tai |access-date=20 February 2022|publisher=BIPM}}</ref>

In July 2022, atomic optical clocks based on [[iodine]] molecules were demonstrated at-sea on a naval vessel and operated continuously in the Pacific Ocean for 20 days in the [[Exercise RIMPAC]] 2022.<ref>{{Cite journal |date=August 23, 2023 |title=Optical Clocks at Sea |arxiv=2308.12457 |last1=Roslund |first1=Jonathan D. |last2=Cingöz |first2=Arman |last3=Lunden |first3=William D. |last4=Partridge |first4=Guthrie B. |last5=Kowligy |first5=Abijith S. |last6=Roller |first6=Frank |last7=Sheredy |first7=Daniel B. |last8=Skulason |first8=Gunnar E. |last9=Song |first9=Joe P. |last10=Abo-Shaeer |first10=Jamil R. |last11=Boyd |first11=Martin M. |journal=Nature |volume=628 |issue=8009 |pages=736–740 |doi=10.1038/s41586-024-07225-2 |pmid=38658684 |pmc=11043038 |bibcode=2024Natur.628..736R }}</ref> These technologies originally funded by the [[United States Department of Defense|U.S. Department of Defense]] have led to the world's first commercial rackmount optical clock in November 2023.<ref>{{Cite web |date=2023-11-13 |title=Vector Atomic brings world's first rackmount optical clock to market |url=https://www.businesswire.com/news/home/20231113157771/en/Vector-Atomic-brings-world%E2%80%99s-first-rackmount-optical-clock-to-market |access-date=2023-11-23 |website=www.businesswire.com |language=en}}</ref>

== See also ==
{{Portal|Electronics}}
* [[Atomic clock]]

== References ==
{{Reflist}}

[[Category:Clocks]]