atomic clock

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atomic clock
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{{About||a clock updated by radio signals which is sometimes incorrectly called an "atomic clock"|Radio clock|the clock as a measure for risk of catastrophic destruction|Doomsday Clock|other topics|Atomic Clock (disambiguation)}}{{Use dmy dates|date=June 2015}}

File:Usno-mc.jpg|thumb|right|The master atomic clock ensemble at the U.S. Naval Observatory in Washington, D.C., which provides the time standard for the U.S. Department of Defense.USNO Master Clock The rack mounted units in the background are MicrosemiMicrosemiAn atomic clock is a clock device that uses a hyperfine transition frequency in the microwave, or electron transition frequency in the optical, or ultraviolet regionBOOK, TIME from Earth Rotation to Atomic Physics, McCarthy, Dennis, Dennis McCarthy (scientist), Seidelmann, P. Kenneth, ch. 10 & 11, Weinheim, Wiley-VCHelectromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time standard>time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS.The principle of operation of an atomic clock is based on atomic physics; it measures the electromagnetic signal that electrons in atoms emit when they change energy levels. Early atomic clocks were based on masers at room temperature. Since 2004, more accurate atomic clocks first cool the atoms to near absolute zero temperature by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity. An example of this is the NIST-F1 atomic clock, one of the national primary time and frequency standards of the United States.The accuracy of an atomic clock depends on two factors: the first is temperature of the sample atoms—colder atoms move much more slowly, allowing longer probe times, the second is the frequency and intrinsic linewidth of the electronic or hyperfine transition. Higher frequencies and narrow lines increase the precision.National standards agencies in many countries maintain a network of atomic clocks which are intercompared and kept synchronized to an accuracy of 10−9 seconds per day (approximately 1 part in 1014). These clocks collectively define a continuous and stable time scale, the International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but has added leap seconds from UT1, to account for variations in the rotation of the Earth with respect to the solar time.


File:Atomic Clock-Louis Essen.jpg|thumb|Louis EssenLouis EssenThe idea of using atomic transitions to measure time was suggested by Lord Kelvin in 1879.BOOK, William, Thomson, Lord Kelvin, Peter Guthrie, Tait, Treatise on Natural Philosophy, 2nd, Cambridge, England, Cambridge University Press, 1879, 1, part 1, 227,weblink Magnetic resonance, developed in the 1930s by Isidor Rabi, became the practical method for doing this.JOURNAL
, M.A. Lombardi, T.P. Heavner, S.R. Jefferts, 2007
, NIST Primary Frequency Standards and the Realization of the SI Second
, Journal of Measurement Science
, 2, 4, 74
, In 1945, Rabi first publicly suggested that atomic beam magnetic resonance might be used as the basis of a clock.See:
  • Isidor I. Rabi, "Radiofrequency spectroscopy" (Richtmyer Memorial Lecture, delivered at Columbia University in New York, New York, on 20 January 1945).
  • "Meeting at New York, January 19 and 20, 1945" Physical Review, vol. 67, pages 199-204 (1945).
  • JOURNAL, Laurence, William L., 2007, NIST primary frequency standards and the realization of the SI second,weblink NCSLI Measure, 2, 4, 74–89, 10.1080/19315775.2007.11721402, The first atomic clock was an ammonia absorption line device at 23870.1 MHz built in 1949 at the U.S. National Bureau of Standards (NBS, now NIST). It was less accurate than existing quartz clocks, but served to demonstrate the concept.
, D.B. Sullivan
, 2001
, Time and frequency measurement at NIST: The first 100 years
, 2001 IEEE International Frequency Control Symposium
, 4–17
, National Institute of Standards and Technology, NIST
, The first accurate atomic clock, a caesium standard based on a certain transition of the caesium-133 atom, was built by Louis Essen and Jack Parry in 1955 at the National Physical Laboratory in the UK.JOURNAL, Essen, L., Louis Essen, Parry, J. V. L., 10.1038/176280a0, 1955Natur.176..280E, An Atomic Standard of Frequency and Time Interval: A Cæsium Resonator, Nature, 176, 4476, 280–282, 1955, WEB,weblink 60 years of the Atomic Clock, National Physical Laboratory, 2017-10-17, Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time (ET).JOURNAL
, W. Markowitz, R.G. Hall, L. Essen, J.V.L. Parry, 1958
, Frequency of cesium in terms of ephemeris time
, Physical Review Letters
, 1, 3
, 105–107
, 10.1103/PhysRevLett.1.105, 1958PhRvL...1..105M
, In 1967, this led the scientific community to redefine the Second in terms of a specific atomic frequency. Equality of the ET second with the (atomic clock) SI second has been verified to within 1 part in 1010.CONFERENCE
, W. Markowitz
, 1988
, Comparisons of ET(Solar), ET(Lunar), UT and TDT'
, A.K. Babcock, G.A. Wilkins, The Earth's Rotation and Reference Frames for Geodesy and Geophysics, International Astronomical Union Symposia #128
, 413–418
, . Pages 413–414, gives the information that the SI second was made equal to the second of ephemeris time as determined from lunar observations, and was later verified in this relation, to 1 part in 1010. The SI second thus inherits the effect of decisions by the original designers of the ephemeris time scale, determining the length of the ET second.Since the beginning of development in the 1950s, atomic clocks have been based on the hyperfine transitions in hydrogen-1, caesium-133, and rubidium-87. The first commercial atomic clock was the Atomichron, manufactured by the National Company. More than 50 were sold between 1956 and 1960. This bulky and expensive instrument was subsequently replaced by much smaller rack-mountable devices, such as the Hewlett-Packard model 5060 caesium frequency standard, released in 1964.In the late 1990s four factors contributed to major advances in clocks:JOURNAL, J. Ye, H. Schnatz, L.W. Hollberg, 2003, Optical frequency combs: From frequency metrology to optical phase control,weblink IEEE Journal of Selected Topics in Quantum Electronics, 9, 4, 1041, 10.1109/JSTQE.2003.819109, File:ChipScaleClock2 HR.jpg|thumb|Chip-scale atomic clocks, such as this one unveiled in 2004, are expected to greatly improve GPS location.]]In August 2004, NIST scientists demonstrated a chip-scale atomic clock.WEB
, 2007
, Chip-Scale Atomic Devices at NIST
, National Institute of Standards and Technology, NIST
, 17 January 2008
,weblink" title="">weblink
, 7 January 2008
, yes
, dmy-all
, Available on-line at: According to the researchers, the clock was believed to be one-hundredth the size of any other. It requires no more than 125 mW,WEB
, 2011
, SA.45s CSAC Chip Scale Atomic Clock (archived version of the original pdf)
,weblink" title="">weblink
, yes
, 25 May 2013
, 12 June 2013
, making it suitable for battery-driven applications. This technology became available commercially in 2011. Ion trap experimental optical clocks are more precise than the current caesium standard.In April 2015, NASA announced that it planned to deploy a Deep Space Atomic Clock (DSAC), a miniaturized, ultra-precise mercury-ion atomic clock, into outer space. NASA said that the DSAC would be much more stable than other navigational clocks.WEB, Landau, Elizabeth, Deep Space Atomic Clock,weblink 27 April 2015, NASA, 29 April 2015,


{{more citations needed|section|date=October 2017}}Since 1967, the International System of Units (SI) has defined the second as the duration of {{gaps|9|192|631|770|u=cycles}} of radiation corresponding to the transition between two energy levels of the ground state of the caesium-133 atom. In 1997, the International Committee for Weights and Measures (CIPM) added that the preceding definition refers to a caesium atom at rest at a temperature of absolute zero.WEB
, 2006
, International System of Units (SI)
, International Bureau of Weights and Measures (BIPM)
, 8th
, This definition makes the caesium oscillator the primary standard for time and frequency measurements, called the caesium standard. The definitions of other physical units, e.g., the volt and the metre, rely on the definition of the second.
, 2007
, FAQs
,weblink" title="">weblink
, yes
, 17 December 2000
, Franklin Instrument Company
, 17 January 2008
, In this particular design, the time-reference of an atomic clock consists of an electronic oscillator operating at microwave frequency. The oscillator is arranged so that its frequency-determining components include an element that can be controlled by a feedback signal. The feedback signal keeps the oscillator tuned in resonance with the frequency of the hyperfine transition of caesium or rubidium.The core of the Radio frequency atomic clock is a tunable microwave cavity containing a gas. In a hydrogen maser clock the gas emits microwaves (the gas mases) on a hyperfine transition, the field in the cavity oscillates, and the cavity is tuned for maximum microwave amplitude. Alternatively, in a caesium or rubidium clock, the beam or gas absorbs microwaves and the cavity contains an electronic amplifier to make it oscillate. For both types the atoms in the gas are prepared in one hyperfine state prior to filling them into the cavity. For the second type the number of atoms which change hyperfine state is detected and the cavity is tuned for a maximum of detected state changes.Most of the complexity of the clock lies in this adjustment process. The adjustment tries to correct for unwanted side-effects, such as frequencies from other electron transitions, temperature changes, and the spreading in frequencies caused by ensemble effects.{{clarify|date=May 2014}} One way of doing this is to sweep the microwave oscillator's frequency across a narrow range to generate a modulated signal at the detector. The detector's signal can then be demodulated to apply feedback to control long-term drift in the radio frequency. In this way, the quantum-mechanical properties of the atomic transition frequency of the caesium can be used to tune the microwave oscillator to the same frequency, except for a small amount of experimental error. When a clock is first turned on, it takes a while for the oscillator to stabilize. In practice, the feedback and monitoring mechanism is much more complex.File:Clock accuracy.svg|thumb|right|Historical accuracy of atomic clocks from NISTNISTA number of other atomic clock schemes are in use for other purposes. Rubidium standard clocks are prized for their low cost, small size (commercial standards are as small as 17 cm3) and short-term stability. They are used in many commercial, portable and aerospace applications. Hydrogen masers (often manufactured in Russia) have superior short-term stability compared to other standards, but lower long-term accuracy.Often, one standard is used to fix another. For example, some commercial applications use a rubidium standard periodically corrected by a global positioning system receiver (see GPS disciplined oscillator). This achieves excellent short-term accuracy, with long-term accuracy equal to (and traceable to) the U.S. national time standards.The lifetime of a standard is an important practical issue. Modern rubidium standard tubes last more than ten years, and can cost as little as US$50.{{Citation needed|date=June 2009}} Caesium reference tubes suitable for national standards currently last about seven years and cost about US$35,000. The long-term stability of hydrogen maser standards decreases because of changes in the cavity's properties over time.Modern clocks use magneto-optical traps to cool the atoms for improved precision.

Power consumption

The power consumption of atomic clocks varies with their size. Atomic clocks on the scale of one chip require less than 30 milliwatt;JOURNAL, Lutwak, Robert, The Chip-Scale Atomic Clock â€” Prototype Evaluation, 36th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting, 26–29 November 2007, Primary frequency and time standards like the United States Time Standard atomic clocks, NIST-F1 and NIST-F2, use far higher power.WEB,weblink NIST Launches a New U.S. Time Standard: NIST-F2 Atomic Clock,,

Evaluated accuracy

The evaluated accuracy u'B reports of various primary frequency and time standards are published online by the International Bureau of Weights and Measures (BIPM). Several frequency and time standards groups as of 2015 reported u'B values in the {{nowrap|2 × 10−16}} to {{nowrap|3 × 10−16}} range.BIPM Annual Report on Time Activities, Volume 10, 2015, {{ISBN|978-92-822-2263-8}}, {{ISSN|1994-9405}}In 2011, the NPL-CsF2 caesium fountain clock operated by the National Physical Laboratory (NPL), which serves as the United Kingdom primary frequency and time standard, was improved regarding the two largest sources of measurement uncertainties â€” distributed cavity phase and microwave lensing frequency shifts. In 2011 this resulted in an evaluated frequency uncertainty reduction from u'B = {{nowrap|4.1 × 10−16}} to u'B = {{nowrap|2.3 × 10−16}};— the lowest value for any primary national standard at the time.Evaluation of the frequency of the H-maser 1401708 by the primary frequency standard NPL-CsF2, National Physical Laboratory, February 2010 At this frequency uncertainty, the NPL-CsF2 is expected to neither gain nor lose a second in about 138 million ({{nowrap|138 × 106}}) years.WEB,weblink NPL's atomic clock revealed to be the world's most accurate : News : News + Events : National Physical Laboratory,, WEB,weblink NPL-CsF2: now the atomic clock with the world's best long-term accuracy - Science Codex,, JOURNAL, 1107.2412, Li, Ruoxin, Improved accuracy of the NPL-CsF2 primary frequency standard: Evaluation of distributed cavity phase and microwave lensing frequency shifts, Metrologia, 48, 5, 283–289, Gibble, Kurt, Szymaniec, Krzysztof, 2011, 10.1088/0026-1394/48/5/007, 2011Metro..48..283L, (File:NIST-F2 cesium fountain atomic clock.jpg|thumb|NIST physicists Steve Jefferts (foreground) and Tom Heavner with the NIST-F2 caesium fountain atomic clock, a civilian time standard for the United States.)The NIST-F2 caesium fountain clock operated by the National Institute of Standards and Technology (NIST), was officially launched in April 2014, to serve as a new U.S. civilian frequency and time standard, along with the NIST-F1 standard. The planned uB performance level of NIST-F2 is {{nowrap|1 × 10−16}}.JOURNAL, S.R. Jefferts, T.P. Heavner, T.E. Parker, J.H. Shirley, 2007, Acta Physica Polonica A, 112, 5, 759 ff,weblink NIST Cesium Fountains − Current Status and Future Prospects, 2007AcPPA.112..759J, 10.12693/APhysPolA.112.759, "At this planned performance level the NIST-F2 clock will not lose a second in at least 300 million years."JOURNAL, New Scientist, 12 April 2014, 7, Time gets an upgrade, NIST-F2 was designed using lessons learned from NIST-F1. The NIST-F2 key advance compared to the NIST-F1 is that the vertical flight tube is now chilled inside a container of liquid nitrogen, at {{convert|-193|C|F}}. This cycled cooling dramatically lowers the background radiation and thus reduces some of the very small measurement errors that must be corrected in NIST-F1.WEB,weblink NIST launches a new US time standard: NIST-F2 atomic clock, 3 April 2014,, 3 April 2014, WEB,weblink Background: How NIST-F2 Works, 2 April 2014,, 4 April 2014, The first in-house accuracy evaluation of NIST-F2 reported a u'B of {{nowrap|1.1 × 10−16}}.Heavner T P, Donley E A, Levi F, Costanzo G, Parker TE, Shirley J H, Ashby N, Barlow S and Jefferts SR, "First accuracy evaluation of NIST-F2," 2014 Metrologia 51, 174–182, May 2014 However, a published scientific criticism of that NIST F-2 accuracy evaluation described problems in its treatment of distributed cavity phase shifts and the microwave lensing frequency shift,JOURNAL, 1505.00649, Li, Ruoxin, Comment on "first accuracy evaluation of NIST-F2", Metrologia, 52, 2015, 163–166, Gibble, Kurt, Szymaniec, Krzysztof, 2015, 10.1088/0026-1394/52/1/163, 2015Metro..52..163G, which is treated significantly differently than in the majority of accurate fountain clock evaluations. The next NIST-F2 submission to the BIPM in March, 2015 again reported a u'B of {{nowrap|1.5 × 10−16}},[ February/March 2015 Evaluation of NIST-F2] but did not address the standing criticism. There have been neither subsequent reports to the BIPM from NIST-F2 nor has an updated accuracy evaluation been published.At the request of the Italian standards organization, NIST fabricated many duplicate components for a second version of NIST-F2, known as IT-CsF2 to be operated by the (:it:Istituto nazionale di ricerca metrologica|Istituto Nazionale di Ricerca Metrologica) (INRiM), NIST's counterpart in Turin, Italy. As of February 2016 the IT-CsF2 caesium fountain clock started reporting a uB of {{nowrap|1.7 × 10−16}} in the BIPM reports of evaluation of primary frequency standards.February 2016 IT-CsF2 TAI evaluation June 2018 IT-CsF2 TAI evaluation


(File:President Piñera receives ESO's first atomic clock.jpg|thumb|A caesium atomic clock from 1975 (upper unit) and battery backup (lower unit).NEWS, President Piñera Receives ESO's First Atomic Clock,weblink 15 November 2013, 20 November 2013, ESO Announcement, )(File:Strontium Clock (12196092854).jpg|thumb|An experimental strontium based optical clock.)Most research focuses on the often conflicting goals of making the clocks smaller, cheaper, more portable, more energy efficient, more accurate, more stable and more reliable.WEB
, Laura Ost
, 4 February 2014
, A New Era for Atomic Clocks
, National Institute of Standards and Technology
, 18 October 2015
, The Atomic Clock Ensemble in Space is an example of clock research.WEB
, Atomic clock ensemble in space (ACES)
, ERASMUS Centre - Directorate of Human Spaceflight and Operations
, 11 February 2017
, NEWS,weblink With better atomic clocks, scientists prepare to redefine the second, 2018-02-28, Science {{!, AAAS|access-date=2018-03-02|language=en}}

Secondary representations of the second

A list of frequencies recommended for secondary representations of the second is maintained by the International Bureau of Weights and Measures (BIPM) since 2006 and is available online. The list contains the frequency values and the respective standard uncertainties for the rubidium microwave transition and for several optical transitions. These secondary frequency standards are accurate at the level of parts in {{nowrap|10−18}}; however, the uncertainties provided in the list are in the range of parts in {{nowrap|10−14}} – {{nowrap|10−15}} since they are limited by the linking to the caesium primary standard that currently (2015) defines the second.{| class="wikitable"!Type!colspan=2| working frequencyin Hz!relative Allan deviationtypical clocksIsotopes of caesium>133Cs 9 192 631 770  by definitionBOOK
, Unit of time (second)
, SI Brochure
, 23 June 2015
, 2014
, 2006, |10−13
Isotopes of rubidium>87Rb 6 834 682 610 .904 32487Rubidium BIPM document|10−12Isotopes of hydrogen>1H 1 420 405 751 .7667JOURNAL
, L, Essen, Louis Essen
, R W, Donaldson, E G, Hope, M J, Bangham
, Hydrogen Maser Work at the National Physical Laboratory
, Metrologia, July 1973, 9, 3, 128–137
, 10.1088/0026-1394/9/3/004, 1973Metro...9..128E
, Proton Zemach radius from measurements of the hyperfine splitting of hydrogen and muonic hydrogen
, Arnaud, Dupays, Alberto, Beswick
, Bruno, Lepetit, Carlo, Rizzo
, Physical Review A, 68, 5, 052503, August 2003
, 10.1103/PhysRevA.68.052503, quant-ph/0308136, 2003PhRvA..68e2503D, |10−15
Isotopes of strontium>87Sr) 429 228 004 229 873 .487Strontium BIPM document|10−17For context, a femtosecond ({{val|1|e=-15|u=s}}) is to a second what a second is to about 31.71 million ({{val|31.71|e=6}}) years and an attosecond ({{val|1|e=-18|u=s}}) is to a second what a second is to about 31.71 billion ({{val|31.71|e=9}}) years.21st century experimental atomic clocks that provide non-caesium-based secondary representations of the second are becoming so precise that they are likely to be used as extremely sensitive detectors for other things besides measuring frequency and time. For example, the frequency of atomic clocks is altered slightly by gravity, magnetic fields, electrical fields, force, motion, temperature and other phenomena. The experimental clocks tend to continue improving, and leadership in performance has been shifted back and forth between various types of experimental clocks.

Quantum clocks

{{further|Quantum clock}}In March 2008, physicists at NIST described a quantum logic clock based on individual ions of beryllium and aluminium. This clock was compared to NIST's mercury ion clock. These were the most accurate clocks that had been constructed, with neither clock gaining nor losing time at a rate that would exceed a second in over a billion years.WEB, Swenson, Gayle, Press release: NIST 'Quantum Logic Clock' Rivals Mercury Ion as World's Most Accurate Clock,weblink NIST, en, 7 June 2010, In February 2010, NIST physicists described a second, enhanced version of the quantum logic clock based on individual ions of magnesium and aluminium. Considered the world's most precise clock in 2010 with a fractional frequency inaccuracy of {{nowrap|8.6 × 10−18}}, it offers more than twice the precision of the original.NIST's Second 'Quantum Logic Clock' Based on Aluminum Ion is Now World's Most Precise Clock, NIST, 4 February 2010WEB
, C.W Chou, D. Hume, J.C.J. Koelemeij, D.J. Wineland, T. Rosenband, yes, 17 February 2010
, Frequency Comparison of Two High-Accuracy Al+ Optical Clocks
, 9 February 2011
, The accuracy of experimental quantum clocks has since been superseded by experimental optical lattice clocks based on strontium-87 and ytterbium-171.

Optical clocks

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.WEB
, D. Lindley
, 20 May 2009
, Coping With Unusual Atomic Collisions Makes an Atomic Clock More Accurate
, National Science Foundation
, 10 July 2009 National Science FoundationThe 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.New technologies, such as femtosecond frequency combs, optical lattices, and quantum information, have enabled prototypes of next-generation atomic clocks. These clocks are based on optical rather than microwave transitions. 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. Before the demonstration of the frequency comb in 2000, 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.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 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.The primary systems under consideration for use in optical frequency standards are:
  • single ions isolated in an ion trap;
  • neutral atoms trapped in an optical lattice and
, W.H. Oskay
, 2006
, Single-atom optical clock with high accuracy
, Physical Review Letters
, 97, 2, 020801
, 16907426
, 10.1103/PhysRevLett.97.020801, 2006PhRvL..97b0801O, etal, WEB
, Fritz Riehle
, On Secondary Representations of the Second
, Physikalisch-Technische Bundesanstalt, Division Optics
, 22 June 2015
  • atoms packed in a three-dimensional quantum gas optical lattice.
These techniques allow the atoms or ions to be highly isolated from external perturbations, thus producing an extremely stable frequency reference.Atomic systems under consideration include Al+, Hg+/2+, Hg, Sr, Sr+/2+, In+/3+, Mg, Ca, Ca+, Yb+/2+/3+, Yb and Th+/3+.171Ytterbium BIPM documentPTB Time and Frequency Department 4.4PTB Optical nuclear spectroscopy of 229ThFile:Ytterbium Lattice Atomic Clock (10444764266).jpg|thumb|One of NISTNISTThe 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.WEB, Blackbody Radiation Shift: Quantum Thermodynamics Will Redefine Clocks,weblink 5 December 2012, The estimated amount of uncertainty achieved corresponds to a Yb clock uncertainty of 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.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 {{nowrap|10 000 atoms}} of strontium-87 were able to stay in synchrony with each other at a precision of at least {{nowrap|1.5 × 10−16}}, which is as accurate as the experiment could measure.WEB
, Ost, Laura, JILA Strontium Atomic Clock Sets New Records in Both Precision and Stability, NIST Tech Beat, National Institute of Standards and Technology, 22 January 2014,weblink 5 December 2014, 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.WEB, Precise atomic clock may redefine time,weblink 9 July 2013, 24 August 2013,
Using ytterbium-171 atoms, a new record for stability with a precision of {{val|1.6|e=-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 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.WEB, NIST Ytterbium Atomic Clocks Set Record for Stability,weblink 22 August 2013, 24 August 2013, 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.WEB, New atomic clock sets the record for stability,weblink 27 August 2013, 19 January 2014, An improved optical lattice clock was described in a 2014 Nature paper.JOURNAL,weblink An optical lattice clock with accuracy and stability at the 10−18 level, 22 January 2014, Nature, 506, 7486, 71–5, 10.1038/nature12941, 24463513, 1309.1137, 2014Natur.506...71B, Bloom, B. J., Nicholson, T. L., Williams, J. R., Campbell, S. L., Bishof, M., Zhang, X., Zhang, W., Bromley, S. L., Ye, J., In 2015 JILA evaluated the absolute frequency uncertainty of a strontium-87 optical lattice clock at {{nowrap|2.1 × 10−18}}, 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".JOURNAL
, T.L. Nicholson, S.L. Campbell, R.B. Hutson, G.E. Marti, B.J. Bloom, R.L. McNally, W. Zhang, M.D. Barrett, M.S. Safronova, G.F. Strouse, W.L. Tew, J. Ye
, 21 April 2015
, Systematic evaluation of an atomic clock at 2 × 10−18 total uncertainty
, Nature Communications
, 6, 6896, 6896
, 10.1038/ncomms7896
, 24 June 2015, 25898253, 4411304bibcode = 2015NatCo...6E6896N, WEB
, JILA Scientific Communications
, 21 April 2015
, About Time
, 27 June 2015
, Laura Ost
, 21 April 2015
, Getting Better All the Time: JILA Strontium Atomic Clock Sets New Record
, National Institute of Standards and Technology
, 17 October 2015
, At this frequency uncertainty, this JILA optical lattice clock is expected to neither gain nor lose a second in more than 15 billion ({{nowrap|15 × 109}}) years.WEB
, James Vincent
, 22 April 2015
, The most accurate clock ever built only loses one second every 15 billion years
, The Verge
, 26 June 2015
, Single-Ion Atomic Clock with 3 × 10−18 Systematic Uncertainty
, N. Huntemann
, C. Sanner
, B. Lipphardt
, Chr. Tamm
, E. Peik
, Physical Review Letters
, 116
, 6
, 063001
, 8 February 2016
, 10.1103/PhysRevLett.116.063001
, 26918984
, 2016PhRvL.116f3001H, 1602.03908,
(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, like the 2015 JILA clock. A synchronous clock comparison between two regions of the 3D lattice yielded a record level of synchronization of {{nowrap|5 × 10−19}} in 1 hour of averaging time.JOURNAL
, A Fermi-degenerate three-dimensional optical lattice clock
, S. L. Campbell
, R. B. Hutson
, G. E. Marti
, A. Goban
, N. Darkwah Oppong
, R. L. McNally
, L. Sonderhouse
, W. Zhang
, B. J. Bloom
, J. Ye
, Science (journal), Science
, 358
, 6359
, 90–94
, 2017
, 10.1126/science.aam5538
, 28983047
, 29 March 2017
, 2017Sci...358...90C
, 1702.01210
The 3D quantum gas strontium optical lattice clock’s centerpiece is an unusual state of matter called a degenerate Fermi gas (a quantum gas for Fermi particles). The experimental data shows the 3D quantum gas clock achieved a precision of {{nowrap|3.5 × 10−19}} 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."JOURNAL
, A Fermi-degenerate three-dimensional optical lattice clock
, Abigail Beall
, Wired UK
, 5 October 2017
, 29 March 2017
, JILA's 3-D Quantum Gas Atomic Clock Offers New Dimensions in Measurement
, 5 October 2017
, 29 March 2017
, The Clock that Changed the World
, Julie Phillips
, 10 October 2017
, 30 March 2017
In 2018 JILA reported the 3D quantum gas clock reached a frequency precision of {{nowrap|2.5 × 10−19}} over 6 hours.JOURNAL
, Imaging Optical Frequencies with 100 μHz Precision and 1.1 μm Resolution
, G. Edward Marti
, Ross B. Hutson
, Akihisa Goban
, Sara L. Campbell
, Nicola Poli
, Jun Ye
, Physical Review Letters
, 120
, 10
, 103201
, 2018
, 10.1103/PhysRevLett.120.103201
, 29570334
, 30 March 2017
, 2018PhRvL.120j3201M
, 1711.08540, JOURNAL
, JILA Team Invents New Way to 'See' the Quantum World
, Laura Ost
, 5 March 2018
, 30 March 2017,
At this frequency uncertainty, this 3D quantum gas clock would lose or gain about 0.1 seconds over the age of the universe.JOURNAL
, Same Clock. New Perspective.
, 13 March 2018
, 23 September 2018
Optical clocks are currently (2018) still primarily research projects, 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 (TAI).WEB,weblink BIPM Time Coordinated Universal Time (UTC), BIPM, 29 December 2013, 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.JOURNAL, N. Poli, C. W. Oates, P. Gill, G. M. Tino, 13 January 2014
, Optical atomic clocks
, Rivista del Nuovo Cimento, 36, 12, 555–624, 1401.2378, 10.1393/ncr/i2013-10095-x
WEBSITE=DAILY MAILACCESSDATE=10 JULY 2013, In the future this might lead to redefine 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.WEB,weblink BIPM work programme: Time, BIPM, 25 June 2015, JOURNAL
, Helen Margolis
, 12 January 2014
, Timekeepers of the future
, 10
, 2
, 82–83
, Nature Physics
, 10.1038/nphys2834
, 2014NatPh..10...82M, JOURNAL
, Realization of a timescale with an accurate optical lattice clock
, Optica
, 3
, 6
, 563–569
, 10.1364/OPTICA.3.000563
, 1511.03888, 2016
, Grebing
, Christian
, Al-Masoudi
, Ali
, Dörscher
, Sören
, Häfner
, Sebastian
, Gerginov
, Vladislav
, Weyers
, Stefan
, Lipphardt
, Burghard
, Riehle
, Fritz
, Sterr
, Uwe
, Lisdat
, Christian
, Optical atomic clocks
, Reviews of Modern Physics
, 87
, 2
, 673
, 10.1103/RevModPhys.87.637
, 1407.3493, 2015
, Ludlow
, Andrew D
, Boyd
, Martin M
, Ye
, Jun
, Peik
, Ekkehard
, Schmidt
, Piet O

Clock comparison techniques

In June 2015, the European National Physical Laboratory (NPL) in Teddington, UK; the French department of Time-Space Reference Systems at the Paris Observatory (LNE-SYRTE); the German German National Metrology Institute (PTB) in Braunschweig; and Italy’s (:it:Istituto nazionale di ricerca metrologica|Istituto Nazionale di Ricerca Metrologica (INRiM) in Turin) labs have started tests to improve the accuracy of current state-of-the-art satellite comparisons by a factor 10, but it will still be limited to one part in {{nowrap|1 × 10−16}}. These 4 European labs are developing and host a variety of experimental optical clocks that harness different elements in different experimental set-ups and want to compare their optical clocks against each other and check whether they agree. In a next phase these labs strive to transmit comparison signals in the visible spectrum through fibre-optic cables. This will allow their experimental optical clocks to be compared with an accuracy similar to the expected accuracies of the optical clocks themselves. Some of these labs have already established fibre-optic links, and tests have begun on sections between Paris and Teddington, and Paris and Braunschweig. Fibre-optic links between experimental optical clocks also exist between the American NIST lab and its partner lab JILA, both in Boulder, Colorado but these span much shorter distances than the European network and are between just two labs. According to Fritz Riehle, a physicist at PTB, "Europe is in a unique position as it has a high density of the best clocks in the world".JOURNAL
, Elizabeth Gibney
, 2 June 2015
, Hyper-precise atomic clocks face off to redefine time - Next-generation timekeepers can only be tested against each other
, Nature
, 522, 7554
, 16–17
, 10.1038/522016a
, 26040875
, 29 August 2015, 2015Natur.522...16G,
In August 2016 the French LNE-SYRTE in Paris and German PTB in Braunschweig reported the comparison and agreement of two fully independent experimental strontium lattice optical clocks in Paris and Braunschweig at an uncertainty of {{nowrap|5 × 10−17}} via a newly established phase-coherent frequency link connecting Paris and Braunschweig, using {{convert|1415|km|mi|0|lk=on|abbr=on}} of telecom fibre-optic cable. The fractional uncertainty of the whole link was assessed to be {{nowrap|2.5 × 10−19}}, making comparisons of even more accurate clocks possible.JOURNAL
, Paul-Eric Pottie, Gesine Grosche
, 19 August 2016
, A clock network for geodesy and fundamental science
, Nature Communications
, 7
, 10.1038/ncomms12443
, 13 November 2016
, 12443
, 27503795
, 4980484
bibcode = 2016NatCo...712443L, Optical fibre link opens a new era of time-frequency metrology, 19 August 2016


The development of atomic clocks has led to many scientific and technological advances such as a system of precise global and regional navigation satellite systems, and applications in the Internet, which depend critically on frequency and time standards. Atomic clocks are installed at sites of time signal radio transmitters. They are used at some long wave and medium wave broadcasting stations to deliver a very precise carrier frequency.{{Citation needed|date=January 2008}} Atomic clocks are used in many scientific disciplines, such as for long-baseline interferometry in radioastronomy.BOOK, 266, McCarthy, D. D., Seidelmann, P. K., Dennis McCarthy (scientist), 2009, TIME—From Earth Rotation to Atomic Physics, Weinheim, Wiley-VCH Verlag GmbH & Co. KGaA, 978-3-527-40780-4,

Global Navigation Satellite Systems

The Global Positioning System (GPS) operated by the US Air Force Space Command provides very accurate timing and frequency signals. A GPS receiver works by measuring the relative time delay of signals from a minimum of four, but usually more, GPS satellites, each of which has at least two onboard caesium and as many as two rubidium atomic clocks. The relative times are mathematically transformed into three absolute spatial coordinates and one absolute time coordinate.WEB,weblink Global Positioning System,, 26 June 2010, yes,weblink" title="">weblink 30 July 2010, GPS Time (GPST) is a continuous time scale and theoretically accurate to about 14 ns.JOURNAL, David W. Allan,weblinkweblink" title="">weblink no, The Science of Timekeeping, Hewlett Packard, 1997, 25 October 2012, dmy, However, most receivers lose accuracy in the interpretation of the signals and are only accurate to 100 ns.JOURNAL,weblink The Role of GPS in Precise Time and Frequency Dissemination, GPSworld, July–August 1990, 27 April 2014, WEB,weblink GPS time accurate to 100 nanoseconds, Galleon, 12 October 2012, The GPST is related to but differs from TAI (International Atomic Time) and UTC (Coordinated Universal Time). GPST remains at a constant offset with TAI (TAI – GPST = 19 seconds) and like TAI does not implement leap seconds. Periodic corrections are performed to the on-board clocks in the satellites to keep them synchronized with ground clocks.WEB,weblink UTC to GPS Time Correction,, WEB, NAVSTAR GPS User Equipment Introduction,weblink Section 1.2.2 The GPS navigation message includes the difference between GPST and UTC. As of July 2015, GPST is 17 seconds ahead of UTC because of the leap second added to UTC on 30 June 2015weblinkweblink Notice Advisory to Navstar Users (NANU) 2012034, 30 May 2012, 2 July 2012, GPS Operations Center, yes,weblink 8 April 2013, Receivers subtract this offset from GPS Time to calculate UTC and specific timezone values.The GLObal NAvigation Satellite System (GLONASS) operated by the Russian Aerospace Defence Forces provides an alternative to the Global Positioning System (GPS) system and is the second navigational system in operation with global coverage and of comparable precision. GLONASS Time (GLONASST) is generated by the GLONASS Central Synchroniser and is typically better than 1,000 ns.WEB,weblink Time References in GNSS,, Unlike GPS, the GLONASS time scale implements leap seconds, like UTC.GLONASS Interface Control Document, Navigation radiosignal In bands L1, L2 (ICD L1, L2 GLONASS), Russian Institute of Space Device Engineering, Edition 5.1, 2008(File:ESA Galileo Passive Hydrogen Maser.jpg|thumb|Space Passive Hydrogen Maser used in ESA Galileo satellites as a master clock for an onboard timing system)The Galileo Global Navigation Satellite System is operated by the European GNSS Agency and European Space Agency and is near to achieving full operating global coverage. Galileo started offering global Early Operational Capability (EOC) on 15 December 2016, providing the third and first non-military operated Global Navigation Satellite System, and is expected to reach Full Operational Capability (FOC) in 2019.WEB,weblink Galileo begins serving the globe, European Space Agency, 15 December 2016, To achieve Galileo's FOC coverage constellation goal 6 planned extra satellites need to be added. Galileo System Time (GST) is a continuous time scale which is generated on the ground at the Galileo Control Centre in Fucino, Italy, by the Precise Timing Facility, based on averages of different atomic clocks and maintained by the Galileo Central Segment and synchronised with TAI with a nominal offset below 50 ns.European GNSS (Galileo) Open Service Signal-In-Space Operational Status Definition, Issue 1.0, September 20151 The Definition and Implementation of Galileo System Time (GST). ICG-4 WG-D on GNSS time scales. Jérôme Delporte. CNES – French Space Agency.WEB,weblink Galileo's clocks, European Space Agency, 16 January 2017, NEWS, Galileo’s contribution to the MEOSAR system,weblink 30 December 2015, European Commission, According to the European GNSS Agency Galileo offers 30 ns timing accuracy.WEB,weblink GALILEO GOES LIVE, European GNSS Agency, 15 December 2016, 1 February 2017, The March 2018 Quarterly Performance Report by the European GNSS Service Centre reported the UTC Time Dissemination Service Accuracy was ≤ 7.6 ns, computed by accumulating samples over the previous 12 months and exceeding the ≤ 30 ns target.WEB,weblink GALILEO INITIAL SERVICES – OPEN SERVICE – QUARTERLY PERFORMANCE REPORT OCT-NOV-DEC 2017, European GNSS Service Centre, 28 March 2018, 28 March 2017, Galileo Open Service and Search and Rescue - Quarterly Performance Reports, containing measured performance statistics Each Galileo satellite has two passive hydrogen maser and two rubidium atomic clocks for onboard timing.WEB,weblink Passive Hydrogen Maser (PHM),, WEB,weblink Rb Atomic Frequency Standard (RAFS),, The Galileo navigation message includes the differences between GST, UTC and GPST (to promote interoperability).GNSS Timescale DescriptionWEB,weblink ESA Adds System Time Offset to Galileo Navigation Message,, The BeiDou-2/BeiDou-3 satellite navigation system is operated by the China National Space Administration and is also near to achieving full-scale global coverage. BeiDou Time (BDT) is a continuous time scale starting at 1 January 2006 at 0:00:00 UTC and is synchronised with UTC within 100 ns.China Satellite Navigation Office, Version 2.0, December 2013Definition and Realization of the System Time of COMPASS/BeiDou Navigation Satellite System, Chunhao Han, Beijing Global Information Center,(BGIC), Beijing, China BeiDou became operational in China in December 2011, with 10 satellites in use,NEWS,weblink China GPS rival Beidou starts offering navigation data, BBC, 2011-12-27, and began offering services to customers in the Asia-Pacific region in December 2012.WEB,weblink China's Beidou GPS-substitute opens to public in Asia, BBC, 27 December 2012, 27 December 2012, On 27 December 2018 the BeiDou Navigation Satellite System started to provide global services with a reported timing accuracy of 20 ns.WEB,weblink China's BeiDou navigation satellite, rival to US GPS, starts global services, PTI, K. J. M. Varma, 2018-12-27,, en, 2018-12-27, The BeiDou global navigation system should be finished by 2020.WEB,weblink BeiDou navigation system covers Asia-Pacific region till 2012, 2010-03-03, Xinhua News Agency, 2010-05-19, Chinese,

Time signal radio transmitters

A radio clock is a clock that automatically synchronizes itself by means of government radio time signals received by a radio receiver. Many retailers market radio clocks inaccurately as atomic clocks;Michael A. Lombardi, "How Accurate is a Radio Controlled Clock?", National Institute of Standards and Technology, 2010. although the radio signals they receive originate from atomic clocks, they are not atomic clocks themselves. Normal low cost consumer grade receivers solely rely on the amplitude-modulated time signals and use narrow band receivers (with 10 Hz bandwidth) with small ferrite loopstick antennas and circuits with non optimal digital signal processing delay and can therefore only be expected to determine the beginning of a second with a practical accuracy uncertainty of ± 0.1 second. This is sufficient for radio controlled low cost consumer grade clocks and watches using standard-quality quartz clocks for timekeeping between daily synchronization attempts, as they will be most accurate immediately after a successful synchronization and will become less accurate from that point forward until the next synchronization.Michael A. Lombardi, "How Accurate is a Radio Controlled Clock?, National Institute of Standards and Technology, 2010.Instrument grade time receivers provide higher accuracy. Such devices incur a transit delay of approximately 1 ms for every 300 kilometres (186 mi) of distance from the radio transmitter. Many governments operate transmitters for time-keeping purposes.

See also

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External links

{{Time Topics}}{{Electric clock technology}} {{Time measurement and standards}}{{Machines}}{{Authority control}}

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