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Atomic clock
| Atomic clock | |
|---|---|
 NIST physicists Steve Jefferts (foreground) and Tom Heavner with the NIST-F2 caesium fountain atomic clock, a civilian time standard for the United States | |
| Classification | Clock |
| Industry | Telecommunications, science |
| Application | TAI, satellite navigation |
| Fuel source | Electricity |
| Powered | Yes |
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions between such states they interact with a very specific frequency of electromagnetic radiation. This phenomenon serves as the basis for the International System of Units' (SI) definition of a second:
The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, , the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.
This definition is the basis for the system of International Atomic Time (TAI), which is maintained by an ensemble of atomic clocks around the world. The system of Coordinated Universal Time (UTC) that is the basis of civil time implements leap seconds to allow clock time to track changes in Earth's rotation to within one second while being based on clocks that are based on the definition of the second, though leap seconds will be phased out in 2035.[2]
The accurate timekeeping capabilities of atomic clocks are also used for navigation by satellite networks such as the European Union's Galileo Programme and the United States' GPS. The timekeeping accuracy of the involved atomic clocks is important because the smaller the error in time measurement, the smaller the error in distance obtained by multiplying the time by the speed of light is (a timing error of a nanosecond or 1 billionth of a second (10−9 or 1⁄1,000,000,000 second) translates into an almost 30-centimetre (11.8 in) distance and hence positional error).
The main variety of atomic clock uses caesium atoms cooled to temperatures that approach absolute zero. The primary standard for the United States, the National Institute of Standards and Technology (NIST)'s caesium fountain clock named NIST-F2, measures time with an uncertainty of 1 second in 300 million years (relative uncertainty 10−16). NIST-F2 was brought online on 3 April 2014.[3][4]
- ^ "USNO Master Clock". Archived from the original on 7 December 2010. Retrieved 23 November 2010.
- ^ Brumfiel, Geoff (27 November 2022). "The world is doing away with the leap second". Weekend Edition Sunday. National Public Radio. Retrieved 30 April 2024.
- ^ "NIST Launches a New U.S. Time Standard: NIST-F2 Atomic Clock". NIST. 3 April 2014 – via www.nist.gov.
- ^ Thomas P. Heavner; Elizabeth A. Donley; Filippo Levi; Giovanni Costanzo; Thomas E. Parker; Jon H. Shirley; Neil Ashby; Stephan Barlow; Steven R. Jefferts (May 2014). "First Accuracy Evaluation of NIST-F2" (PDF). Metrologia. 51 (3): 174–182. Bibcode:2014Metro..51..174H. doi:10.1088/0026-1394/51/3/174.
Currently, the type B fractional uncertainty in NIST-F1 is 0.31×10−15 and is dominated by the uncertainty in the blackbody radiation (BBR) shift correction, which is 0.28×10−15 (this corresponds to a 1 degree uncertainty in the radiation environment as seen by the atoms in NIST-F1). To improve the performance of the NIST primary frequency standard, we sought to reduce the uncertainty due to the BBR effect. To accomplish this goal and to better understand the accepted model of the BBR shift, we developed NIST-F2, a laser-cooled Cs fountain primary frequency standard in which the microwave cavity structure and flight tube operate at cryogenic temperatures (80 K).
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