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Dr. Judah Levine:

Atomic clock

From Wikipedia, the free encyclopedia

For a clock updated by radio signals which is sometimes called an "atomic clock", see Radio clock. For the clock as a measure for risk of catastrophic destruction, see Doomsday Clock. For other topics, see Atomic Clock (disambiguation).

Atomic clock
FOCS 1, a continuous cold caesium fountain atomic clock in Switzerland, started operating in 2004 at an uncertainty of one second in 30 million years.
Classification	Clock
Industry	Telecommunications, science
Application	GPS
Fuel source	Electricity
Powered	Yes

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. The rack mounted units in the background are Symmetricom (formerly HP) 5071A caesium beam clocks. The black units in the foreground are Symmetricom (formerly Sigma-Tau) MHM-2010 hydrogen maser standards.

An atomic clock is a clock device that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate 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 not based on nuclear physics, but rather on atomic physics; it uses the microwave signal that electrons in atoms emit when they change energy levels. Early atomic clocks were based on masers at room temperature. Currently, the most 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 factor is temperature of the sample atoms—colder atoms move much more slowly, allowing longer probe times. The second factor is the frequency and intrinsic width of the electronic 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, International Atomic Time (TAI). For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but approximately synchronised, by using leap seconds, to UT1, which is based on actual rotation of the Earth with respect to the solar time.