Atomic clock
A device that uses an internal resonance frequency of atoms (or molecules) to measure the passage of time. The terms atomic clock and atomic frequency standard are often used interchangeably. A frequency standard generates pulses at regular intervals. It can be made into a clock by the addition of an electronic counter, which records the number of pulses.
Most methods of timekeeping rely on counting some periodic event, such as the rotation of the Earth, the motion of a pendulum in a grandfather clock, or the vibrations of a quartz crystal in a watch. An atomic clock relies on counting periodic events determined by the difference of two different energy states of an atom. A transition between two energy states with energies E1 and E2 may be accompanied by the absorption or emission of a photon (particle of electromagnetic radiation). The frequency ν of this radiation is given by the equation 
where h is Planck's constant. A basic advantage of atomic clocks is that the frequency-determining elements, atoms of a particular isotope, are the same everywhere. Thus, atomic clocks constructed and operated independently will measure the same time interval. See Atomic structure and spectra, Energy level (quantum mechanics), Quantum mechanics
An atomic frequency standard can be either active or passive. An active standard uses as a reference the electromagnetic radiation emitted by atoms as they decay from a higher energy state to a lower energy state. A passive standard attempts to match the frequency of an electronic oscillator or laser to the resonant frequency of the atoms by means of a feedback circuit. Either kind of standard requires some kind of frequency synthesis to produce an output near a convenient frequency that is proportional to the atomic resonance frequency. See Laser, Maser
Two different gages of the quality of a clock are accuracy and stability. The accuracy of a frequency standard is defined in terms of the deviation of its frequency from an ideal standard. The stability of frequency standard is defined in terms of the constancy of its average frequency from one interval of time to the next.
The three most commonly used types of atomic clock are the cesium atomic beam, the hydrogen maser, and the rubidium gas cell. The cesium clock has high accuracy and good long-term stability. The hydrogen maser has the best stability for periods of up to a few hours. The rubidium cell is the least expensive and most compact and also has good short-term stability.
The cesium atomic-beam clock uses a 9193-MHz transition between two hyperfine energy states of the cesium-133 atom. Both the atomic nucleus and the outermost electron have magnetic moments; that is, they are like small magnets, with a north and a south pole. The two hyperfine energy states differ in the relative orientations of these magnetic moments. The cesium atoms travel in a collimated beam through a series of evacuated regions, where they are exposed to microwave radiation near their resonance frequency and are deflected into different trajectories by nonuniform magnetic fields. See Electron spin, Hyperfine structure, Molecular beams, Nuclear moments
Cesium has become the basis of the international definition of the second; the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine states of the ground state of the cesium-133 atom. The cesium clock is especially well suited for applications such as timekeeping, where absolute accuracy without recalibration is necessary. Measurements from many cesium clocks throughout the world are averaged together to define an international time scale that is uniform to parts in 1014, or about 1 microsecond in a year. See Physical measurement
The hydrogen maser is based on the hyperfine transition of atomic hydrogen, which has a frequency of 1420 MHz. Atoms in the higher hyperfine energy state enter an evacuated storage bulb inside a microwave cavity, and are induced to make a transition to the lower hyperfine state by a process called stimulated emission.
The rubidium gas cell is based on the 6835-MHz hyperfine transition of rubidium-87. The rubidium atoms are contained in a glass cell together with a buffer gas, where they are subjected to optical pumping and microwave radiation at the hyperfine transition frequency; this results in a detectable decrease in the light transmitted through the cell.
Many other kinds of atomic clocks, such as thallium atomic beams and ammonia and rubidium masers, have been demonstrated in the laboratory. The first atomic clock, constructed at the National Bureau of Standards in 1949, was based on a 24-GHz transition in the ammonia molecule. Some laboratories have tried to improve the cesium atomic-beam clock by replacing the magnetic state selection with laser optical pumping and fluorescence detection. One such standard, called NIST-7, is in operation at the U.S. National Institute of Standards and Technology and is the primary frequency standard for the United States. Atomic frequency standards can also be based on optical transitions. One of the best-developed optical frequency standards is the 3.39-micrometer (88-THz) helium-neon laser, stabilized to a transition in the methane molecule. Frequency synthesis chains have been built to link the optical frequency to radio frequencies.
Atomic clocks are used in applications for which less expensive alternatives, such as quartz oscillators, do not provide adequate performance. In addition to maintaining a uniform international time scale, atomic clocks are used to keep time in the Global Positioning System, various digital communications systems, radio astronomy, and navigation of space probes.