An atomic clock is a type of clock that uses an atomic frequency standard as its counter. Early atomic clocks were masers with attached equipment. Today's best atomic frequency standards (or clocks) are based on more advanced physics involving cesium beams and fountains. National standards agencies maintain an accuracy of 10-9 seconds per day, and a precision equal to the frequency of the radio transmitter pumping the maser. The clocks maintain 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 synchronized with the passing of day and night based on astronomical observations.
The most accurate time scales are moderated by precise astronomical measurements and the insertion or removal of leap seconds. "Atomic clocks" are really frequency standards that provide pulses at precise intervals, typically 1 Hz or 1, 5, or 10 MHz. They must be carefully synchronized with the beginning of a UTC second, and leap seconds must be accounted for, in order to produce "atomic time".
The first atomic clock was built in 1949 at the U.S. National Bureau of Standards.
For information about how to get the time signals of an atomic clock, see radio clock.
How they work
Frequency reference masers use glowing chambers of ionized gas, most often caesium, because caesium is the element used in the official international definition of the second.
Since 1967, the International System of Units (SI) has defined the second as 9,192,631,770 cycles of the radiation which corresponds to the transition between two energy levels of the ground state of the Caesium-133 atom. This definition makes the cesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see Cesium standard). Other physical quantities, like the volt and metre, rely on the definition of the second as part of their own definitions.
A microwave radio transmitter fills the chamber with a standing wave of radio waves. When the radio frequency matches the hyperfine transition frequency of cesium, the cesium atoms absorb the radio waves and emit light. The radio waves make the electrons move farther from their nuclei. When the electrons are attracted back closer by the opposite charge of the nucleus, the electrons wiggle before they settle down in their new location. This moving charge causes the light, which is a wave of alternating electricity and magnetism.
A photocell looks at the light. When the light gets dimmer because the frequency of the excitation has drifted from the true resonance frequency, electronics between the photocell and radio transmitter adjusts the frequency of the radio transmitter. This adjustment process is where most of the work and complexity of the clock lies. For example, the driving frequency could be deliberately cycled sinusoidally up and down to generate a modulated signal at the photocell which can then be demodulated in order to apply feedback to control the excitation frequency. In practice, the feedback and monitoring mechanism is much more complex than described above. When a clock is first turned on, it takes a while for it to settle down before it can be trusted.
A counter counts the waves made by the radio transmitter. A computer reads the counter, and does math to convert the number to something that looks like a digital clock, or a radio wave that is transmitted. Of course, the real clock is the original counter.