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 caesium 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 first atomic clock was built in 1949 at the U.S. National Bureau of Standards. The first accurate atomic clock, based on the transition of the caesium-133 atom, was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. This led to the internationally agreed definition of the second being based on atomic time.
In August 2004, NIST scientists demonstrated a chip-scaled atomic clock. According to the researchers, the clock was believed to be one hundredth the size of any other atomic. It was also claimed that it requires just 75 mW, making it suitable for battery-driven applications.
Modern radio clocks are referenced to atomic clocks, and provide a way of getting high-quality atomic-derived time over a wide area using inexpensive equipment.
How they work
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 Cesium-133 atom. This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see caesium standard). Other physical quantities, like the volt and metre, rely on the definition of the second as part of their own definitions.
The core of the atomic clock is a microwave cavity containing the ionized gas, a tunable microwave radio oscillator, and a feedback loop which is used to adjust the oscillator to the exact frequency of the absorption characteristic defined by the behavior of the individual atoms.
The microwave transmitter fills the chamber with a standing wave of radio waves. When the radio frequency matches the hyperfine transition frequency of caesium, the caesium 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 this way, the ultra-precise quantum-mechanical properties of the atomic transition frequency can be used to tune the microwave oscillator to the same frequency (except for a small amount of experimental error). 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 mechanism of cavity, oscillator and feedback loop that maintains the frequency standard on which the clock is based.
- Radio clock
- Optical Atomic Clock 
- Web pages on atomic clocks by The Science Museum (London)
- NIST website
- National Physical Laboratory (UK) time website
- NIST Internet Time Service (ITS): Set Your Computer Clock Via the Internet
- NIST press release about chip-scaled atomic clock
- Atomic Clock United Kingdom