Elements of Atomic Clocks

An atomic clock is a kind of clock that uses an atomic resonance frequency standard as its counter. While the time base of a conventional clock is the oscillating pendulum, the atomic clock is based on the properties of atoms transitioning between different energy states.

The first person to express the concept of the atomic clock was the Columbia University Isidor Rabi in the year 1945. He suggested that a clock could be made from a technique he developed in the 1930s, called atomic beam magnetic resonance. By 1949, the National Bureau of Standards introduced the world's first atomic clock using the ammonia molecule as the source of vibrations, and the first atomic clock using cesium atoms as the vibration source was announced in 1952.

Both conventional clocks and atomic clocks use oscillations to keep track of passing time. The oscillation frequencies within the atom depend on the mass of the nucleus and the gravity and electrostatic "spring" between the positive charge on the nucleus and the electron cloud around it.

The most accurate atomic clocks available today use the cesium atom and normal magnetic fields and detectors. Cesium 133 is the element most commonly chosen for atomic clocks. The operation of a cesium-beam atomic clock is based on the transition between two states in the cesium atom. Rubidium is also used in atomic clocks, and it lowers the cost; however, rubidium clocks are less stable. Hydrogen masers have short-term stability and low long-term accuracy. Mercury ions are also used in some atomic clocks. A particular isotope of Ytterbium has a particularly precise resonance frequency in one of its hyperfine transitions. Strontium has a hyperfine transition that is not precise. But strontium can be driven by solid-state lasers that can be utilized to make a very inexpensive, long-lasting, and compact clock. Cesium atomic clocks are used in laboratories like the National Institute of Standards and Technology to maintain national time scales.

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