Caesium standard

An atomic frequency standard that uses the caesium-133 hyperfine transition to realize and maintain precise time and frequency. The transition defines the SI second.

Caesium-133: A stable isotope of caesium with a nucleus containing 55 protons and 78 neutrons. Its ground-state hyperfine transition supplies the reference frequency.
SI second: The SI base unit of time, defined by fixing the caesium-133 hyperfine transition frequency at 9,192,631,770 hertz. The caesium transition provides the second's defining reference.
Isidor Rabi: A physicist who developed molecular-beam magnetic resonance and received the 1944 Nobel Prize in Physics. His resonance method established a foundation for atomic frequency standards.
Hydrogen maser: An atomic frequency standard that uses stimulated emission from hydrogen atoms to produce a highly stable microwave signal. It often offers better short-term stability, while caesium standards provide a direct second realization.
Clock accuracy: How closely a clock's realized frequency agrees with its ideal reference frequency. Systematic shifts limit how accurately a caesium clock realizes the defined transition.
Hyperfine structure: Small energy-level splittings caused by interactions between an atom's nucleus and its electrons. The standard interrogates the two ground-state hyperfine levels of caesium-133.
International Atomic Time: A continuous time scale computed from measurements by atomic clocks worldwide. Caesium clocks contribute measurements used to form this global time scale.
Louis Essen: A British physicist who built an early caesium atomic clock at the National Physical Laboratory. His 1955 clock demonstrated caesium's practical use as a time standard.
Rubidium frequency standard: An atomic frequency standard based on a hyperfine transition in rubidium-87. Rubidium standards are compact and economical, but generally less accurate than primary caesium standards.
Clock stability: The degree to which a clock's frequency remains consistent over time. Statistical fluctuations determine how long caesium measurements need to average.

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