Fig 1: This is the core of the National Institute for Standards and Technology NIST-F1 primary standard atomic clock the extensive support systems are not shown. Primary-standard atomic clocks require supercooled plumbing near absolute zero, lasers, optics, and electronics, as well as considerable operating power and attention, and occupy a large room (Figure 1). In the SI (Système international- International System of Units) definition of base units, the second is defined by setting the fixed numerical value of the cesium frequency Δν Cs (the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom) to be 9,192,631,770 when expressed in the unit Hertz (Hz, the reciprocal of seconds). This cesium “fountain” atomic clock contributes to the international group of atomic clocks that define Coordinated Universal Time (UTC), the official world time. In the United States, the National Institute for Standards and Technology (NIST) developed, operates, and maintains NIST-F1, the nation’s primary time and frequency standard located in Boulder, Colorado. They are the absolute standard of time in meteorology. Atomic clocks have shrunk from room-sized assemblies to PC-board mountable modules, admittedly with lower performance than their big siblings but still far better than crystal-based timing alternatives.Ītomic clocks are based on the transitions of energy states of specific elements such as cesium, ytterbium, rubidium, and others under specific and tightly controlled conditions.
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