Atomic clock

An atomic clock counts a transition inside an atom instead of the swing of a balance. The transition in question is written into the SI: BIPM's SI Brochure fixes the caesium frequency, "the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz". NIST states the same history plainly: "The resonance frequency of the hyperfine splitting in cesium has defined the second since 1967." This Codex's entry on the second sets out that definition in full.

NIST describes how its caesium fountain standards find that frequency. "Six infrared laser beams then are directed at right angles to each other at the center of the chamber. The lasers slow down the atoms, and a cold cloud of cesium atoms forms"; the cloud is launched upward through a microwave cavity and falls back through it, and the microwave frequency that excites the most atoms is "the natural resonance frequency of the cesium atom (9,192,631,770 Hz), or the frequency used to define the SI second". NIST states that with such clocks "the definition of the second can now be realized with an uncertainty approaching 1×10-16".

These clocks are the reference behind ordinary civil time rather than instruments anyone wears. NIST notes that "A primary frequency standard contributes to the calibration of the official international time scale Coordinated Universal Time (UTC), and to its local realization UTC(NIST) which is synthesized and distributed from the NIST laboratories in Boulder", and the CGPM's own definition has TAI as "a continuous time scale produced by the BIPM based on the best realizations of the SI second". A radio-controlled watch is not an atomic clock; it is a watch that receives a signal traceable to one.