Atomic clocks

Caesium-based atomic clocks observe electromagnetic transitions in the hyperfine structure of caesium-133 atoms and use it as a reference point. The first accurate caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK.[63] Since then, they have been improved repeatedly over the past half-century, and form the basis for standards-compliant time and frequency measurements. These clocks measure frequency with an accuracy of 2 to 3 parts in 1014, which would correspond to a time measurement accuracy of 2 nanoseconds per day, or one second in 1.4 million years. The latest versions in the United States and France are accurate to 1.7 parts in 1015, which means they would be off by about 4 seconds since the extinction of the dinosaurs 65 million years ago,[6] and has been regarded as "the most accurate realization of a unit that mankind has yet achieved."[60]
Caesium clocks are also used in networks that oversee the timing of cell phone transmissions and the information flow on the Internet.[64]


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