Keeping time with caesium

In 1993, the National Institute of Standards and Technology (NIST) brought into use a caesium-based atomic clock called NIST-7 which kept international standard time to within one second in 10⁶ years; the system depends upon repeated transitions from the ground to a specific excited state of atomic Cs, and the monitoring of the frequency of the electromagnetic radiation emitted. In 1995, the first caesium fountain atomic clock was constructed at the Paris Observatory in France. A fountain clock, NIST-F1, was introduced in 1999 in the US to function as the country’s primary time and frequency standard; NIST-F1 is accurate to within one second in 20 × 10⁶ years. While earlier caesium clocks observed Cs atoms at ambient temperatures, caesium fountain clocks use lasers to slow down and cool the atoms to temperatures approaching 0 K.