A caesium clock draws on the hypothesis of stable atomic properties, in particular the frequency of radiation corresponding to the transitions between two energy levels. The atomic frequency standard comprises an atomic resonator which gives the ultimate reference and an electronic part which, using a 5 MHz oscillating quartz, generates an exciter signal of around 9.191 GHz. This interacts with the atomic "stream" inside the resonator to generate an error signal which corrects the frequency of the quartz oscillator to maintain it permanently on the atom transition frequency.
Accuracy of an atomic clock:
- Commercial clocks have an accuracy of approximately 10-12 (atomic clocks at the Physikalisch-Technische Bundesanstalt (Germany) and the National Institute of Standards and Technology (USA).
- Accuracy of 10-14 for laboratory caesium clocks (fountain clock made by the Laboratoire Primaire du Temps et des Fréquences at the Observatoire de Paris in 1995): 3 x 10-15 (accurate to 0.26 nanoseconds per day)).
- Other types of atomic clock are, less accurate, hydrogen masers (the most stable are accurate to within 1 s over 3,000 years), rubidium masers (the least costly), and confined mercury ions (stable in the long-term; undergoing research).
Thanks to the atomic clock, in 1983 the standard metre was redefined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.