An atomic fountain is a cloud of atoms that is tossed upwards in the Earth's gravitational field by

lasers A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

. If it were visible, it would resemble the water in a fountain. While weightless in the toss, the atoms are measured to set the frequency of an

atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...

.https://www.nist.gov/public_affairs/releases/n99-22.cfm How the NIST-F1 Caesium Fountain Clock Works The primary motivation behind the development of the atomic fountain derives from the Ramsey method of measuring the frequency of atomic transitions. In broad strokes, the Ramsey method involves exposing a cloud of atoms to a brief

radiofrequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upper ...

(rf) electromagnetic field; waiting a time ''T''; briefly exposing the cloud to the rf field again; and then measuring what fraction of the atoms in the cloud have transitioned. If the frequency of the rf field is identical to the atomic transition frequency, 100% of the atoms will have transitioned; if the frequency of the field differs slightly from the transition frequency, some of the atoms will not have transitioned. By repeatedly sending clouds of atoms through such an apparatus, the frequency of the field can be adjusted to match the atomic transition frequency. The precision of the Ramsey method can be increased by increasing the wait time ''T'' of the cloud. The use of an atomic fountain with a cooled atomic cloud allows for wait times on the order of one second, which is vastly greater than what can be achieved by performing the Ramsey method on a hot atomic beam. This is one reason why

NIST-F1 NIST-F1 is a cesium fountain clock, a type of atomic clock, in the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and serves as the United States' primary time and frequency standard. The clock took less than four ye ...

, a caesium fountain clock, can keep time more precisely than

NIST-7 NIST-7 was the atomic clock used by the United States from 1993 to 1999. It was one of a series of Atomic Clocks at the National Institute of Standards and Technology. Eventually, it achieved an uncertainty of 5 × 10−15. The caesium beam clock ...

, a caesium beam clock.

History

The idea of the atomic fountain was first proposed in the 1950s by

Jerrold Zacharias Jerrold Reinach Zacharias (January 23, 1905 – July 16, 1986) was an American physicist and Institute Professor at the Massachusetts Institute of Technology, as well as an education reformer. His scientific work was in the area of nuclear physics ...

. Zacharias attempted to implement an atomic fountain using a thermal beam of atoms, under the assumption that the atoms at the low-velocity end of the Maxwell–Boltzmann distribution would be of sufficiently low energy to execute a reasonably sized parabolic trajectory. However, the attempt was not successful because fast atoms in a thermal beam struck the low-velocity atoms and scattered them.

References

Atoms Atomic physics Atomic clocks Nuclear technology {{AMO-physics-stub