Atomic fountain

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Rubidium atomic fountain.

An atomic fountain is a cloud of atoms that is tossed upwards by lasers in the Earth's gravitational field. 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.[1]

The primary motivation behind the development of the atomic fountain derives from the Ramsey method of measuring the frequency of atomic transitions.[2] In broad strokes, the Ramsey method involves exposing a cloud of atoms to a brief radiofrequency (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.[2] 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.[2] By repeatedly sending clouds of atoms through such an apparatus, the frequency of the field can be adjusted to match the atomic transition frequency.[3]

The precision of the Ramsey method can be increased by increasing the wait time T of the cloud.[2] 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.[2] This is one reason why NIST-F1, a cesium fountain clock, can keep time more precisely than NIST-7, a cesium beam clock.[1]

History[edit]

The idea of the atomic fountain was first proposed in the 1950s by Jerrold Zacharias.[4] 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.[5] However, the attempt was not successful because fast atoms in a thermal beam strike the low-velocity atoms and scatter them.[5] The first successful realization of an atomic fountain clock, using a thermal beam of cesium atoms, was achieved in 1959 by Louis Essen and J V L Parry at the National Physical Laboratory, England.[6]

References[edit]

  1. ^ a b http://www.nist.gov/public_affairs/releases/n99-22.cfm How the NIST-F1 Cesium Fountain Clock Works
  2. ^ a b c d e C. J. Foot (2005). Atomic Physics. p. 212. 
  3. ^ "NIST Launches a New U.S. Time Standard: NIST-F2 Atomic Clock" on YouTube
  4. ^ M. A. Kasevich; et al. (1989). "Atomic fountains and clocks". Optics News 15 (12): 31–32. 
  5. ^ a b S. Chu (1998). "The manipulation of neutral particles" (PDF). Rev. Mod. Phys. 70: 685–706. Bibcode:1998RvMP...70..685C. doi:10.1103/RevModPhys.70.685. 
  6. ^ L. Essen (1959). "An improved cesium frequency and time standard". Nature 184: 1791–1792. Bibcode:1959Natur.184.1791O. doi:10.1038/1841791b0. 

Further reading[edit]

External links[edit]