National Physical Laboratory

Operation of Atomic Fountain

Principles of operation of an atomic fountain

Diagram of an atomic fountain
Diagram of an atomic fountain

The atomic fountain is given its name because atoms are launched upwards and fall back under gravity, spreading outwards in the horizontal plane.

The fountain is pulsed and the cycle, which takes approximately one second, can be considered in five stages:

  1. The atoms are collected, cooled and prepared in one of two quantum states, and launched upwards in a vacuum;
  2. They pass through a microwave cavity;
  3. They continue their ballistic flight upwards and fall back under gravity;
  4. They pass once again through the microwave cavity;
  5. Finally their quantum state is detected.

The atoms interact with a microwave field in the cavity and the microwave frequency is adjusted so as to cause the maximum number of atoms to change quantum state on passing through the cavity.The caesium atoms are collected from a caesium background vapour at 10-9 Torr, trapped, and cooled by three pairs of counter-propagating laser beams either in a magneto-optical trap or as optical molasses. Up to 108 atoms can be captured and cooled to a temperature of approximately 1 μK. The atoms are optically pumped into one of the two ground state hyperfine levels (either g1 or g2) and launched upwards. The interaction of the atoms with the microwave field transfers them into a state which is a quantum superposition of the two levels g1 and g2. The atoms continue their ballistic flight in a highly homogenous magnetic field achieved by surrounding the region with several shields constructed from μ-metal. As the atoms fall down through the cavity the second interaction stimulates the transition to either g1 or g2, depending on the phase difference between the field and atomic coherence. Finally, the populations of both sub-levels are measured by fluorescence induced by two laser probe beams.

Last Updated: 23 Sep 2013
Created: 7 Aug 2007

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