Atomic fountain

An atomic fountain measures an atomic hyperfine transition by letting a cloud of laser-cooled atoms fall through an interaction region under the influence of gravity. The atomic cloud is cooled and pushed upwards by a counter-propagating lasers in an optical molasses configuration. The atomic transition is measured precisely with coherent microwaves while the atoms pass through the interaction region. The measured transition can be used in an atomic clock measurement to high precision.

The measurement of the atomic transition in an atomic fountain uses the Ramsey method. 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 been driven from the initial state to final state. When the frequency of the rf field is resonant with the atomic transition, atoms are detected in the final state. The microwave frequency is swept across the atomic transition over many repeated measurements.

The precision of the Ramsey method is inversely proportional to 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, which may have interaction times on the order of tens of microseconds. This is one reason why NIST-F1, a caesium fountain clock, with a fractional instability of $$2 \times 10^{-16} $$ can keep time more precisely than atomic clocks that use interrogate atomic beams, for example the NIST-7 caesium beam clock, with a fractional instability of $$1 \times 10^{-14}$$.

History
The idea of the atomic fountain was first proposed in the 1950s by Jerrold Zacharias. 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.