The USNO Cesium Fountain Project
We are pursuing a research project to build
and study atomic fountain frequency standards. This work initially dealt
with the processes of laser cooling and trapping of cesium atoms.
Background
Cesium atoms are widely used in atomic
clocks. One of the transitions
in Cesium has an oscillation
frequency of 9 192 631 770 Hz, which is used to define the second. In a standard atomic clock, the cesium atoms in a hot beam are interrogated
twice by microwave radiation. The first pulse starts the oscillation between
two hyperfine states and a second, later pulse stops the oscillation. The information about the frequency of the microwaves is encoded in
the population of the two states of the cesium. This type of clock is a passive device.
The Atomic Fountain
The fountain geometry increases the time between the two interrogations by gently tossing
the atoms up and letting them fall back down under the influence of gravity,
all under high vacuum. Atoms are collected and then launched through a single
microwave cavity, which interrogates the atoms both on the way up and again on
the way down. The atoms are then detected optically to determine the
information about the microwave frequency. This cycle is
then repeated. The longer time between interrogations improves the precision of the measurement, as does the use of
a single microwave cavity.
We use light pressure
to manipulate the atoms: collect them, launch them at a controllable velocity, and keep them cold
to minimize the expansion of the atom cloud during its ballistic flight.
The USNO Fountain Design Goals
The USNO fountain is designed to be a
reference device, not a primary frequency standard. In other words, our device
does not have to "tick" at precisely one second, (the job of defining
the second in the US belongs to NIST)
but must be as stable as possible. This means
that we must minimize fluctuations in factors that might affect the fountain,
such as temperature, magnetic field, and number of atoms in the signal.
Short- and long-term performance goals
The short-term performance of the fountain is
a combination of two factors: the interrogation time, which determines the
spacing of the interference fringes generated by scanning the external
microwave frequency, and the resolution of an individual fringe, which depends
on the signal-to-noise ratio.
Our launching allows a one-half second
interrogation of the atoms. This means that the interference fringe peaks will
be spaced 2 Hz apart. If our microwaves are on the cesium resonance, this means
we know the microwave frequency to better than 1 Hz, or about 1 part in 1010
(ten billion). We can generate signal-to-noise sufficient to resolve the fringe
to about one part in a thousand, so our overall short-term performance is between one and two parts in 1013 (ten
trillion) at one second.
As the clock runs over a period of time, we
are able to average out any random noise that is present, so the performance
improves until non-random (systematic) noise sources begin to dominate. This systematic floor depends on how well we limit fluctuations in the
systematic noise terms, and a long-term performance of a few parts in 1016
should be achievable.
In a recent data run we were able to steer out
the drift of the Hydrogen Maser with which the fountain is compared, revealing
the underlying fountain's medium-term performance.
email questions or comments to theanswerguy at atom.usno.navy.mil
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Take a look at the Fountain! A schematic and photo of the fountain A closeup of the atom trapping chamber The laser system The fountain without the magnetic shields The fountain and the web
page are still under construction Info on the new Rubidium Fountain New pictures! (3/15/04) |
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Last updated 8/13/04
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