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Schematic of the Experiment. We
collide a clock atom with atoms in another state (labeled |4, 4>).
When the clock atoms collide, they experience a discrete jump,
which is the difference of two s-wave phase shifts. The s-wave
phase shifts can be measured with atomic-clock accuracy and give
precise information about the atom-atom interactions.
Schematic of Fountain Clock
Juggling Cesium Atoms. Two clouds of cesium atoms are tossed
upward with a short time delay so that they collide after passing
through the microwave cavity (silver) on the way up and before
they return down through the cavity. The microwaves in the cavity
excite and probe the atoms, resulting in an atomic fountain clock.
Images by Kurt Gibble, Penn State |
These phase shifts, which cause jumps in the
atom's ticks, limit the accuracy of the world's most accurate atomic
clocks. Until this study, these shifts had been impossible to measure
with high precision because earlier techniques relied on knowing the
atom's density, which cannot be measured accurately. "Atomic clocks
detect the entire wave-function of the atom, and this gives a
frequency shift that is proportional to the density of the atoms,"
Gibble explains. "But, in our new technique, we detect only the part
of each atom's wave function that is scattered during a collision with
another atom, and these atoms see a huge frequency shift that is
independent of the density of the atoms."
The method detects s-wave shifts, or jumps in time,
that the atoms experience during a collision, which Gibble explains "makes
some probability for the atoms to travel spherically outward, which is
known as an s-wave." These s-wave phase shifts are of vital importance
in many areas of research in contemporary atomic physics; for example,
efforts to make use of such exotic concoctions as Bose-Einstein
condensates and degenerate Fermi gasses. "The dreams for Bose-Einstein
condensates include using them to make an atom laser that would be
orders of magnitude more sensitive than a regular laser to enable
ultra-precise navigation and measuring gravity so precisely that you
perhaps could detect oil or other gravity anomalies underground,"
Gibble explains. "Degenerate Fermi gasses may shed light on important
condensed-matter physics problems, including high-temperature
superconductivity, which could have enormous technological
implications including dramatically improving the efficiency and speed
of electronic circuits and computers."
The research also could help scientists understand
how the very early universe may have behaved differently from the way
it behaves today. "Our method will give the most direct and precise
measurement of ultracold interactions between atoms and will place
stringent limits on a fundamental constant that some cosmologists say
is likely to have had a large change over cosmic time -- the
electron-proton mass ratio," Gibble says.
The experiment by Gibble's research group involves
juggling two ball-shaped clouds of cesium atoms in an atomic fountain
clock. Each cloud is about the diameter of a penny and contains about
a billion atoms. Before the atoms in the clouds collide, they approach
each other like balls on a pool table but, because quantum physics
rules their behavior, they act nothing like balls on a pool table
after they collide. The atoms pass through each other undeflected,
without any change in their direction, as if they were unaffected by
the collision. At the same time, they also "scatter" by radiating
outwardly as a symmetric sphere centered at the point where they
collided. This outwardly expanding sphere is known as an s-wave. "Each
atom is in the quantum-mechanical superposition of being both
scattered and undeflected after a collision at these extremely low
temperatures," Gibble explains.
At temperatures that are a millionth of a degree
above absolute zero, quantum effects dominate the atomic collisions.
"In atomic fountains, the atoms are so cold that they collide at such
low energies that they don't have enough energy to have even one
quantum unit of angular momentum about one another," Gibble explains.
This condition greatly simplifies their scattering by producing just a
spherical s-wave "The only effect of a collision at low energies is a
shift of the phase of the outgoing spherical s-wave, which we are able
to isolate and detect with a couple of laser pulses," Gibble says.
In atomic fountain clocks, clouds of atoms rise and
fall like water droplets in a water fountain. But unlike water
fountains, atomic clocks include a region of microwaves that the atoms
pass through twice, once as they are shooting up and a second time as
they are falling back down. The microwaves allow the tick rate of the
atoms to be detected. "By comparing the phase of the microwaves with
the phase of the s-waves of the scattered atoms detected after a
collision, we can directly see the phase shifts that occur when the
s-waves scatter," Gibble explains. "This new technique can be used
with a wide variety of atoms, and our first results already are
comparable to the best results from previous techniques. Future
versions may be 100 times more accurate." |
