| The
first sighting of atoms flying in formation has been reported
by physicists at the Department of Commerce’s National
Institute of Standards and Technology (NIST) and the University
of Colorado at Boulder (CU-Boulder) in the Aug. 13 issue of
Physical Review Letters.* While the Air Force and
geese prefer a classic “V,” the strontium atoms—choreographed
in this experiment with precision laser pulses and ultracold
temperatures—were recorded flying in the shape of a
cube.
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Next-Generation
Atomic Clocks
Atomic
clocks get their “ticks” from the natural
oscillations of atoms, a physical property caused by
electrons jumping between orbits of various shapes and
distances from the nucleus. The international definition
of the second, for example, is based on an oscillating
frequency in the microwave range for cesium atoms. In
simple terms, cesium atoms vibrate back and forth more
than 9 billion times a second and this frequency is
used as a type of natural pendulum to keep track of
time.
Strontium’s
two “resonant” absorption frequencies, on
both ends of the higher-frequency, visible radiation
spectrum, provide more “ticks” per second
and make it a potentially more precise “pendulum”
than cesium. An atomic clock based on strontium could
be as much as 100 times more accurate than cesium clocks.
NIST’s
cesium-based F1 atomic clock is already so accurate
it would have to run for 50 million years before it
would be off by one second. Ultraprecise timekeeping
is critical to space travel and other precision navigation
tasks, telecommunications, computing efficiency, advanced
chemistry research and many other fields.
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This
“really bizarre” behavior is believed to occur
with all atoms under similar conditions, says physicist Jun
Ye of NIST, who led the research at JILA, a joint institute
of NIST and CU-Boulder. Ye is also a faculty member of the
CU-Boulder physics department."
Atoms
have not previously been seen flying in formation, says Ye.
Strontium’s unique physical properties make the observations
possible. In particular, the configuration of strontium’s
electrons and the resulting atomic properties allow it to
efficiently absorb laser energy in two very specific “resonant”
wavelengths—a strong resonance at a wavelength of blue
light and another, much weaker resonance for longer-wavelength
red light. This makes strontium a promising candidate for
a next-generation atomic clock based on optical rather than
microwave frequencies, and is the reason the JILA team is
studying the atom’s quantum behavior (see text box at
right).
The experiment
was conducted with a dense gaseous cloud of 100 million strontium
atoms. The atoms were held in the center of a vacuum chamber
with both a magnetic field and six intersecting laser beams,
in three sets of facing pairs aligned at right angles to each
other. The atoms, which were very hot initially at 800 Kelvin
(980 degrees F), were trapped with the magnetic field and
blue laser light and cooled to 1 milliKelvin. Then with red
laser light, the atoms were rapidly cooled further to about
250 nanoKelvin (almost absolute zero, or minus 459 degrees
F).
At this
point, the magnetic field was turned off, and the red laser
beams were tuned to a slightly higher frequency than strontium’s
weak, red resonance frequency. This caused the atoms to fly
apart in cubic formation. The shape was observed in a series
of images (see graphic) by hitting the atoms with blue laser
light. The atoms absorb the laser energy by shifting an outermost
electron to a higher energy orbit but then very quickly decay
back to the lower energy state by re-emitting blue light.
Even though different atomic packets are flying away at different
speeds and directions, the strong blue fluorescence signals
emitted from the atoms can be recorded with a camera, and
all of the atoms can be visualized at the same time.
The flying
structure was created in part by a recoil effect—the
momentum kick received by the atoms as they absorbed or emitted
each particle of light, or photon. This effect is similar
to the recoil received when shooting a gun, Ye says. When
an atom absorbs photons from a laser beam, it is pushed in
the same direction as the incoming beam. The Doppler effect—the
same reason that a train’s whistle sounds lower as it
moves away from you—causes the incoming laser beam,
as the atoms fly away, to appear to have a slightly lower
frequency, moving closer to the atom's resonant frequency.
Meanwhile, the opposing laser beam appears to have a higher
frequency as the atoms rush toward it, and falls farther out
of resonance. Strontium atoms very quickly stop responding
to the latter beam and fly off in the same direction as the
resonant beam.
The combined
effect of all six incident beams makes the atoms fly away
in cube-shaped clusters. The research team captured images
of the atoms flying in clusters at speeds of about 10 to 15
centimeters per second. The cubic arrangement of the atomic
clusters changes as the intensity and frequency of the red
light varies. The corners of the cube appear first, when the
laser light is tuned relatively close to the atomic resonance.
As the red light is tuned further above the atomic resonance
frequency, atomic clusters appear at the mid-points of all
sides of the cube, as well as eventually at the centers of
each face.
Ye conducted the
work and co-authored the paper with JILA postdoctoral research
associates Thomas H. Loftus and Tetsuya Ido, and CU-Boulder
doctoral candidates Andrew D. Ludlow and Martin M. Boyd.
Funding
for the project was provided by the Office of Naval Research,
the National Science Foundation, the National Aeronautics
and Space Administration, and NIST.
As a non-regulatory
agency of the U.S. Department of Commerce’s Technology
Administration, NIST develops and promotes measurement, standards
and technology to enhance productivity, facilitate trade and
improve the quality of life.
* Thomas H. Loftus, Tetsuya Ido, Andrew D. Ludlow, Martin
M. Boyd, and Jun Ye. 2004. Narrow Line Cooling: Finite Photon
Recoil Dynamics. Physical Review Letters 93(7), Aug.
13.
These
colorized images show strontium atoms forming a “cube”
as the frequency of laser light used to manipulate them changes.
(Left) Atoms become visible at the eight
corners of a cube. (Middle) Atoms also appear
at the midpoints of the lines forming each cube face and begin
to appear at the center of each cube face. Right)
Atoms appear at the corners, as well as at the midpoints and
more clearly at the centers of each cube face.
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