Physicists
at JILA have demonstrated a novel “superradiant” laser design, which has the
potential to be 100 to 1,000 times more stable than the best conventional
visible lasers. This type of laser could boost the performance of the most
advanced atomic clocks and related technologies, such as communications and
navigation systems as well as space-based astronomical instruments.
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JILA’s superradiant laser
traps 1 million rubidium atoms in a space of about 2 centimeters between two
mirrors. The atoms synchronize their internal oscillations to emit laser light. Credit: Burrus/NIST
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Described
in the April 5, 2012, issue of Nature,*
the JILA laser prototype relies on a million rubidium atoms doing a sort of
synchronized line dance to produce a dim beam of deep red laser light. JILA is
a joint institute of the National Institute of Standards and Technology (NIST)
and the University of Colorado Boulder (CU).
JILA/NIST
physicist James Thompson says the new laser is based on a powerful engineering
technique called "phased arrays" in which electromagnetic waves from
a large group of identical antennas are carefully synchronized to build a
combined wave with special useful features that are not possible
otherwise.
"It's
like what happens in the classical world but with quantum objects,"
Thompson explains. "If you line up lots of radio antennas that each emit
an oscillating electric field, you can get all their electric fields to add up
to make a really good directional antenna. In the same way, the individual
atoms spontaneously form something like a phased array of antennas to give you
a very directional laser beam."
An
ordinary laser relies on millions of particles of light (photons) ricocheting
back and forth between two mirrors, striking atoms in the lasing material and
generating copies of themselves to build up intense light. Photons with
synchronized wave patterns leak out of the mirrored cavity to form a laser
beam.** The laser frequency, or color, wobbles slightly because the mirrors are
vibrating due to either the motion of atoms in the mirrors or environmental
disturbances—which can be as subtle as people walking past the room or cars
driving near the building.
That
doesn't happen in the new JILA laser simply because the photons don't hang
around long enough. The atoms are constantly energizing and emitting
synchronized photons, but on the average,
very few—less than one photon, in fact —stick around between the mirrors. This
average, which scientists calculate indirectly based on the laser beam's output
power, is just enough to maintain an oscillating electric field to sustain the
atoms' synchronized behavior. Nearly all photons escape before they have a
chance to become scrambled by the mirrors and disrupt the synchronized
atoms—thus averting the very effect that causes laser frequency to wobble in a
normal laser.
Thompson
engineered a system that first traps the atoms in laser light between two
mirrors and then uses other low-power lasers to tune the rate at which the
atoms switch back and forth between two energy levels. The atoms emit photons
each time their energy level drops. The atoms ordinarily would emit just one
photon per second, but their correlated action boosts that rate
10,000-fold—making the light superradiant, Thompson says. This "stimulated
emission" meets the definition of a laser (Light Amplification
by the Stimulated Emission of Radiation).
"This
superradiant laser is really, really dim—about a million times weaker than a
laser pointer," Thompson says. "But it is much brighter than one
would expect from the ordinary uncoordinated emissions from individual atoms."
Thompson's
measurements show that the stability of the laser beam frequency is less than
1/10,000th as sensitive to mirror motion as in a normal optical laser. This
result suggests the new approach might be used in the future to improve the
best lasers developed at NIST as much as 1,000-fold. Just as important, such
lasers might be moved out of the vibration-controlled laboratory environment to
be used in real-world applications.
Despite
its dim light, the extraordinary stability of the superradiant laser can be
transferred by using it as part of a feedback system to "lock" a
normal laser's output. The bright laser, potentially 100 to 1,000 times more
stable than today's best lasers, could then be used in the most advanced atomic
clocks to induce the atomic oscillations that are the pendulum ticks of
super-accurate clocks. The added stability allows for a better match to the
atoms' exact frequency, significantly boosting the precision of the clock. The
improvement would extend to atomic clock-based technologies such as GPS,
optical communications, advanced geodetic surveys and astronomy.
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