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Nearly Lightless Laser Has Bright Future
Apr 2012
BOULDER, Colo., April 4, 2012 — A new “superradiant” laser design that traps 1 million rubidium atoms into a 2-cm space between two mirrors produces a deep-red laser beam that could boost the performance of most advanced atomic clocks, communications and navigation systems, and space-based astronomical instruments.

Artist's concept of the Thompson group's new superradiant laser. (Image: The Thompson Group and Brad Baxley, JILA)

Scientists at JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, developed the prototype, which has the potential to be 100 to 1000 times more stable than the best conventional visible lasers. The device synchronizes the rubidium atoms with an engineering technique called “phase 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 otherwise possible.

“It’s like what happens in the classical world, but with quantum objects,” said physicist James Thompson of JILA/NIST. “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.”

JILA’s superradiant laser traps 1 million rubidium atoms in a space of about 2 cm between two mirrors. The atoms synchronize their internal oscillations to emit laser light. (Image: Burrus/NIST)

Ordinary lasers rely on millions of photons bouncing 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 wobbles as the mirrors vibrate under motion of the atoms or environmental disturbances.

In the new laser, this does not occur because the photons are not around long enough to wobble. 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, calculated by the scientists based indirectly on the laser beam’s output power, is enough to maintain an oscillating electric field to sustain the atom’s synchronized behavior. Almost every photon escapes before it has a chance to bounce around the mirrors and disrupt the synchronized atoms, which causes laser frequency to wobble in standard lasers.

In Thompson’s system, atoms are trapped in laser light between two mirrors and then tuned to a rate at which they can switch back and forth between two energy levels using low-power lasers. Each time their energy level drops, the atoms emit photons. The atoms ordinarily would emit one photon per second, but their correlated action boosts that rate ten-thousandfold, making the light superradiant, Thompson said. This “stimulated emission” meets laser definition.

“This superradiant laser is really, really dim — about a million times weaker than a laser pointer,” Thompson said. “But it is much brighter than one would expect from the ordinary uncoordinated emissions from individual atoms.”

The scientists’ measurements found that the stability of the laser beam frequency is less than 1/10,000th as sensitive to mirror motion as in standard optical lasers, which suggests the new approach could improve NIST lasers as much as a thousandfold. Just as important, such lasers could be moved out of the vibration-controlled laboratory and into real-world applications.

Although dim, the stability of the superradiant laser can be transferred by using it as part of a feedback system to “lock” a normal laser’s output. It could then be used in atomic clocks to induce the atomic oscillations that are the pendulum ticks of superaccurate timekeepers. The added stability would enable a better match to the atoms’ exact frequency, significantly boosting the clock’s precision. The improvement also could extend to GPS, optical communications, astronomy and advanced geodetic surveys.

For the superradiant laser design to reach its full stability potential and to be of practical use, Thompson stresses that it will need to be recreated using different atoms, such as strontium, which are better suited for advanced atomic clocks.

The research appeared in the April 5 issue of Nature.

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