Physicists at JILA have demonstrated a novel "superradiant" laser that works on a subtly different principle than ordinary lasers. In principle, the new JILA laser could be 100 to 1,000 times more stable than the best conventional visible lasers. This superior stability could boost the performance of the most advanced atomic clocks and related technologies such as communications and navigation systems.
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).
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.
The new JILA laser, says physicist James Thompson, is based on a powerful radio technique called phased arrays. "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 [in the JILA laser] spontaneously form something like a phased array of antennas to give you a very directional laser beam."
The atoms in the JILA laser are constantly energizing and emitting synchronized photons, but on the average, very few—less than one photon, in fact—stick around between the mirrors. 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. 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."
The new approach might be used in the future to improve the best lasers developed at NIST as much as 1,000-fold. 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, which in turn could 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.
For more, see the NIST April 4 news story, "JILA Team Demonstrates 'A New Way of Lasing': A 'Superradiant' Laser" at www.nist.gov/pml/div689/superradiant-040412.cfm.
* J.G. Bohnet, Z. Chen, J.M. Weiner, D. Meiser, M.J. Holland and J.K. Thompson. A steady state superradiant laser with fewer than one intracavity photon. Nature. Apr. 5, 2012.
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