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Physics News Update
Number 665, December 10, 2003 by Phillip F. Schewe, Ben Stein, and James Riordon

Light Frozen in a Hall of Atomic Mirrors

In a new experiment a pulse of light has been stopped without losing its optical energy. A few years ago, two different Harvard groups succeeded in slowing and then storing a pulse of light in atomic vapor. In that work the propagation of light pulses was halted by vesting the properties of incoming photons into the spin orientations of the atoms in the vapor.

Thus light pulses had been stopped by ceasing to exist in the form of electromagnetic energy while ceding all of its signal qualities to the atomic vapor. Later they could be reconstituted into propagating light beams (Update 521).

Now, a new experiment, also conducted at Harvard, brings light to a halt but leaves the pulse intact as an optical entity. Mikhail Lukin and his colleagues begin as before by converting the incoming light pulse into a corresponding ensemble of spins in a vapor. But then something else is added: a pair of counter-propagating laser beams ease the pulse back into existence. But the control beams also serve to herd the atoms in just such a way as to cause them to act like a stack of mirrors.

In this hall of atomic mirrors, the original pulse still exists as electromagnetic radiation, but it cannot move---it persists within a fixed stationary envelope. Thus the light pulse containing optical photons is literally frozen in space. It can be held and released into motion again on command.

The present experimental work follows a theoretical proposal published last year in Physical Review Letters (A. André and M. D. Lukin, 30 September 2002). Researchers believe that the new phenomenon that they demonstrated may be used to controllably localize, shape and guide stationary photonic pulses in three spatial dimensions. This can create ideal conditions for different light beams to interact or "talk" to each other since localized light electromagnetic energy can be held in one place for a relatively long time. Such techniques may enable nonlinear interactions between faint laser pulses that could be useful for processing light signals.

For example, this process might serve in optical computing, where calculations are carried out not with electrons but with photons. Another ambitious goal would be to perform logic operations between individual photons in future quantum computers. But the researchers say that much further work is still needed to determine if the present work can aid of any of these applications. For now, it's just another step toward ultimate control of light. (Bajcsy, Zibrov, and Lukin, Nature, 11 December 2003.)

Do Quantum Measurements Change If the Detector Moves?

For example, could a count of the number of photons in a burst of light depend on the location of the detector in an extreme gravitational field? These ideas, long pondered by physicists, might be verifiable in the lab, according to a new theory in which a Bose Einstein condensate (BEC) of cold atoms acts as a stand-in for the universal vacuum.The related notion that potential energy residing in the vacuum can influence the geometry of spacetime and thus the expansion of the cosmos could also be testable in a tabletop experiment here in Earth.

The pertinent phenomenon that would facilitate this line of research is called the Unruh-Davies effect, which suggests that a detector accelerating (not just moving at a constant speed but actually moving ever faster) through a vacuum will effectively encounter photons coming out of the vacuum. (A related phenomenon is the Gibbons-Hawking effect, in which photons, "Hawking radiation," can be detected in the gravitationally intense region of a black hole.)

In the Unruh effect the energy needed to turn virtual photons into real photons would be supplied by the accelerating detector itself. The detector would see the vacuum not as an empty space but as a thermal bath of photons. The same effect can disrupt quantum teleportation (see Update 660). The "temperature" of this bath would be proportional to the detector's acceleration.

Actually observing such a thermal bath (equivalent to an effective temperature of something like 10-15 K for a detector acceleration one hundred thousand times more than that felt by us on the surface of the Earth) with any foreseeable manmade detector is close to impossible, but two physicists at the Leopold-Franzens-Universitaet in Innsbruck, Petr Fedichev (peter.fedichev@uibk.ac.at) and Uwe Fischer (uwe.fischer@uni-tuebingen.de), believe the effect could be probed by studying how sound waves ripple through BECs in the lab. The superfluid condensate of atoms would correspond to the vacuum and phonons would be analogous to photons moving through a curved space-time. Before the experiment can be performed, larger BECs than used so far will be needed, as well as sharper optical manipulation of atoms in the BEC. (Fedichev and Fischer, Physical Review Letters, 12 December 2003)