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.)