http://www.sciam.com/2001/0701issue/0701hau.html

Frozen Light
By Lene Vestergaard Hau

[excerpts only - long article]

In response, the light pulse comes to a grinding halt and turns off. But the information that was in the light is not lost. That information was already imprinted on the atoms' states, and when the pulse halts, that imprint is simply frozen in place, somewhat like a sound recorded on a magnetic tape. The stopping process does not compress the pattern of states, because all of it slows down in unison, unlike the earlier stage wherein the pulse gradually entered the gas.

The frozen pattern imprinted on the atoms contains all the information about the original light pulse. For example, the ratio of states 1 and 2 relates to the intensity of the pulse at each location. We effectively have a hologram of the pulse written in the atoms of the gas. This hologram is read out by turning the coupling laser on again. Like magic, the light pulse reappears and sets off in slow motion again, along with the wave of atoms' states, as if nothing had interrupted it.

The process of slowing and stopping light opens up many interesting experiments. For example, we could send a light pulse through a Bose-Einstein condensate with the light speed adjusted to match the speed of sound in the condensate (around one centimeter a second). Atoms of the condensate should surf along with the light pulse, setting off oscillations of the entire condensate. This would be a completely new way to study superfluid properties of condensates. Condensates can also be produced in a vortex state, wherein the gas rotates, reminiscent of water going down the drain. A pulse of slow light traveling through a vortex would find itself dragged along with the gas--very similar to a phenomenon expected to occur near black holes. With slow light, we can study this and some other black hole phenomena in the laboratory.

Slow light also enables a new kind of nonlinear optics, which occurs, in particular, when one laser beam alters the properties of another beam. Nonlinear optics is a huge field of research, both of fundamental interest and with applications from imaging to telecommunications. Extremely intense beams are usually needed to achieve nonlinear optical effects, but with slow light the corresponding phenomena can be produced with a very small number of photons. Such effects could be useful for creating ultrasensitive optical switches.

Another application for slow and stopped light could be quantum computers, in which the usual definite 1's and 0's are replaced with quantum superpositions of 1's and 0's called qubits. Such computers, if they can be built, would be able to solve certain problems that would take an ordinary computer an enormously long time. Two broad categories of qubits exist: those that stay in one place and interact with one another readily (such as quantum states of atoms) and those that travel rapidly from place to place (photons) but are difficult to make interact in the ways needed in a quantum computer. The slow-light system, by transforming flying photons into stationary dark state patterns and back, provides a robust way to convert between these types of qubits, a process that could be essential for building large-scale quantum computers. We can imagine imprinting two pulses in the same atom cloud, allowing the atoms to interact, and then reading out the result by generating new output light pulses.

Even if frozen light doesn't prove to be the most convenient and versatile component for building a quantum computer, it has opened up more than enough research applications to keep us--and other groups--busy for many more all-night sessions in the years to come.

The Author

Lene Vestergaard Hau is Gordon McKay Professor of Applied Physics and professor of physics at Harvard University and heads the Atom Cooling Group at the Rowland Institute for Science in Cambridge, Mass., where the experiments detailed in this article were performed. She received her Ph.D. in theoretical solid state physics from the University of Århus in Denmark. The author wishes to thank the wonderful Rowland Institute team of Zachary Dutton, Chien Liu, Cyrus H. Behroozi, Brian Busch, Christopher Slowe and Michael Budde, as well as Stephen E. Harris of Stanford University, for an extremely fruitful collaboration.