PAUL E. PRESTON
Science may have finally caught up with science fiction. Harvard physicist Lene Vestergaard Hau and her group at the Rowland Institute for Science (Cambridge, MA) have for the first time slowed down, stopped, and restarted a beam of light. Back in February 1999 they slowed light, she says, "to bicycle speed—about 38 miles per hour—and we compressed a light pulse 1 km in length to only 0.05 mm inside a cloud of sodium atoms. Now, it's almost quite like magic; it comes to a grinding halt." At a few billionths of a degree above absolute zero and in a near-perfect vacuum, the magnetically suspended sodium atoms appear as a tiny, brightly shining sun as they absorb and emit a yellow fluorescence reminiscent of sodium street lamps.
Using a dye laser tuned to emit at 589 nm, a beamsplitter, and two acousto-optic modulators, the researchers create two beams—a probe pulse and a coupling beam (see figure). The coupling beam enters the ultracold sodium cloud at a temperature just above that for which Bose-Einstein condensates form. The beam excites the atoms from their ground state into a superposition state called electromagnetically induced transparency, where they are much less able to absorb the probe pulse. Next, a probe pulse, 1 to 2 km long and containing on the order of 27,000 photons, enters the atomic cloud; before the probe pulse has exited the cloud, the coupling beam is shut off, trapping the probe pulse inside the medium.
Hau has stopped the light pulse for up to 2 ms—a virtual eternity for light. At any time within those two milliseconds, if the coupling beam is turned back on, the probe light instantly emerges from the cloud as though nothing had happened. "It exits with its original speed, wavelength, shape, and everything," says Hau. "All the coherent information in the light was stored in the atomic cloud." The researchers can also halt two or three portions of a single light pulse, a possible application for a quantum computer in which it would read in, store, and read out different channels of optical information.
When the light stops, it imprints its optical properties into the sodium atoms by tinkering with their energy levels. "It's like writing a holographic phase grating into the cloud," said Hau. When atoms are very cold they start to behave like waves—they show wave nature and have a wavelength equal to the distance between the atoms. The ingoing optical field generates an internal atomic field, which generates an outgoing optical field, all without any memory loss. "The experiment has created a completely new optical medium: a mixture of atoms and light and the ability to control its optical properties," says Hau.
"We can generate a fantastically large nonlinear refractive index—14 orders of magnitude greater than what you have in an optical fiber, a hundred thousand billion times more, even at bicycle speeds—with no pulse spreading at all, and it becomes even greater at slower speeds," she adds. "This would permit nonlinear optics to be used with low-power lasers for infrared to visible conversion. Or, put on a single chip, it could be a very sensitive optical switch that you can open and close with extremely low switching energy."
Fellow researcher Chien Liu says they will continue to study the properties of cold atoms, nonlinear optics, and the quantum manipulation of atoms and light in optical fibers and silicon chips. He added that they did not break any known physical laws, such as "stopping time," a question he has been asked. Liu said, "To stop time we'd have to stop all the light in the universe, but all we stopped was a small piece of laser pulse in the lab."
Currently, Hau's team is working on a 5 x 16-ft. tabletop, which is, according to Hau, "filled with all kinds of lasers, electromagnets, a vacuum chamber, and optical gizmos to cool the cloud." In further research, they will decrease the size of the apparatus to 1 cm2, essentially to match the actual size of the cloud.
Paul Preston is a freelance technology writer living in Rancho Dominguez, CA.