Light had been slowed experimentally before by a factor of 165 (to 1,816,923 meters per second or so) using the transparency technique Hau employs. But “by observing light going 17 meters per second, it gave impetus to a worldwide effort in that direction,” says Stephen E. Harris of Stanford University, who collaborated with Hau and first demonstrated electromagnetically induced transparency and slowed light with it in the early 1990s. Researchers have now slowed light in hot gases as well as in crystals and semiconductors at room temperature.
Slowing light led Hau to stopping and starting it. In 2001 she and her colleagues turned off the coupling laser and discovered that the light pulse in the condensate disappeared; its characteristic shape, amplitude and phase, however, were imprinted on the sodium atoms. When the coupling laser came back on, the incoming jolt of energy caused the altered sodium atoms to shift energy levels, in the process releasing a light pulse of the exact phase and amplitude as the one originally sent in by the probe laser. Light had come in with information, conveyed that information to matter and disappeared. Then matter had produced light with that same information. “That is how we preserve information in the system. It is not some random thing that you have no control over,” Hau says.
This year Hau and two members of her lab, Naomi S. Ginsberg and Sean R. Garner, took matters a step further by transmitting the light pulse’s characteristics between two condensates. They sent a pulse from the probe laser into the first condensate, where, as expected, it slowed. Next they turned off the coupling laser. The light pulse from the probe disappeared, but not before it had communicated information about its amplitude and phase to the sodium atoms. These atoms also had momentum from the photonic collision, momentum that propelled them out of the first condensate, across a tiny gap and into the second condensate. Once the atoms—a matter copy of the extinguished light pulse—arrived, the coupling laser was turned back on; the atoms, eager to join the second condensate, shifted energy levels, releasing photons with the exact phase and amplitude of those that had entered the first condensate.
As Hau and Lloyd note, transferring light into matter and back again means that quantum information could be processed. “Basically, the probe light would carry quantum information over long distances in optical fibers,” Hau explains. “Then if you want to do something to it, you read it into matter. We can use matter dynamics to change optical information.” Light interactions in Bose-Einstein condensates have also produced unexpected phenomena—for example, tornadolike storms in the condensates sometimes act like billiard balls, bouncing off one another, and sometimes annihilate one another. “It is a total zoo,” Hau says excitedly. “The experiments show much more detail than the calculations did.”
Hau’s many experiments kept her from the special blue of midsummer’s eve again this year. But she brought Scandinavia to her new suite of labs: the walls are yellow and orange, and there is plenty of light wood. “Colors are very important,” she says. “Colors and light, they are the way you feel how happy you are.” Hau and poet Robert Frost seem of the same mind:
“The light was what it was all about
I would not go in till the light went out
It would not go out till I came in.”
This article was originally published with the title What Visions in the Dark of Light.