Lene Vestergaard Hau’s favorite time of year is midsummer’s eve, when the sky in her native Denmark turns a light metallic blue and the sun stays set for only a few hours. “It never really gets dark,” she says one May morning in her sunny office at Harvard University. “You have these long, light nights. It is just a wonderful time of year. That is the thing I really miss here.” Hau came to the U.S. for postdoctoral work two decades ago, vaulted into a new realm of physics, ignited another one, and has been here since, making the world think differently about the qualities of light.
The speed of light—299,792,458 meters per second in a vacuum—“is an incomprehensibly high speed,” Hau says. “If you could somehow tame that to a human level, it would be completely fascinating.” That is exactly what the 47-year-old physicist has done: she has forced light to plod, pile up and squeeze into a tiny cage, stay docile in that cage and even vanish, only to reappear some distance off. Light slows all the time: photons passing through water decelerate to roughly 224,844,344 meters per second, and they stop and are obliterated when they hit opaque surfaces. But before Hau’s work, light had never lagged to 17 meters per second and, in the same manner, been snuffed out and then revived intact.
Because photons travel far and fast without degrading, they have become the focus of research to develop quantum computers and improve optical communication. Hau’s work is not directly applicable, because her experiments unfold in Bose-Einstein condensates—clusters of supercold atoms acting as one giant collective. Yet her research gets at the root of the challenge of using light to store and process information. By stopping the light, “you are storing a quantum bit. Conceptually, it is a new kind of memory unit,” says Seth Lloyd, a quantum physicist at the Massachusetts Institute of Technology.
Hau, who won a MacArthur Fellowship in 2001, did not plan to be an experimental physicist. Her training was in the theoretical side, although in the 1980s, at home in Denmark and then at CERN near Geneva, she worked on condensed matter. “In doing that, I discovered that people had started to use new techniques of using lasers to cool atoms down to extremely cold temperatures,” she recalls. In 1988 Hau traveled to the U.S. to meet researchers, give talks and satisfy a desire to “see if this country was really like the movies.” Which, she decided, it was: big, with big cars and talkative, open people.
One of Hau’s visits was to the Rowland Institute in Cambridge, Mass., a small nonprofit that joined Harvard five years ago. There she met physicists Michael Burns and Jene A. Golovchenko; both encouraged her to explore cold matter, even though neither worked in that emerging field. “I could have gone to a more established place, but it seemed that that would be too predictable,” Hau says.
Hau set about designing a way to get a constant supply of sodium atoms in a vacuum. She then started cooling her sodium atoms toward absolute zero, and on midsummer’s eve in 1997 she made “some really big, fat” Bose-Einstein condensates. This form of matter had been hypothesized but never created until three scientists—now Nobel laureates—managed to do so in 1995. Hau intended to use light to probe the properties of this new species when she decided to use the condensate to play with light instead. In 1999, in a now famous finding, Hau shone laser light on a condensate, causing photons to creep along inside it. “It was a very, very tricky experiment because it was just on the borderline of what was possible,” she says.
What happens is this: The condensate contains sodium atoms held in place by a magnetic field and illuminated by a “coupling” laser that serves to make the condensate transparent to a specific frequency of light. When photons of that frequency, emitted in a short pulse by a “probe” laser, hit the condensate, they trigger a quantum dark state. This means the sodium atoms enter superposition—they are in two energy states simultaneously. As the photons encounter these atoms, they become entangled with them. The front edge of the light pulse slows, and the back edge catches up, compressing the light like a concertina into the 100-micron-thick condensate.
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.”