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.