''The scientific papers were written in English, but the meetings announcing the availability of various particle beams were all in French, and without French I would have been lost,'' Dr. Hau said.

''I loved it at CERN. The people were really passionate about their work. Even at 3 in the morning you would always see people in the labs.''

In 1988, while completing her Ph.D. work, Dr. Hau made her first trip to the United States, looking for a post-doctoral job. By then she had received a one-year stipend from Carlsberg so she could accept a post-doctoral university appointment without pay. Among the scientists she met was Dr. Jene A. Golovchenko, a physicist at Harvard, who also worked at the Rowland Institute.

''I told Jene what I had been doing and also that I wanted a complete change in direction. I wanted to work on cooling atoms,'' she said. ''He told me he didn't know anything about cooling atoms but said we could work together on it, so I was given a post-doctoral appointment. Later, the Rowland Institute gave me a staff job and my own laboratory.''

Although she is a permanent United States resident, Dr. Hau has not applied for citizenship.

''I would like to be a citizen so I could vote,'' she said, ''but I would have to give up Danish citizenship, and that would make me feel disloyal. Last week the Danish comedian and pianist Victor Borge performed in Boston for two straight hours -- and it was hilarious -- even though he is 90 years old. Now there's a Dane for you, one to make you feel proud.''

Although much of Dr. Hau's work is done on a blackboard, she is also a ''dirty hands'' physicist who builds and adjusts much of her own complex apparatus.

In 1994, she and Dr. Golovchenko designed and built an ingenious atomic beam source called the ''candlestick,'' which is now a part of Dr. Lau's light-slowing apparatus. The device incorporates a ''wick'' made of gold-plated stainless-steel cloth, which soaks up hot molten sodium metal and wicks the sodium up to a heater that vaporizes it. A jet of hot sodium atoms then shoots out of a pinhole into the cooling apparatus that chills them to a tiny fraction of a degree above absolute zero.