March 30, 1999
SCIENTIST AT WORK
Lene Vestergaard Hau: She Puts the Brakes on Light
By MALCOLM W. BROWNE
CAMBRIDGE, Mass. -- Her hair is boyishly cropped, her clothes are
casual, and while riding her bike around Cape Cod she could pass as a
But Dr. Lene Vestergaard Hau is no freshman. A teacher at Harvard
University and an accomplished experimenter, she has gained a
formidable reputation in a branch of science traditionally dominated
by men: physics.
Most of all, she is known as the scientist who slowed light down to a walk.
Dr. Lene (pronounced LEE-nuh) Hau, at 39, is rapidly reaching the top
of her profession, her colleagues agree. On Feb. 18 the prestigious
scientific journal Nature selected for its cover article a paper of
which she was the leading author, and it caused a mild sensation.
In their paper, Dr. Hau, with Dr. Steve E. Harris of Stanford
University and two of Dr. Hau's Harvard students, reported the
results of their experiment in which a beam of laser light was slowed
to the astonishingly low speed of 38 miles an hour. (By comparison,
light in a vacuum travels about 186,000 miles per second.)
Dr. Hau's laboratory at the Rowland Institute for Science in
Cambridge (where she conducts research with the help of her graduate
and post-doctoral students from Harvard) ) is one of a handful of
organizations studying the interactions of lasers with a very
peculiar kind of matter called a Bose-Einstein condensate.
It was by shining precisely tuned lasers on such a condensate, or
cloud, of ultra-cold sodium atoms that Dr. Hau and her team reduced
the speed of a light beam to a pace slower than her bicycle.
The achievement, noted by many newspapers, magazines and
broadcasters, was a tour de force of pure physics, and it may also
herald great practical applications.
Among them could be the development of optical switches that could
enormously improve the performance of computers.
Dr. Hau presides over a half million dollars' worth of apparatus at
the Rowland Institute, a research organization founded by the late
Edwin H. Land, the inventor of Polaroid instant photography, to
support innovative research. Unmarried, Dr. Hau lives near the
laboratory and spends most of her time either at the laboratory or
pondering the problems her next experiment will present.
"I find that the shower is a very good place to think things out,"
she said, "but sometimes I forget what I'm doing. Once I even stepped
into the shower with my clothes on."
Although increasing numbers of women have entered the sciences in
recent years, their participation in physics remains very limited. Of
several hundred scientists attending a typical physics meeting, only
a half dozen or so are likely to be women. Dr. Hau is often asked how
she came to be one of them.
"For one thing," she said, "I was lucky to be a Dane. Denmark has a
long scientific tradition that included the great Niels Bohr, one of
the founders of quantum theory.
In Denmark, physics is widely respected by laymen as well as
scientists, and laymen contribute to physics. For instance, research
in quantum mechanics has been supported in Denmark by the makers of
Carlsberg beer since the 1920's. I myself was supported as a graduate
student for one year by a Carlsberg scholarship."
Dr. Hau was born in Vejle, a small town not far from the University
of Aarhus, on the east coast of Jutland.
"Neither of my parents had any background in science," she said. "My
father was in the heating business and my mother worked in a store.
But both of them believed in giving me the same advantages as my
brother, which was very important to my education."
A prodigy who skipped 10th grade, she was immediately accepted at a
"gymnasium," a European upper school roughly equivalent to the first
two years of college in the United States.
geometry. "All my life I have needed to visualize things, even
abstractions. Without a visualization in my head I'm lost, and
geometry is very visual."
This approach to physics works, she said, even for visualizing
mathematical abstractions like Hilbert spaces (named for David
Hilbert, a German mathematician), which combine a variety of physical
states (or "wave functions") into a single mathematical entity.
"When I first entered Arhus University," Dr. Hau recalled, "I was
bored by physics. They just taught us thermodynamics and classical
mechanics, and that bored me. But I loved mathematics. I would rather
do mathematics than go to the movies in those days.
"But after a while I discovered quantum mechanics, and that got me
interested in physics again, and I've been hooked ever since."
Quantum mechanics is a system of probabalistic rules governing the
discontinuous jumps in behavior of very small particles of matter and
energy. Transistors and many other electronic devices work according
to these rules.
Dr. Hau was awarded her Ph.D. from the University of Arhus in 1991
after completing a dissertation on the "channeling" of electrons
along strings of atoms in a silicon crystal, as if the atomic strings
were miniature wave guides, like the optical fibers used to guide
Along the way she acquired a working knowledge of English, German and
French. French proved to be vital to her work during seven months of
research she conducted at CERN, the European Laboratory for Particle
Physics near Geneva.
"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
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
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
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
An atom at room temperature moves at high speed, but when it is
bombarded from three directions by laser beams it loses energy and
slows down: it cools off. In a complicated series of stages, Dr.
Hau's apparatus uses lasers to cool the sodium atoms part way, and
then evaporates the fastest (and therefore hottest) of them, saving
the coolest in the trap. At the end of this cooling operation (which
takes 38 seconds), the cloud of atoms in the trap has been reduced to
only 50 one-billionths of a degree above absolute zero, a temperature
far colder than any in nature, even in the depths of space.
Because of the Uncertainty Principle -- one of the fundamental rules
of quantum mechanics -- the more precisely the momentum (or velocity)
of a particle is known, the less precisely is it possible to measure
its position, and vice versa. At exactly zero degrees (which in
practice could never be reached), a particle would have zero
momentum, a precise value. This would mean that the particle's
position would be highly uncertain; it might be found anywhere within
a large volume of possible places.
The volume occupied by each ultra-cold sodium atom in Dr. Hau's trap
expands enormously, so much so that atoms in the trap are forced to
overlap and merge into what physicists call a Bose-Einstein
condensate (named for the theorists Satyendra Nath Bose and Albert
Einstein), in which the atoms' quantum wave functions are combined.
Once the condensate is created, a "coupling" laser tuned to resonate
with the trapped mass of atoms is beamed into the trap chamber so
that the atoms and photons of light become "entangled" with each
other, behaving as if they were a single entity. A pulsed laser probe
is then shot into the "laser dressed" condensate from a different
direction, and some of its light passes through, but at a speed 20
million times as slow as the speed of light in a vacuum.
A table covered with a labyrinth of little mirrors guides a network
of beams requiring perfect adjustment, and Dr. Hau and her team work
long and tedious hours, sometimes around the clock. Their eyes become
strained from exposure to the glaring yellow laser spots reflected
from the apparatus.
"Of course, we have to eat while we work, and we like pepperoni
pizza, but we have to be careful to keep the optical system clean,"
she said. "Each cooling cycle for the sodium atoms takes 38 seconds,
and that gives us just enough time to take a bite of pizza, change
hands and flip up one of the mirrors in the optical system to prepare
for the next cooling cycle. It takes some practice."
Dr. Hau is considering an experiment to measure localized wave
functions in the cooled atom trap, allowing her to explore the
structure of the condensate.
"Some people think it might be dangerous, that the trap might
explode. But the most worthwhile physics is often a bit dangerous,"
"When you're looking at the computer monitor and you suddenly see
what you had hoped you would find," she added, "it's the greatest
feeling in the world."