From the archive, originally posted by: [ spectre ]

Harvard Physicist Plays Magician With the Speed of Light
By Erin Biba 10.23.07 | 12:00 AM

Lene Vestergaard Hau can stop a pulse of light in midflight, start it
up again at 0.13 miles per hour, and then make it appear in a
completely different location. “It’s like a little magic trick,” says
Hau, a Harvard physicist. “Of course, in all magic tricks there’s a
secret.” And her secret is a 0.1-mm lump of atoms called a Bose-
Einstein condensate, cooled nearly to absolute zero (-459.67 degrees
Fahrenheit) in a steel container with tiny windows. Normally — well,
in a vacuum — light goes 186,282 miles per second. But things are
different inside a BEC, a strange place where millions of atoms move —
barely — in quantum lockstep.

About a decade ago, Hau started playing with BECs — for a physicist,
that means shooting lasers at them. She blew up a few. Eventually, she
found that lasers of the right wavelengths could tune the optical
properties of a BEC, giving Hau an almost supernatural command over
any other light shined into it. Her first trick was slowing a pulse of
light to a crawl — 15 mph as it traveled through the BEC. Since then,
Hau has completely frozen a pulse and then released it. And recently
she shot a pulse into one BEC and stopped it — turning the BEC into a
hologram, a sort of matter version of the pulse. Then she transferred
that matter waveform into an entirely different BEC nearby — which
emitted the original light pulse. That’s just freaky. Hey, Einstein
may have set that initial speed limit of light, but he only theorized
about BECs. “It’s not breaking relativity,” Hau says. “But I’m sure he
would have been rather surprised.”

Lene Vestergaard Hau
hau [at] physics [dot] harvard [dot] edu


Hau wins MacArthur:
Physicist recognized for work with light
BY William J. Cromie  /  October 25, 2001

Lene Hau, the woman who stopped light completely, then released it at
will, has won a $500,000 MacArthur Fellowship. She and 22 other
winners will receive $100,000 a year for the next five years to spend
as they wish. No accounting of how the money is spent is required by
the giver of the awards, the John D. and Catherine T. MacArthur
Foundation of Chicago.

No application or interview occurs prior to these awards, and those
who nominate and select the winners remain anonymous. The Foundation
makes only one call, and that call often changes a person’s life.

“I was totally stunned when I got the call,” says Hau, Gordon McKay
Professor of Applied Physics and Professor of Physics. “I didn’t have
a clue. It’s a tremendous honor.” Hau hasn’t decided what to do with
the money yet. “I have to think about it very carefully,” she says.
“It gives me such tremendous freedom.”

Jonathan Fanton, president of the MacArthur Foundation, said that Hau
and the selected fellows have provided “the imagination and fresh
ideas that can improve people’s lives and bring about movement on
important issues.”

Earlier this year, Hau, 41, accomplished what even Albert Einstein
thought was impossible. She slowed light to a standstill then released
it. The feat adds greatly to our understanding of the interaction
between light and matter, and opens the way to building new types of
computers that could make the supercomputers of today look puny. Other
practical applications include news ways to communicate solely by
light and coding methods to protect military and personal information.

Making new matter

“The Fellowship will allow me to keep moving into new fields of
research,” Hau says. “Typically, it is very difficult to get funding
when you want to make a 90-degree change in the kind of research you
are known for doing.”

Hau received her B.S., M.S., and Ph.D. degrees from the University of
Aarhus in Denmark. Her training focused on theoretical physics, but
her interest turned to experimental efforts to create a new form of
matter known as a Bose-Einstein condensate. Einstein and Indian
physicist Satyendra Nath Bose theorized that it should be possible to
cool atoms to the point where they lose their individuality and lock
together into a single superatom that would have properties like no
other type of matter. Hau applied to the National Science Foundation
for funds to make a batch of this condensate but was rejected on the
grounds that she was a theorist for whom such experiments would be too
difficult to do.

However, Hau got funding from the Rowland Institute of Science in
Cambridge, Mass., and 15 months later she became one of a handful of
scientists who made the superatom soup. In 1998, she used the
condensate to slow light from its natural speed of 186,282 miles a
second to a pokey 38 mph.

Hau continued these experiments and last year succeeded in bringing
light to a full stop. After parking it for a thousandth of a second,
she and her colleagues brought the beam of light up to full speed

This work gained Hau a tenured professorship at Harvard and a new
laboratory there where she can further explore the basic and practical
implications of controlling the speed of light.

“If I discover a totally new area of research that I want to work in,
the fellowship gives me the funds to pursue it without being told that
it’s not my field,” Hau says.

That’s a good example of what Daniel Socolow of the MacArthur
Foundation meant when he said “the fellowships will provide new
freedom and opportunity … in support of these fellows’ demonstrated
potential for still greater achievement.”


Shocking superfluids
How shock waves travel through a superfluid provides clues to
understanding the deeper
nature of Bose-Einstein condensation. An optical analogue that behaves
as a pure superfluid
could tell us what these clues mean.

nature physics | VOL 3 | JANUARY 2007 |
BY Lene Vestergaard Hau

A Bose-Einstein condensate is one of the most intriguing states of
matter. Its existence was predicted by Bose and Einstein in the 1920s
and it has been particularly clearly demonstrated in recent years with
laser-cooled, trapped atomic gases (1). In these condensates, millions
of atoms occupy the same quantum state — all atoms are in ‘lock step’
with each other and behave in many ways as a single entity — which
leads to a number of interesting properties (2). For example, a
condensate shows superfluidity and can therefore flow without damping
or dissipation. Similarly, Bose-Einstein condensation is responsible
for the superfluid properties of helium below 2.17 K, and for the
lossless conduction of electrical currents in superconductors. Yet
there is much we do not know about Bose-Einstein condensation and
superfluid behaviour.

A particularly interesting way of probing the inner workings of Bose-
Einstein condensates (BECs) is to study how shock waves are generated
and propagate within them. In real BECs this is challenging, and there
have been only a few experimental studies (3-6). In these experiments
done in cooled atom clouds, a small component of non-condensed atoms
co-exists with the condensate and creates interactions with
propagating shock waves that are complex and of fundamental importance
for the understanding of superfluidity. To identify these
interactions, it is important to separate out the shock wave dynamics
due entirely to the condensate component. On page 46 of this issue,
Wan and colleagues describe a system for studying superfluid-like
phenomena (7). By taking advantage of the fact that the interaction of
laser light with certain types of nonlinear optical crystals is
governed by similar equations to those that govern the dynamics in
BECs, they demonstrate an experimentally simple, all-optical system
for studying superfluid-like shock waves. Because laser fields are
perfectly coherent, the optical system mimics the behaviour of a pure
condensate — one that is free from the influence of non-condensed

Shock waves have startling effects, experienced in everyday life when
sonic booms are generated by supersonic aircraft , and they have been
studied in the context of classical gases and fluids for many years.
When a fluid body is subject to a localized disturbance in its
density, such as that caused by an object passing through it or by an
injected laser pulse, it will generate sound waves that propagate
outwards from the disturbance. If this disturbance is sufficiently
strong, for instance when caused by a high-speed impact or by a high-
energy laser pulse, the speed of sound, which increases with density,
varies greatly across the disturbance. For a local density increase
(‘hump’), the central, dense part of the propagating wave will rapidly
catch up with the leading edge of the wave. This in turn generates the
steep front edge of a shock wave’s characteristic asymmetric shape. If
a localized density depletion (‘dark hump’) is induced instead, the
back edge of the disturbance develops a shock front (see Fig. 1a).

In a classical fluid, wave dynamics is dominated by dissipative
effects caused by viscosity in the fluid. This results in well-
defined, propagating shock fronts where density and velocity change
abruptly in a localized region across the front. But in superfluid
BECs, the dynamics of shock waves is governed by dispersion rather
than dissipation, and this greatly alters the behaviour. As a
consequence of the coherent (lock-step) nature of a BEC, when a
density hump is induced, the longitudinal shape of the resulting
propagating shock fronts develops pronounced wiggles. These wiggles
are a result of nonlinear wave mixing and interference effects in the
coherent fluid.

In addition to the potential insights they provide into the physics of
BECs and related systems, superfluid shock waves are of interest in
their own right for their rich nonlinear excitation behaviour (Fig. 1b
and c). The creation of particle-like excitations such as ‘dark
solitons’ and quantized vortices (the superfluid equivalent of
classical tornadoes) represents just some of the many peculiar
phenomena that have been seen to emerge from the propagation of shock
waves through BECs (3). But, just as in high-energy physics where the
most interesting physics is found when two particles collide, the real
fun begins when multiple superfluid shock waves interact. For example,
in shock collision experiments in sodium BECs, new, compound
‘particles’ with a very complex structure have been discovered (4).
Clearly, a rich field is emerging where interesting discrepancies with
theory exist. In this respect, the platform for studying superfluid-
like phenomena presented by Wan et al. could be very useful.

The crystal that forms the heart of Wan’s system is one that shows a
negative, Kerr-type optical nonlinearity. The authors illuminate the
input face of the crystal with a laser field that consists of a
gaussian peak superimposed on a uniform background (a ‘hump-on-
background’ profile). As the field propagates through the crystal, the
nonlinear response of the crystal causes the gaussian peak to spread
in a way that directly mimics the outward propagation of a shock wave
in a two-dimensional BEC. The light field at the output face of the
crystal is imaged by a CCD camera. The strength of the shock is
controlled simply by changing the amplitude of the gaussian peak with
respect to the background field. And more importantly, by
superimposing multiple peaks on this field, the system allows the
generation and study of multiple colliding shock fronts.

The theory with which the authors analyse their results is based on
Maxwell’s equations. Although the Gross Pitaevskii equation, with
which shock behaviour in atomic condensates is usually analysed,
provides a similar mean-field description, inevitable differences
between the two exist. Moreover, it should be noted that the steep
gradients in density and velocity that develop at shock fronts could
lead to a local breakdown of the mean-field description, which could
exacerbate these differences. Recent calculations indicate that a
depletion of an atomic BEC may take place at shock fronts originating
from dark humps induced in the condensate (8). This depletion is
associated with local excitations of atoms out of the condensate and
would cause non-condensed atoms to fill the otherwise depleted cores
of dark solitons and, probably, of vortices as well.

At the very least, being able to compare shock waves propagating in an
atomic BEC with those in the optical system should allow the
contribution to shock dynamics from the BEC component to be identified
and separated from dynamics induced by, for example, interactions of
quantized vortices with non-condensed atoms. Indeed, such insight
could be of great value to our understanding of the breakdown of
superfluidity and superconductivity — an issue that even on its own is
of tremendous importance, from a fundamental perspective and for
practical applications.


1.  Anderson, M. H., Ensher, J. R., Matthews, M. R., Wieman, C. E.
& Cornell, E. A. Science 269, 198-201 (1995).
2.  Pitaevskii, L. & Stingari, S. Bose-Einstein Condensation
(Clarendon, Oxford, 2003).
3.  Dutton, Z., Budde, M., Slowe, C. & Hau, L. V. Science
293, 663-668 (2001).
4.  Ginsberg, N., Brand, J. & Hau L. V. Phys. Rev. Lett.
94, 040403 (2005).
5.  Simula, T. P. et al. Phys. Rev. Lett. 94, 080404 (2005).
6.  Hoefer, M. A. et al. Phys. Rev. A 74, 023623 (2006).
7.  Wan, W., Jia, S. & Fleischer, J. W. Nature Phys. 3, 46-51 (2007).
8.  Damski, B. Phys. Rev. A 73, 043601 (2006).

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