Capturing
the "Holy Grail"
Eric
A. Cornell of the National Institute of Standards and Technology
and Carl E. Wieman of the University of Colorado at Boulder
led a team of physicists at JILA, a joint institute of NIST
and CU-Boulder, in a research effort that culminated in
1995 with the creation of the world's first Bose-Einstein
condensatea new form of matter. The work earned them,
along with Wolfgang Ketterle, a researcher at the Massachusetts
Institute of Technology who did early studies on the properties
of the BEC, the 2001 Nobel Prize in physics.
Predicted
in 1924 by Albert Einstein, who built on the work of Satyendra
Nath Bose, the condensation occurs when individual atoms
meld into a "superatom" behaving as a single entity
at just a few hundred billionths of a degree above absolute
zero. The
71-year quest to confirm Bose and Einstein's theory was
likened by many physicists to the search for the mythical
Holy Grail.
The
BEC allows scientists to study the strange and extremely
small world of quantum physics as if they are looking through
a giant magnifying glass. Its creation established a new
branch of atomic physics that has provided a treasure-trove
of scientific discoveries.
The
condensation was first achieved at 10:54 a.m. on June 5,
1995, in a laboratory at JILA. The apparatus that made it
is now at the Smithsonian Institution.
The
atoms within the condensate obey the laws of quantum physics
and are as close to absolute zerominus 273.15 Celsius
or minus 459.67 degrees Fahrenheitas the laws of physics
will allow. The physicists likened it to an ice crystal
forming in cold water.
"It
really is a new form of matter," Wieman said. "It
behaves completely differently from any other material."
Recipe for a BEC
The
team led by Cornell and Wieman used laser and magnetic traps
to create the BEC, a tiny ball of rubidium atoms that are
as stationary as the laws of quantum mechanics permit. The
condensate was formed inside a carrot-sized glass cell.
Made visible by a video camera, the condensate looks like
the pit in a cherry except that it measures only about 20
microns in diameter or about one-fifth the thickness of
a sheet of paper.
Wieman
started searching for the BEC in about 1990 with a combination
laser and magnetic cooling apparatus he designed. He pioneered
the use of $200 diode lasersthe same type used in
compact disc playersshowing they could replace the
$150,000 lasers others were using. Cornell joined the effort
about a year later.
Wieman's
tactics in pursuing the condensation initially were met
with skepticism in the scientific community. But as his
and Cornell's methods began to show the goal was achievable,
several other teams of physicists joined the chase.
Beginning
with atoms of rubidium gas at room temperature, the JILA
team first slowed the rubidium and captured it in a trap
created by light from the lasers. The infrared beams were
aligned so that the atoms are bombarded by a steady stream
of photons from all directionsfront, back, left, right,
up and down. The wavelength of the photons was chosen so
that they would interact only with atoms that moved toward
the photons.
For
the atoms, "It's like running in a hail storm so that
no matter what direction you run the hail is always hitting
you in the face," Wieman said. "So you stop."
Where
No Temperature Has Gone Before
Wieman's
technique cooled the atoms to about 10 millionths of a degree
above absolute zero, still far too hot to produce Bose-Einstein
condensation. About 10 million of these cold atoms were
captured in the light trap. Once the atoms were trapped,
the researchers turned off the laser and kept the atoms
in place by a magnetic field. Most atoms act like tiny magnets
because they contain spinning charged particles like electrons.
The atoms can be trapped, or held in place, if a magnetic
field is properly arranged around them.
The
atoms were further cooled in the magnetic trap by selecting
the hottest atoms and kicking them out of the trap. This
works in a way similar to the evaporative cooling process
that cools a hot cup of coffeethe hottest atoms leap
out of the cup as steam.
The
trickiest part was trapping a high enough density of atoms
at a cold enough temperature. Cornell came up with an improvement
to the standard magnetic trapcalled a time-averaged
orbiting potential trapthat was the final breakthrough
which allowed them to form the condensate.
Because the coldest atoms had a tendency to fall out of
the center of the standard atom trap like marbles dropping
through a funnel, Cornell designed a technique to move the
funnel around.
"It's
like playing keep-away with the atoms because the hole kept
circulating faster than the atoms could respond," Cornell
said.
The
result was a Bose-Einstein condensate of about 2,000 rubidium
atoms that lasted for 15 to 20 seconds. New machines can
now make condensates of much greater numbers of atoms that
last up to three minutes.
Working
with Cornell and Wieman on the initial BEC were postdoctoral
researcher Michael Anderson and CU-Boulder graduate students
Jason Ensher and Michael Matthews. Over the six years preceding
the discovery, the experiment involved eight graduate and
three undergraduate students at CU-Boulder.
Further
Journeys into the Supercold
As
of September 2001, some three dozen laboratories worldwide
had replicated the BEC discovery and were conducting a wide
variety of experiments.
In
1997, researchers under 2001 Nobel Physics Laureate Ketterle
at MIT developed an atom laser based on the Colorado discovery
that was able to drip single atoms downward from a micro-spout.
In March 1999, scientists at the NIST facility in Gaithersburg,
Md., under 1997 Nobel Physics Laureate William Phillips
created a device that shoots out streams of atoms in any
direction, just as a laser shoots out streams of light.
Made
possible by nudging super-cold atoms into a beam, the breakthrough
could lead to a new technique for making extremely small
computer chips. Eventually, such a device might be able
to construct nanodevices one atom at a time.
In
February 1999, a team of researchers from Harvard University
led by Lene Vestergaard Hau used the BEC to slow lightwhich
normally travels at 186,000 miles per secondto just
38 miles per hour by shining a laser light through the condensate.
In 2001, Hau's team announced that it had briefly brought
a light beam to a complete stop.
On
June 18, 1999, JILA researcher Deborah Jin of NIST and CU-Boulder
graduate student Brian DeMarco used the technique in achieving
the first Fermi degenerate gas of atoms, a state of matter
in which atoms behave like waves. While the Bose-Einstein
experiments used one class of quantum particles known as
bosons, Jin and DeMarco cooled atoms that are fermions,
the other class of quantum particles found in nature. This
was important to physicists because the basic building blocks
of matterelectrons, protons and neutrons -- are all
fermions.
BEC
pioneers Cornell and Wieman are continuing to explore the
properties of their discovery. In 1999, they were the leaders
of a group that created the first vortices ever seen in
the condensates. They also have been doing extensive studies
of two-component condensates.
In
July 2001, Cornell and Wieman were part of a CU-Boulder/JILA
team that was able to make a BEC shrinkan event which
was followed by a tiny explosion. The team said the phenomenon
was similar in some ways to a microscopic supernova explosion
and dubbed it a "Bosenova." About half of the
original atoms appear to vanish during the process.
"We
have gotten down to the nitty-gritty science and have been
able to study the behavior of a new material by manipulating
it in new and different ways," Wieman said. In doing
so, the JILA team cooled the matter to 3 billionths of a
degree above absolute zero, currently the lowest temperature
ever achieved.
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