WOLFGANG KETTERLE gazes at the cloud of atoms floating in his lab with the
pride of someone who has done the seemingly impossible. And justly so: he has
managed to trap matter in a container made out of nothing but light. The cloud
floats near the centre of a tiny vacuum chamber, and just inside that chamber’s
walls, a thin laser beam rushes round the cloud on all sides, cradling it in an
optical corral.
The container created by Ketterle, a physicist at the Massachusetts Institute
of Technology, might be the world’s strangest, but what it contains is even more
peculiar. For his circling fence of infrared light induces millions of sodium
atoms stuck inside to pull off an impressive trick. At less than a millionth of
a degree above absolute zero, the bizarre laws of quantum mechanics dictate that
the atoms lose their identity, condense into a single quantum state, and form a
giant “superatom”. Instead of bouncing about haphazardly within the trap like
minuscule Ping-Pong balls, they drift in perfect lock step.
This peculiar globule is a new form of matter called a Bose-Einstein
condensate (BEC). It could have all sorts of uses, from making “atom lasers” and
incredibly accurate instruments to providing tools to control matter with a
precision never achieved before. But it might also be used to probe the basic
laws of the quantum world—and could perhaps even overturn the orthodox
quantum theory that has prevailed since the 1920s.
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Before June 1995, no one had ever seen such exotic stuff. Then Eric Cornell
and Carl Wieman, working at the former Joint Institute for Laboratory
Astrophysics in Boulder, Colorado, unleashed an entirely new branch of physics
by creating a BEC in superchilled rubidium gas. “At that moment, the fact that
an atom acts as both a particle and a wave was no longer just scientific
mumbo-jumbo,” says Cornell. “Quantum mechanics was something we could actually
see.” A group led by Randall Hulet at Rice University in Houston, Texas, also
succeeded in creating a BEC that summer with a vapour of lithium atoms, as did
Ketterle’s group at MIT with sodium atoms.
Quantum jelly
But in the past two years, Ketterle and his colleagues have gone even
further. They’ve managed to make two separate BEC-globules collide, showing
that they can interfere with each other, forming crests and troughs like
intersecting water waves. And they have used a BEC to build an atom laser, a
device that pumps out a beam of matter waves rather than light. Today, nearly a
dozen groups routinely make BECs, and quantum mechanics writ large has become
familiar. Even so, the physicists are still genuinely awe-struck as they
confront a new face of nature. “It is rather mysterious to see a blob nearly a
millimetre across in which all the atoms are in an identical state,” says Keith
Burnett, a theorist at Oxford University. “One might think of the condensate as
a quantum-mechanical jelly. We can squash it, squeeze it, and move it around. It
has an appealing weirdness.”
Albert Einstein predicted this weirdness in 1925. His theory, based on
calculations by the Indian physicist Satyendra Nath Bose (after whom bosons were
named), showed that a surprising effect should occur in gases made of identical
particles cooled to within a fraction of absolute zero—almost all of the
particles should condense into a single quantum state of very low energy. But it
was 70 years before physicists had precisely designed magnetic fields and
delicately tuned lasers that could damp atoms’ motions from a frantic slam-dance
to a stately ballet and so bring one of these condensates into existence.
In the now standard recipe for making a BEC, physicists first inject gas into
an evacuated glass cell, and suspend the atoms in the centre with magnetic
fields and weak laser beams. This creates a fancy thermos flask that keeps the
atoms away from the cell’s relatively warm walls. Next, they chill the atoms to
the coldest temperatures ever achieved, which means slowing them down nearly to
a halt. To do this, the researchers tune their lasers so the light slows the
atoms drifting toward each beam, as a barrage of ball-bearings would retard an
oncoming bowling ball. Then they adjust the magnetic fields to put the atoms in
a kind of a steep, narrow bowl. Bursts of radio waves knock the most energetic
atoms over the bowl’s brim, leaving colder atoms behind.
Einstein’s atomic glob
In this way, physicists produce temperatures as low as 50 billionths of a
degree above absolute zero in less than half a minute. At such temperatures, the
region of space within which a particle might be found—known as its wave
function—spreads out and grows larger than the particle’s size. Confine
many such atoms in a tiny volume, and their wave functions overlap. Out of the
gas comes Einstein’s glob of atoms.
This supercold glob is nothing like a dense gel or even an atomic
vichyssoise, but is matter unlike anything else in the Universe. In a cubic
centimetre of BEC, there are just 1014 atoms, about 100 000 times fewer than in
a similar sample of the air we breathe. Getting it to form was a remarkable
challenge: to begin with, most collections of atoms turn into liquids or solids
under such extreme conditions, and bang goes any chance of making a BEC. And
before the first BEC was made, reserachers couldn’t be sure that it would form
even under the best of conditions.
Seeing the newly formed BEC wasn’t easy either. At first this was achieved by
measuring how the particles were distributed according to their velocities, but
this required turning the trap off and watching what happened. Using this
method, the condensate shows up as a sharp spike, as the particles in the BEC
have almost zero velocity (see Diagram).
Now physicists can study BECs without destroying them, by sending a weak laser beam
through the gas and making it interfere with a similar laser beam bounced round the gas
by mirrors. The “probe” beam has the positions of its peaks and troughs shifted slightly
backward or forward, relative to the reference wave, by its interactions with
the gas. As a result, the interference pattern between the probe and reference
beams reflects the properties of the medium through which the “probe” laser
travelled. Images made this way clearly show the condensate in the centre of the
trap (see Diagram).
“Initially, we thought the condensates would be fragile,” says Ketterle. “But
it’s proven totally the opposite. They are extremely sturdy and robust, and we
can play with them in any number of ways.” Researchers now routinely make BECs
containing a million atoms, and under the right conditions they persist for
nearly a minute. Ketterle has pushed up to 10 million atoms, and he thinks
billion-atom BECs are just around the corner.
Theorists are racing to work out how a BEC ought to behave—how it
should flow, what kinds of sound waves should travel through it and so on. They
speculate, for example, that pulses and swirls, once set into motion in a BEC
cloud, will never die down. Similar quantum flows, which result from the lack of
any internal “friction”, or viscosity, occur in superfluid helium and
superconducting materials, in which some of the particles display BEC behaviour.
They would be far more exciting in a gaseous BEC. For now, however, experiments
are driving the field. “The progress has been nothing short of spectacular,”
says Steven Chu of Stanford University, who shared the 1997 Nobel Prize in
physics for his role in developing the laser-cooling techniques integral to
making BECs. Adds Burnett: “It’s sheer delight to see the artistry in these
laboratories. These people are experimental geniuses.”
One of Ketterle’s stunts made the entire field take notice last year, when
his team demonstrated “interference fringes” between two separate BECs. “That’s
the most interesting experiment so far,” says Anthony Leggett, a theorist at the
University of Illinois in Urbana-Champaign. The work showed unambiguously that a
BEC acts not like a group of particles, but like a wave with precisely located
peaks and troughs separated by less than a millimetre.
Caged condensates
To display this startling effect, Ketterle devised a special cage to keep two
BECs apart. Then he removed the barrier and let the BECs expand. The results
matched those in the classic physics experiment where a light beam passes
through two narrow slits and recombines to create bands of dark and light
fringes. Where the two BECs overlapped, their waves alternately reinforced and
cancelled each other out. “That was an amazing demonstration of the coherent
behaviour of atoms,” says Daniel Heinzen of the University of Texas at
Austin.
Working in a different direction, Cornell and Wieman last year cooked up BECs
that contain two types of atoms by mixing two different condensates. All atomic
nuclei and electrons have a basic property called “spin”. Cornell and Wieman
found a way to condense two separate clumps of rubidium atoms with subtly
different spin states. In one clump, the spins of each atom’s nucleus and its
outer electron line up; in the other, they are opposite. The slight energy
difference between these states makes the BECs coexist uneasily, like oil and
vinegar. However, tiny changes in external conditions, such as temperature,
magnetic field or gravity, can cause ebbs and flows of atoms between the two
condensates. “Mixed condensates could become a very sensitive detector for
measuring those changes,” says Wieman.
Back at MIT, Ketterle earlier this year upped the experimental ante with his
all-optical trap, which can hold BECs in three different spin states using only
light. He wasn’t merely trying to perform a fancy technical trick. Whereas the
traditional BEC recipe requires magnetic fields to hold the condensate,
Ketterle’s trap enables him and his colleagues to make “tunable” condensates by
grasping a BEC with light and holding it in an applied and changeable magnetic
field. Ketterle likens this field to a knob that alters the forces between his
sodium atoms. Changing the field strength is equivalent to changing the size of
the electron clouds surrounding each atom, and so changing the forces that act
between atoms.
So far, Ketterle has tuned this interaction force by a factor of ten, but he
hasn’t yet managed to make the sodium atoms attract each other rather than
repel. If he can, he expects to see something startling—a violent
implosion and burst of energy somewhat like a miniature supernova.
These experiments are aimed at fleshing out the basic behavioural antics of
BECs. But physicists are also thinking about ways in which BECs might be put to
practical use. Last year, Ketterle’s group scored another first when it made the
first prototype atom laser. The laser works by generating a sodium condensate
and releasing it to form a pulse that falls under the force of gravity as a
uniform wave (see Diagram). Although this seems to be a far cry from
optical lasers, the physics in both cases is very similar.
In a laser, the light in the beam induces the laser’s “active medium” to emit
more light of exactly the same frequency. Similarly, in a BEC, as the number of
atoms in the condensate grows it becomes ever more likely that other atoms in
the gas will fall into the condensate. In principle, this amplification effect
could be used to make tightly aimed beams of coherent matter waves. A team led
by Nobel laureate William Phillips at the National Institute of Standards and
Technology (NIST) near Washington DC, is using optical lasers to split a BEC and
propel bits of it in the same direction, in rapid succession. Steven Rolston,
part of the NIST team, likens this device to an “atom machine gun”. By making
larger condensates, Rolston says, the team hopes to fire its gun for minutes at
a time.
Will the atom laser revolutionise technology? “We have dealt mainly with
photons and electrons in our technology,” says Allan Griffin, a theorist at the
University of Toronto. “Now we will have a third tool: matter waves. We’re at
the earliest stages, but already it’s clear we can manipulate them to an
astonishing degree.”
Breathtaking accuracy
The first applications of Bose condensates may be in the science of
measurement. The world’s best atomic clocks now use garden variety laser-cooled
atoms, and lose or gain no more than a second every three million years. That
might seem rather good, but it’s not good enough for space navigators, satellite
operators, astronomers and others who must track the passage of time within a
gnat’s eyelash of perfection. The extraordinary stillness of BEC atoms may make
possible clocks thousands of times more accurate. BECs also may spawn a new
generation of interferometers—devices that use interference patterns
between sets of waves to gauge distances. The tiny wavelengths of atoms promise
to yield sensitive new inertial navigation gyroscopes and gravimeters, as well
as tools that could measure the fundamental constants of nature with
breathtaking precision.
But whether a BEC-based atom laser will have any impact on industry is less
clear. “We will never see the same widespread use of atom lasers as optical
lasers, for the simple reason that the beams don’t propagate through air,” says
Phillips. “And yet, they will be as different from conventional atomic beams as
laser light is different from the light of a regular light bulb. There are many
potential uses, but it’s all very speculative.”
Much talk revolves around using BECs as sources for atom
lithography—”airbrushing” patterns onto computer chips or other surfaces.
Because the matter waves in a BEC are “coherent”, which means that they have
well-defined peaks and troughs, they can be controlled more precisely and made
to interfere to produce patterns with high resolution. The problem, says David
Pritchard of MIT, is that existing atomic beams, using noncoherent atoms, have
fluxes of up to 1013 atoms per second. Today’s BECs, he notes, consist of only
107 atoms—and take 20 seconds to accumulate. But Ketterle has confidence.
“In 1994 we were a factor of millions away from achieving BEC with heavy atoms.
We have seen 12 to 13 orders of magnitude improvement since then.”
BECs may find more fundamental uses too. As a giant blob of quantum stuff, a
BEC could be exploited to test the limits of quantum theory itself. Anthony
Leggett points out that quantum theory explains the behaviour of tiny things
such as electrons and atoms very well, it doesn’t explain the behaviour of large
everyday objects.
For example, single quantum particles are notorious for being able to enter
into weird “superpositions of states”, in which they exist in more than one
place at a time. According to quantum theory in its usual form, the same should
be true for much larger things such as footballs or cars. But we never see such
things in big objects. Why?
One possibility is that the “usual form” of quantum theory is wrong, and that
the rules of the game change when a large number of particles become involved.
This is the nub of so-called “spontaneous localisation” theories, a group of
alternative quantum theories which grew out of the work done in the 1970s by
Philip Pearle of Hamilton College, New York State. Roughly speaking, these
theories suggest that quantum superpositions become unstable in large objects
and quickly collapse into one specific state or another. This would explain why
we never see them—they collapse too quickly.
To test this idea, Leggett proposes splitting a BEC into a superposition of
states. These two states might, for example, correspond to the BEC rotating
slowly clockwise or counterclockwise. In such a setup, the BEC cloud would be
analogous to a quantum football with its ghostly essences revolving both ways at
once. Earlier this year, a team of physicists led by J van Cirac of the
University of Innsbruck published a report on the experimental prospects for
making such a state. Building on this work, Leggett estimates that the technical
feat might be achieved within a couple of years.
Are there unknown natural laws that would prevent such a state? Or would
ordinary quantum theory prove true? There’s no way of predicting the outcome,
but if the standard theory is undone by one of its own bizarre offspring, you
can be certain that whatever takes its place will be just as weird and
wonderful.
- Further reading:
Mark Edwards of Georgia Southern University maintains a comprehensive BEC site with links to
every lab and numerous popular articles. Go to http://amo.phy.gasou.edu/bec.html