Download Bose-Einstein condensates

Bose-Einstein
condensates
Ultra-cold groups of atoms
t h a t a c t a s s i n g l e wave - l i ke
e n t i t i e s o ffe r a n ew p a t h i n t o
t h e q u a n tu m wo r l d
VISIONS 16
B R I E F I N G P A P E R S F O R
P O L I C Y M A K E R S
A. Arnold/Strathclyde University
A
decade ago, physicists
created a new kind of atomic
matter called a Bose-Einstein
condensate (BEC). The
phenomenon is not only
providing new insights into
quantum theory – which
underpins our understanding of
the Universe at the microscopic
level – but also opens the door
to a host of applications such as
atom lasers, improved atomic
clocks and quantum computers.
Quantum theory is a
mathematical framework
which describes the wave-like
behaviour of atoms (as well as
photons of light). Eighty years
ago, Albert Einstein and the
Indian physicist Satyendra Nath
Bose predicted that a cloud of
identical atoms would reveal
their quantum character in an
A rubidium BEC
bouncing off a soft
magnetic mirror
particles with spins having a
whole-number value – now
called bosons.
The first evidence for BoseEinstein condensation was seen
in liquid helium-4 in 1938. Below
2.12 kelvin, helium-4 atoms,
which are bosons, form a fluid
with no viscosity, or superfluid.
Theorists believe that superfluidity is part of the BEC effect.
However, no-one had
observed a BEC in atomic gases
because it wasn’t possible to
cool them down to the required
nanokelvin temperatures. Then
in the 1980s, physicists began
developing ingenious optical and
magnetic techniques for
isolating and trapping atoms,
and slowing down their motions
to a practical standstill. Alkalimetal atoms were ideal BEC
awarded the Nobel Prize in 2001.
During the past decade, activity
in the field has exploded, with
more than 50 research groups
around the world now making
BECs. They have been produced
from all the alkali metals,
hydrogen, helium and even the
heavy metal ytterbium.
The technique is ingenious.
The atoms are slowed down
with laser beams so they can be
caged in a magnetic trap; the
fastest atoms are then released
so that only the most sluggish
and therefore coolest remain.
Methods of confinement
evolved quickly with new
magnetic configurations
developed that could capture
and retain millions of atoms.
More recently, all-optical traps
consisting of a lattice of beams
Making matter behave as waves
unusual way. When cooled to
extremely low temperatures,
they would sink to their lowest
energy state. As each ‘atom
wave’ stretched and overlapped,
their separate identities would
blur to form a single wave-like
‘super-atom’ – a BEC. This
behaviour applies only to
A video chip
used at Imperial
College London
to create a BEC
THANKS
GO
TO
candidates to be cooled by
these methods: their internal
energies are easy to manipulate,
and they are bosons. The race
was on to create a BEC.
In June 1995, Eric Cornell,
Carl Wiemann and their team at
the JILA laboratory in Boulder,
Colorado made the breakthrough.
They cooled 2000 rubidium-87
atoms to less than 2 billionths of
a kelvin, glimpsing the shadow
of a BEC in a flash of laser light.
Shortly afterwards, Wolfgang
Ketterle and colleagues at the
Massachusetts Institute of
Technology created a sodium-23
condensate. They were all
produced by criss-crossed lasers
have been employed.
Captured in such devices,
BECs soon revealed their
quantum wave-like behaviour.
Ketterlee showed that two
separate BECs could be
combined to create an
interference pattern, just as light
waves do. BECs have been
made to quiver like a jelly, ‘ring’
like a bell, and form minitsunamis and smoke rings. They
can be stirred up to form the
persistent vortices tellingly
characteristic of a superfluid.
ED HINDS, KEITH BURNETT, CHRIS FOOT
AND
ERLING RIIS
FOR
Making a
BEC at Oxford
University
One dramatic result arose from
a crucial set of experiments to
manipulate how the atoms
interact. While rubidium-87
atoms form a stable BEC because
they slightly repel each other,
BECs of mutually attracting
lithium-7 atoms tend to implode.
However, judicious use of a
magnetic field can change the
repulsion to attraction. For
example, a self-attracting
rubidium-85 BEC can be made
to be self-repelling so it swells;
if the magnetic field is altered
to make atoms attractive, the
condensate shrinks and then
suddenly explodes like a supernova (dubbed a bosenova) leaving
behind a tiny core of atoms.
More recently, researchers
have exploited this kind of
manipulation to explore new
territory in ultracold quantum
gases. Some types of atoms
have spins with a half-number
value and are classified as
quantum particles called
fermions. These behave quite
differently from bosons
in that they always arrange
themselves so that no two
particles are in the same energy
state. They would not be
expected to form a BEC.
Nevertheless, fermions can and
do pair up into bosons – such
composite particles, derived
from electrons, are responsible
for the related phenomenon of
superconductivity.
In 2003, two research groups,
one at JILA and one at the
THEIR
HELP
WITH
THIS
University of Innsbruck, created
molecular BECs of bosons,
by pairing up fermion atoms of
potassium-40 and lithium-6
respectively under the influence
of a magnetic field. Such
sophisticated experiments allow
physicists to test quantum theory
at its most fundamental level.
Applications
Meanwhile, many research
groups have been exploring how
to exploit BECs. Since an atomic
BEC is a coherent wave, just like
laser light, an ‘atom laser’
becomes a possibility. Such a
device could be exploited in a
similar way to optical lasers to
create a new field of ‘atom
optics’. Atom lasers could be
used holographically to ‘paint’
integrated circuits at the nano
scale, while interferometers
based on atom lasers could
provide a new method of making
precision measurements.
A working atom laser is still
a way off yet. It requires an
efficient method of releasing the
BEC, while also replenishing the
cold atoms and amplifying their
number to provide a continuous
Computer image
of a neutron star
BECS IN THE UNIVERSE
Bose-Einstein condensation is a fundamental
phenomenon of Nature. It is thought to have played
a part in the evolution of the early Universe when
the particles and forces formed, and the outer layers of a neutron star may well
consist of a BEC of exotic subatomic particles. The properties of some advanced
electronic and magnetic materials being explored today are mediated by
composite bosons with quantum characteristics related to BEC behaviour.
PAPER
NASA
Controlling cool atoms
beam. Immense effort has gone
into developing approaches using
magnetic fields, radiofrequency
pulses and optical lasers, but
these are extremely challenging
experiments.
One promising ‘real world’
approach to creating BECs
quickly involves using magnetic
fields generated just above the
surface of a microchip. Ed Hinds
at Imperial College London has
gone one step further, generating
BECs in an array of magnetic
traps on a videotape surface,
and also in a glass-fibre guide.
His aim is to develop a logic
device for quantum computing.
Other UK researchers are also
actively investigating BECs.
Keith Burnett’s group at Oxford
University explores the many
theoretical aspects of BECs,
while Chris Foot and colleagues
are working to cut out a regular
array of ultracold atoms from a
BEC squashed into a thin sheet,
with the aim of producing a
quantum version of analogue
computing (where numerical data
are represented by a changing
physical quantity). Erling Riis and
Aidan Arnold at Strathclyde
University are developing a
magnetic ring to store BECs for
an atom interferometer.
Physicists are now in the throes
of a revolution to manipulate this
new state of matter in a wide
variety of applications. We can
expect exciting
developments in
the future.
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Front:
The growth of a
Bose-Einstein
condensate
Imperial College London
Inset:
Strathclyde University