When bosons behave like fermions


Physicists
have
blurred
the
distinction
between
bosons
and
fermions
by

coaxing
a
one-dimensional
gas
of
atoms
into
a
Tonks-Girardeau
gas
for
the

first
time.
The
creation
of
this
novel
quantum
state,
which
was
first
predicted
about
40
years
ago,
represents
yet
another
breakthrough
in
the
field
of

ultracold
physics
(B
Paredes
et
al.
2004
Nature
429
277).

All
atoms
are
either
fermions
or
bosons
depending
on
whether
they
possess

half-integer
or
integer
spin,
and
the
difference
between
the
two
becomes

clear
when
they
are
cooled
to
near
absolute
zero.
Fermions
obey
the
Pauli

exclusion
principle,
which
means
that
two
of
them
cannot
occupy
the
same

quantum
state,
but
no
such
restrictions
apply
to
bosons.
This
means
that
large
numbers
of
bosonic
atoms
can
collapse
into
the
same
quantum
ground
state

in
a
process
known
as
Bose-Einstein
condensation.

Belén
Paredes
of
the
Max
Planck
Institute
for
Quantum
Optics
in
Garching
and
co-workers
in
Munich,
Mainz,
Paris
and
Amsterdam
first
made
a
Bose-Einstein
condensate
from
rubidium-87
atoms.
This
condensate
was
then
transferred

to
a
two-dimensional
optical
lattice

an
array
of
potential
wells
created
by

the
interference
of
multiple
laser
beams

so
that
the
atoms
could
only
move
in
one
dimension
along
narrow
potential
“tubes”
(see
figure).

When
bosons
are
confined
in
this
way,
the
repulsive
interactions
between

them

which
are
normally
weak
in
an
ordinary
three-dimensional
cold
gas

become
important
and
tend
to
separate
the
atoms
from
one
another.
As
a

result,
the
bosons
are
prevented
from
occupying
the
same
position
in
space

and
so
effectively
mimic
the
Pauli
exclusion
principle
for
fermions.
To
observe

the
Tonks-Girardeau
regime,
where
this
“fermionization”
is
even
more

pronounced,
Paredes
and
co-workers
introduced
an
additional
optical
lattice

along
the
tubes
that
further
increased
the
repulsive
interactions
between
the

bosons.

To
confirm
that
a
Tonks-Girardeau
gas
had
been
created
the
team
measured

the
momentum
distribution
of
the
atoms
in
the
tubes
and
found
that
it
agreed
with
theoretical
predictions.
Paredes
and
co-workers
now
hope
to
tune
the

bosonic
interactions
in
order
to
observe
behaviour
similar
to
that
shown
by

correlated
fermions.
For
instance,
pairs
of
bosons
might
be
coaxed
into

forming
Cooper
pairs
like
the
electrons
in
a
superconductor.

Author
Belle
Dumé
is
Science
Writer
at
PhysicsWeb

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