Laser Cooling Puts the Freeze on Fast-Moving Atoms
Written by the Microgravity News staff at Hampton University for the Summer 2001 Newsletter
At the beginning of the 20th century, scientists armed with the latest discoveries
from the fields of chemistry, electricity and magnetism, radioactivity,
and quantum mechanics were on the verge of uncovering a number of major
mysteries regarding the atomic world. They understood that atoms were made of
even smaller particles of varying masses that carried both positive and negative
charges, that light given off when an electric current passes through the atoms of
a gaseous substance produces a unique pattern of dark and colored bands (and
that these bands could be used to identify that and
only that particular gas), and they were even beginning
to have some idea about the arrangement of subatomic
particles within atoms.
It was clear that atoms were small particles and
usually behaved the way particles behave, existing in a
single point in time and space. But under certain conditions
the atoms exhibited a “wave-like” behavior. That is,
sometimes an atom exists not as a single-point object,
but rather is spread out over a region of space, known
as the wavelength of the atom. The exact location of the
atom along the length of the wave is uncertain.
Because an atom’s wave properties are inversely
proportional to the speed of the atom, the slower the
atom moves, the longer its wavelength – and the
greater the uncertainty about the atom’s position along
the wave. In order to observe the wave-like behavior of an
individual atom, however, scientists have to reduce the
atom’s speed. They do this by cooling the atom down,
since the cooler the atom, the slower it moves.
 |
| Slowing down atoms enough to produce a Bose-Einstein condensate is a tricky undertaking that involves the trapping and cooling of atoms to extremely low temperatures. (In this artist’s rendering, time progresses from the lower left to the upper right corner. Six cylinder-shaped magnets keep the atoms in place as they cool to a condensate, shown as the central peaks in this series of images.) |
Even more fascinating than the behavior of single
atoms with wave-like properties is the collective behavior
of atoms in large numbers under very unique and
difficult-to-attain conditions. Since atoms of the same
material all display the same type of wave, under unique
conditions, they lock together, like troops marching in
formation. At extremely cold temperatures (when the
length of an atom’s wave increases), dense clouds of
certain types of atoms enter this lock-step formation,
known as Bose-Einstein condensation. (This phenomenon
is named for Satyendra Nath Bose, who formulated rules
to determine when two photons [particles of light] should
be classified as identical or different, and Albert Einstein,
who predicted that these same rules might apply to
atoms.) Bose-Einstein condensates display unusual
“quantum” phenomena, including superfluidity, in which
fluid substances show no resistance to flow. Dense clouds
of ultracold atoms may also mimic certain classes of stars,
offering insight into the reasons stars do not collapse
under the force of their own gravitational attraction.
The trick of getting atoms to reveal these traits,
however, lies with investigators like microgravity funda-mental
physics Principal Investigator Randall Hulet, of
Rice University. Hulet and his research team are learning
more about atomic phenomena by cooling, and therefore
slowing down, these fast-moving particles. In fact, together
with his team of researchers, Hulet has cooled atoms among
the lowest temperatures ever attained.
Just how cold is cold? Approximately 100 billionths
a degree above absolute zero (the value assigned as the
hypothetical lowest limit of physical temperature, or
approximately -273 deg C) is the lowest temperature Hulet
has achieved so far in his laboratory. “In principle,” says
Hulet, “you can get arbitrarily close to absolute zero, but,
course, nobody has [actually reached absolute zero]. The
techniques we are using, laser cooling and atom trapping,
have produced the lowest temperatures to date.”
But even 100 billionths of a degree above absolute zero
still too warm for the studies Hulet hopes to conduct
using atoms vaporized from the lightest of all metals,
lithium. That’s 10 million times colder than liquid helium,
which can be used to instantly freeze most substances.
“We need to get very cold,” says Hulet, “and we aren’t
there yet.” To achieve superfluidity with lithium-6
particular isotope of lithium), Hulet and his group
will have to find a way to make the atoms five times
colder than they already have.
At these almost impossible-to-imagine temperatures,
pairs of lithium atoms should begin to act collectively.
Hulet hopes to produce atoms with wavelengths of 3
microns (millionths of a meter), which is approximately
the mean spacing between atoms in their gas. As the
wavelengths increase, the atoms will overlap one another
and begin to act as a single unit.
 |
| Hulet (third from left) and his team of researchers have cooled lithium atoms to among the coldest temperatures ever achieved, using a combination of laser and evaporative cooling. The object on the table covered in foil is the ‘atomic oven,’ where the experiment starts. |
This behavior, known to physicists as collective
quantum mechanics, has been studied in detail in sub-stances
like liquid helium. At 2 degrees above absolute
zero, liquid helium behaves as a superfluid. It can flow
through a tube the width of a minute blood vessel without
any resistance. It can also climb the walls of a container or
maintain a tornado-like vortex indefinitely once stirred.
Until the advent of laser cooling and atom trapping
-techniques that Hulet and his team have used to cool
and contain atoms – almost all materials exhibiting
quantum collective effects were liquids or solids.
Hulet and his co-workers, on the other hand, are studying
these effects in gases. The quantum aspects of the phase
transition to superfluidity are more readily revealed in
gases, because the effects of strong particle interactions
among atoms that are dominant in solids and liquids
are minimized.
For Hulet’s research, a solid piece of lithium is heated
up to approximately 600 deg C, at which point it becomes a
vapor. The lithium vapor produces a narrow stream, or
beam, of lithium atoms. The laser cooling technique, which
was the topic that resulted in the award of the 1997 Nobel
Prize in physics to Steven Chu, Claude Cohen-Tannoudji,
and William Phillips, slows them down. Laser cooling
works by bombarding the lithium atoms head-on with
photons of light from a laser beam. When the lithium
atoms come into contact with the laser light, the light
bounces off the atoms and scatters in many directions.
“Any time that the atoms scatter some of the light,”
explains Hulet, “they get pushed backward a little bit.
Imagine that the atoms are like bowling balls and the
photons are like ping-pong balls. If you throw enough
ping-pong balls at an oncoming bowling ball, the ping-pong
balls will eventually slow the bowling ball down
and stop it. That’s what we did. We kept bouncing photons
off the atoms, and eventually the atoms stopped moving;
that is, they cooled.”
At this point, Hulet and his team of researchers need
to contain the atoms in order to study their unique behavior.
“But you can’t just put atoms in a box,” says Hulet.
Remember that the natural state of lithium at low
temperature is a metallic solid. “If you put them into a
physical box,” explains Hulet, “they would condense
onto the walls of the container.” Atoms stuck to the
walls of the container would be of no use because this
contact would cause the atoms to heat back up. “In
order to avoid that,” he says, “we put them into magnetic
traps.” A magnetic trap is a carefully arranged magnetic
field that allows atoms to be suspended in space and
cooled without having contact with any container wall.
A few atoms do heat up when disturbed by a chance
encounter with another gaseous molecule. Although care
is taken to rid the trapping container of other unwanted
gases by using vacuum pumps, a random collision with a
stray nitrogen molecule, for instance, will knock some
of the atoms out of the trap. For the most part, however,
once the atoms are cooled and trapped, they tend to stay cool
for several minutes, as long as the pressure of background
gas atoms and molecules is maintained below one hun-dredth
of a trillionth of atmospheric pressure.
After the atoms have been trapped, they may be
cooled to even lower temperatures. The first step is to use
six laser beams, in counter-propagating pairs along three
orthogonal directions, directed at the atoms, so that if
atoms move in any given direction, they get a push back-ward
from the photons of light. “It’s like a viscous fluid
that the atoms have to move through,” Hulet describes.
“Every direction that they try to move, they feel this
resistance.” These laser beams themselves can be precisely
tuned to produce a very narrow band of light. The final
step is to evaporate the hottest atoms out of the trap. This
evaporative cooling leaves the remaining atoms colder,
and if everything works as planned, a phase transition
that produces superfluidity will be observed.
Although a number of elements and compounds have
been found to undergo the superfluid phase transition at
low temperatures, lithium is of particular interest to
Hulet because lithium exists in two different forms, or
isotopes, known as bosons and fermions. Bosons, it
turns out, reach the superfluid transition at higher
temperatures than fermions. The difference lies in the
way each type of atom can be arranged. Atoms that are
fermions resist being crowded together, so only a single
atom can occupy a particular energy level. The force
that keeps atoms from occupying the same place at the
same time is known as Fermi pressure. Bosons, on the
other hand, don’t have this restriction. Any number of
boson atoms can occupy an energy level.
Lithium-6 and lithium-7 are each composed of
three electrons and three protons; however, lithium-6
has three neutrons in its nucleus, and lithium-7 has four.
These two isotopes of lithium are chemically identical,
but the difference in the number of neutrons is enough
to make lithium-6 a fermion and lithium-7 a boson.
Hulet has already seen that bosons cooled to ultracold
temperatures undergo the quantum phase transition to
become Bose-Einstein condensates. His challenge is to
encourage the fermion atoms to also reach this superfluid
transition by forming pairs with other fermions. Pairs
of fermions are bosons, exhibiting the same superfluid
behavior as naturally occurring bosons.
“The prediction is,” says Hulet, “that you have to cool
the lithium-6 atoms to near 50 billionths of a degree above
absolute zero in order to see the pairing transition.” Cooling
the fermions to a sufficiently low temperature has proven
difficult, however, because the fermions resist pairing. If
this pairing could be achieved, it could help researchers
better understand superconductors, explains Hulet.
“Paired fermion atoms would give us a test bed
for the theory of superconductivity, which is not at
all well-understood in some situations,” he says. “It
will give us the ability to look at the phenomena of
superconductivity in a system that we can control
extraordinarily well. We can control the interactions
between atoms, we can probe the atoms, and we can
change the density of the atoms. That’s exquisite control
over a lot of different parameters. With a gaseous sub-stance
we have a kind of ‘designer system’ where we
can fine-tune things.”
An inspired technique demonstrated by Hulet and
his team of researchers just might prove key to cooling
the fermions to temperatures low enough for the pairing
of atoms to occur. The technique involves a lithium atom
cloud containing a mixture of boson and fermion atoms.
The bosons, which readily respond to evaporative cooling,
act as refrigerants for the fermions, which are further
cooled by contact between the two isotopes.
Hulet and his team conduct their research on lithium
atoms in their laboratory at Rice University. In the
normal-gravity conditions in the lab, however, the
clouds of atoms are big enough that their weight causes
them to sag. The magnetic field in the atom trap helps
to suspend the atoms, but the sagging is not completely
eliminated. The distortion of the shape of the atom cloud
caused by the sagging may be enough to compromise
the pairing of electrons between fermion atoms that
Hulet is after. “So we are not sure yet whether the
experiment is going to work in gravity,” he cautions.
Conducting the experiment in microgravity, where the
mass of the atoms becomes a negligible factor, may
allow Hulet to approach the phase transition in which
the fermion atoms abruptly form pairs to become
superfluid bosons.
 |
| Fermi pressure prevents the cloud of lithium-6 atoms (the fermions) from condensing nearly as much as the lithium-7 atoms (the bosons) as the temperature drops from 810 nanokelvins to 240 nanokelvins. |
Even though mixing
bosons and fermions has
not created fermion pairs
yet, it has given Hulet’s
team a snapshot of another
amazing phenomenon.
Images of the atom cloud
containing lithium-6
and lithium-7 atoms
inside the magnetic
trap have revealed
some fascinating
insights into the forces
responsible for stabilizing
stars. As the cloud of
atoms is cooled, it tends to fall to the bottom of the trap
and shrink in size. At a temperature of 500 nanokelvins
(500 billionths of a degree above absolute zero), the
difference between the amount of shrinking that takes
place in the bosons in the atom cloud and that in the fermions
becomes noticeable. The bosons compress more because
multiple atoms are capable of occupying the same energy
level. The fermions, restricted from this kind of
shrinking by Fermi pressure, cannot compress beyond
a certain point. If cooling continues, the bosons will
still shrink to a fraction of the size they were at 500
nanokelvins. The fermions will not. Fermi pressure,
then, is responsible for keeping the fermions in the
atom cloud from condensing further.
 |
| A white dwarf star, a type of star at the end of its life, is stabilized from collapsing from its own gravitational attraction by Fermi pressure. The dwarf stars are circled in this image. |
White dwarf stars, a type of star at the end of its
life, are stabilized against collapsing from their own
gravitational attraction by this same Fermi
pressure, in this case produced
by a high-density electron gas in
the stars. Fermion atoms, even
in vastly different spatial and
energy scales, can exhibit the same
effect responsible for stabilizing
stars, which are much bigger,
much hotter, and have much
higher energy than the atom
clouds in Hulet’s laboratory.
Photographs of the atom
cloud in the magnetic trap clearly
demonstrate the stabilizing power
of Fermi pressure on fermion atoms
and the corresponding compression
of the boson atoms to smaller and
smaller diameters. Hulet’s group
is able to photograph the atom cloud by shining a
laser beam through it and using cameras to capture
shadows cast by the atoms as they absorb some of the
light. Boson and fermion atoms, although mixed in
the same atom cloud, can be imaged separately by
fine-tuning the laser frequency. The lithium-6 and
lithium-7 atoms respond to slightly different colors of
light, so very precise adjustments of the laser beam
can reveal the bosons and the fermions selectively,
and each isotope can be photographed separately.
The next step for Hulet and his team in approaching
cooler temperatures for his atom clouds is to work on
a different type of atom trap, one that uses a focused
laser beam instead of magnetic fields. Hulet hopes that
this laser trap will offer more flexibility in encouraging
the phase transition that will result in the pairing of
fermion atoms. “It’s a technique-driven field,” says
Hulet. “You have to get lower and lower temperatures
and higher and higher densities [of atoms].”
Hulet and his team are encouraged in their pursuits
by the success they have already achieved in their
experiments with lithium atoms. “There’s a lot of
basic fundamental physics to try to understand,”
says Hulet, when it comes to the behavior of atoms
at low temperatures. “When we set out to use laser
cooling and atom trapping back in 1991, they were
still in their infancy,” he recalls. Now these techniques
are taking researchers ever closer to the lowest limits
of temperature possible.
