Microgravity in Bremen, Germany, has
dropped its reusable cold atom experiments down a 146-meter tower into a bed
of polystyrene pellets. The plunge produces nearly 5 seconds of weightless free
fall in which to work—and twice as long
if researchers catapult the experiment up
the tower and allow it to fall back to Earth.
Those few seconds enabled the team to
reach temperatures of 50 picokelvins, the
coldest ever attained, says Ernst Rasel, a
physicist at the Gottfried Wilhelm Leibniz
University of Hanover in Germany and
leader of the QUANTUS collaboration.
(The acronym comes from the German for
“quantum gases under weightlessness.”)
Earlier this year, QUANTUS researchers
launched their apparatus on a sounding
rocket from Kiruna, Sweden. Rising to
an altitude of more than 240 kilometers,
the rocket flight offered 6 minutes of free
fall. During that time, the automated
machinery performed 85 distinct experiments, Rasel says, including producing the
first BEC in space. The ISS, however, will
give CAL far more time—a year or longer—
letting users do even more.
For starters, CAL physicists aim simply to try to reach the lowest temperatures
possible, which might allow delicate new
quantum effects to emerge. Researchers are confident they can dip down to
100 picokelvins and possibly lower, Sackett
says. That may not be quite as low as the
QUANTUS team claims in its drop-tower
result. But the QUANTUS team can perform
just three runs a day, whereas CAL can perform experiments continuously.
In another experiment, Nathan Lundblad,
a physicist at Bates College in Lewiston,
Maine, and colleagues hope to make hollow shells of BECs, something that gravity
squashes on Earth. The bubbles can be fashioned by applying radio waves of the right
frequency to a BEC, Lundblad explains, and
at first, researchers hope to simply jiggle the
bubbles and see how they react.
But the shells might also enable them to
probe the wave nature of the BEC in a new
way. Mathematical consistency demands
that the undulating quantum wave in the
BEC wrap around the sphere and merge
smoothly with itself. As it does so, it might
generate tiny whirlpools called vortices.
Physicists have already produced vortices by
spinning a BEC. In Lundblad’s experiment,
however, vortices would emerge in a new
way—through the interplay of the quantum
wave and the geometry of the bubble.
Others on the CAL team plan to probe
an odd bit of quantum mechanics known
as the Efimov effect that enables certain
atoms to form weakly bound three-atom
molecules, even though no two atoms
CAL science module ISS
to atom chip.
988 8 SEPTEMBER 2017 • VOL 357 ISSUE 6355 sciencemag.org SCIENCE
SPECIAL SECTION ULTRACOLD MATTER
Out in the cold
In 2018, NASA’s Cold Atom
Laboratory (CAL) will rocket to
the International Space Station
(ISS). There, the $70 million
device will chill clouds of atoms
to less than a billionth of a
degree above absolute zero and
create Bose-Einstein condensates
(BECs), in which the atoms
behave like a single quantum
wave of matter and can flow
without any resistance. In orbit,
the atoms will hover weightlessly,
giving physicists more time to
Four steps to frigid temperatures
To chill a gas nearly to absolute zero, CAL employs a multistep process within a vacuum chamber about
the size of a stick of butter. A microchip known as an “atom chip” drives the final cooling steps.
The complete package
The size of an ice chest,
CAL will contain the lasers,
magnetic coils, pumps, and
vacuum chamber needed for
the experiments. Physicists
will run the lab remotely,
doing their experiments
in turns, like the users of a
1. Magneto-optical trapping
Within a magnetic trap, lasers in six directions
counteract the motion of the atoms, slowing
and cooling them.
2. The atom chip takes over
After the atoms are chilled to about 100 microkelvins,
they’re transfered to a purely magnetic trap created
by electric currents in the atom chip.
4. A chilling expansion
To push temperatures lower, the magnetic trap is
weakened. The cloud of atoms expands and cools
while remaining a BEC.
3. Evaporative cooling
Like blowing on hot soup, radio waves from the chip
nudge the hottest atoms out of the trap, leaving
behind cooler ones. A BEC can form at this stage.