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The Thin WIre electroStatic Trap.
Credit: University of Rochester
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Traditionally, a complex process of creating and trapping is required
to produce these molecules, akin to repeatedly emptying and refilling
the ice cube trays in your freezer, says Kleinert. A MOT with a TWIST,
however, can create and store the chilled molecules in one place,
instantly - more like a refrigerator with an
automatic icemaker.
While polar molecules are literally as common as water, and dozens of
laboratories around the world can cool atoms to such extreme
temperatures, creating an ultracold polar molecule is difficult.
Ultracold atoms can combine into molecules, but since only one type of
atom can usually be cooled at once, the molecules it makes are
electrically symmetric, not polar. Physicists have to either chill
regular polar molecules, or chill several types of atoms at the same
time and force them to join into molecules. Both processes are so
complex that Kleinert says only four laboratories in the world do them,
and the yield of ultracold polar molecules until now has been very low.
The TWIST, developed with Kleinert's advisor, Nicholas P. Bigelow, Lee
A. DuBridge Professor of Physics at the University of Rochester, makes
the complex process much more efficient, and thus makes available many
more of these molecules.
The secret to the TWIST is the precise thickness of the tungsten wires
that loop around the molecule-production area. In Kleinert's design,
atoms are chilled with the lasers of a MOT, which drains away the
atoms' energy, chilling them to nearly 460 degrees Fahrenheit below
zero.
So far, this is exactly the same as the traditional method, but
Kleinert surrounds his target area with tungsten loops that create an
electric field. The field has no effect on the chilled atoms, but as
the atoms are grouped into polar molecules by a process called
photoassociation, the new polar molecules, with a positive charge on
one side and a negative charge on the other, are affected by the field.
The electric field has a gradient, and due to some of the strange
properties of the quantum world, polar molecules tend to "slide down"
that gradient, collecting in the center of the field. As a result,
says Kleinert, the TWIST collects and holds the low-field seeking
polar molecules but lets other unaffected particles, such as atoms or
other molecules, simply drift away.
Those tungsten loops have to be thick enough that they can withstand
the electrostatic forces they generate, but thin enough that they
don't block the MOT laser initiating the cooling. After months of
trial and error and a lot of burned-out tungsten wire, Kleinert found
that wires just the width of a hair provided the perfect balance.
"The coldest molecules so far have been produced from MOTs, but until
the TWIST came along, electric field trapping and MOTs just didn't go
together," says Kleinert. "Now we can accumulate these polar molecules
continuously, without switching from creation to storage and back
again."
With a good supply of ultracold polar molecules, computer scientists
would have a new tool with which to tackle the creation of quantum
computers, says Kleinert.
Quantum computer scientists are attracted to ultracold particles
because their temperatures reduce decoherence, a phenomenon where your
system decays from the carefully prepared quantum configuration you
started with, to a classical physics state, which loses all the
advantages quantum computers hold.
Ultracold polar molecules in particular are especially attractive
because their strong polarity allows them to interact with each other
over much larger distances than other atomic particles, and the
stronger the interaction between particles, the faster a quantum
computer can perform certain operations.
Ultracold polar molecules may even allow scientists to venture into an
unknown quarter of the Standard Model of Physics -
the size of the electron, says Kleinert. The answer to whether
the electron has a definite size or is just a dimensionless point in
space could support the Standard Model, or support one of the many
alternate models. Trying to approximate the electron's size would
likely require ultracold polar molecules, which can have 100 times the
sensitivity of simple ultracold atoms. That difference could be enough
to make a definitive measurement supporting or chipping away at the
Standard Model altogether.
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