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Similar
patterns of "noise" are evident in these two
images. The images are taken directly after molecules
have been split into entangled atom pairs. Each of the
pictures shows the absorption of laser light by potassium
atoms in one out of two different energy states. High
concentrations of atoms absorbing light are circled
in yellow, and areas with fewer atoms are circled in
green. The similar pattern in the two images directly
shows the correlation between atoms in the different
states.
Click
here for a higher resolution version of these images.
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Patterns
of noise—normally considered flaws—in images
of an ultracold cloud of potassium provide the first-ever
visual evidence of correlated ultracold atoms, a potentially
useful tool for many applications, according to physicists
at JILA, a joint institute of the National Institute of
Standards and Technology (NIST) and the University of Colorado
at Boulder.
Described in the
March 21 online issue of Physical Review Letters,* the
noise analysis method could, in principle,
be used to identify and test the limits of entanglement,
a phenomenon Einstein called “spooky action at a distance.” With
entangled atom pairs, for example, the properties of one
atom instantaneously affect the properties of its mate, even
when the two are physically separated by substantial distances.
Such tests of the basic rules of quantum physics could be
helpful, for example, in efforts to design quantum computers
that would use the properties of individual neutral atoms
as 1s and 0s for storing and processing data.
The method demonstrated
at JILA also could enable scientists to “see,” for
the first time, other types of correlations between atoms
in fermionic condensates, a new
quantum state first created by the same JILA research group
(see http://www.nist.gov/public_affairs/releases/fermi_condensate.htm),
in which thousands of pairs of atoms behave in unison. And
it could perhaps be applied in highly sensitive measurement
techniques using beams of entangled atoms.
“There are a number of interesting quantum states
that are not obviously seen if you just take a picture,” says
Deborah Jin of NIST, leader of the research group that developed
the new method and also previously created fermionic condensates. “A
Fermi condensate, for example, would not show up in an ordinary
image. However, correlations between atoms should actually
show up in the noise in these images.”
The noise
appears as speckles in images of a cloud of ultracold potassium
atoms made under very specific conditions. This noise is not
random, as would be expected ordinarily, but rather appears
in duplicate patterns suggesting, although not proving, that
pairs of atoms are entangled with each other—even when
separated by as much as 350 micrometers. (For comparison,
a human hair is about 70 micrometers wide.)
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Ultracold
molecules (center) are split into entangled pairs of
atoms flying apart in opposite directions. A laser beam
(left) is used to create shadow images of the cloud
(right). The pairs of entangled atoms can then be found
by carefully studying the noise pattern in these pictures.
(credit: Markus Greiner/JILA)
Click
here for a higher resolution version of this image.
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In the
JILA method, Markus Greiner, Cindy Regal and Jayson Stewart
use a laser
to trap and cool a cloud of about half
a million potassium atoms to near absolute zero temperature.
Then a second laser is shined on the atoms, which absorb
some of the light, and an image is made of the shadow pattern
behind the atoms. The darkest areas have the highest concentrations
of atoms that absorb the light. The grainy or dappled pattern
of lighter and darker areas represent the so-called “atom
shot noise.”
The JILA atom
imaging system is designed to minimize other sources of
noise, such as from the laser. For instance, the
set-up ensures that a relatively large amount of light is
captured per pixel (or dot) in the digital image, and that
each atom absorbs a relatively large amount of light. In
addition, image-processing techniques are used to filter
out laser noise and to find the optimal pixel size for “seeing” the
noise pattern.
For the
experiments, the atoms are prepared in two groups, one
at the lowest of 10 possible energy levels in potassium,
and the other at the next-lowest energy level. A magnetic
field is swept across the trapped mixture of the two groups
to combine pairs of atoms of different energy levels into
weakly bound molecules. (In this way a molecular version
of a Bose Einstein condensate can be created, a state of
matter first realized with atoms in 1995 at JILA; see http://www.bec.nist.gov/index.html.)
Then the magnetic field is increased to split the molecules
and create pairs of atoms that are, based on previous studies
and fundamental quantum mechanics laws, known to be entangled.
In one
experiment, the JILA team made images of the two groups of
atoms separately by tuning the laser to a frequency of light
absorbed by only one group at a time. The two images were
physically overlaid so that the shot noise in sets of corresponding
pixels could be compared. Using mathematical techniques to
analyze the images, the scientists found similar patterns
of dark and light areas, clear evidence for correlated atoms.
In a
second experiment, scientists split the molecules with
a radio wave
pulse into pairs of entangled atoms flying apart with equal
momentum but in opposite directions. The scientists again
took images of each set of atoms and overlaid them. But this
time, they systematically rotated one image to check for
correlations
in noise patterns. Similar patterns were found after a 180-degree
rotation, in pixels on opposite sides of the cloud, clearly
indicating correlated atom pairs. In this experiment the
atom pairs are detected as far as 350 micrometers apart,
and as
a result fascinating quantum phenomena like the “spooky
action at a distance” could be studied.
The research was supported in part by the National Science
Foundation and National Aeronautics and Space Administration.
As a non-regulatory
agency of the U.S. Department of Commerce’s
Technology Administration, NIST develops and promotes measurement,
standards and technology to enhance productivity, facilitate
trade and improve the quality of life.
*M. Greiner,
C.A. Regal, J.T. Stewart, and D.S. Jin. 2005. Probing
Pair-Correlated Fermionic Atoms through Correlations
in the Atom Shot Noise. Physical Review Letters,
posted online March 21, 2005.
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