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Noisy
Pictures Tell a Story of 'Entangled' Atoms, JILA
Physicists Find
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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.
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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)
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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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This
story has been adapted from a news release -
Diese Meldung basiert auf einer Pressemitteilung -
Deze
tekst is gebaseerd op een nieuwsbericht - |
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