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Spooky action at a distance
Staff
writer of The Christian Science Monitor
The cat is the potentially tragic hero(ine) in a quantum-physics paradox penned by Austrian physicist Erwin Schödinger in 1935. The cat finds itself enclosed in a box with an atom. When the atom decays, the cat will die. In quantum physics, the cat
and atom exist in both states—alive and dead, decayed and undecayed—until
someone opens the box and checks. Regardless of what the observer sees inside
the box, the states of the cat and the atom are inextricably interlinked. Among the notions Schrödinger
tried to illumine with his paradox is a property he dubbed entanglement. In
effect, the state of the cat “knows” the state of the atom - even at a
distance. Thus, if an experimenter measures the state of one, he or she will
know the state of the other without making the additional measurement—the
relationship between their two states remains constant. Entanglement forms the basis
for key elements in the burgeoning field of quantum computing and communication.
Whether quantum computers will ever be built remains an open question, some
researchers say. But if such computers are built, achieving and maintaining
entanglement will be critical for everything from processing data to
transmitting it. Hence the excitement over a
report this week that physicists in Denmark have entangled two large clusters of
atoms in neighboring containers. The feat, the team says, represents the first
demonstration of entanglement between separated, large clusters of atoms, at
room temperature, and for relatively long periods of time. The Danish team’s effort
is not the first time scientists have entangled atoms, notes Eugene Polzik, who
led the team at the University of Aahrus in Aarhus, Denmark. Last year, for example,
researchers at the National Institute of Standards and Technology in Boulder,
Colo., reported that they had achieved entanglement with four atoms. The
entangled atoms in the NIST experiment established their relationships through
close-up interactions. Milestone
for quantum computing.
“That was a milestone for
quantum computing,” says Dr Polzik. “But it’s not so good for quantum
communications, where you need to have entangled particles miles apart.” His team advanced that
prospect by using a laser to entangle atoms in two containers a few millimeters
apart. His team gathered cesium atoms and confined them in a pair of glass
containers. Each held a trillion atoms. The researchers treated each
sample with a laser to give each cluster’s overall magnetic “spin” its own
orientation. Then the team sent a single laser beam through the samples to
entangle the disparate clouds. A similar laser shot half a millisecond later
showed that while the orientation of each cloud’s spin had shifted somewhat,
the original relationship between the two clouds’ orientations remained the
same. “This is a real step
forward,” says William Wootters, a physicist at Williams College in Williams,
Mass., who studies quantum interactions and did not take part in the experiment. The team’s use of a laser
to entangle the disconnected clouds of atoms holds the promise for
longer-distance quantum communication, which requires a set of entangled
particles at each end of the quantum “connection.” Like kids at an egg-toss
contest, the team plans to continue to widen the gap between samples to see how
far they can separate the clouds and still trigger entanglement. Entanglement has an
embattled history in physics, Dr Wootters says. Back when Herr Schrödinger
was writing about stuffing cats and atoms into boxes, he also held that
entanglement was the one feature of quantum theory that distinguished it from
“classical” physics, in which cause and effect could be distinguished and
one object is forbidden from influencing another object at a distance
instantaneously. By contrast, according to
quantum mechanics, an experimenter could entangle a pair of particles, separate
them by vast distances, then instantaneously change the state of one by changing
the state of the other—even at distances of millions of light years. This “spooky action at a
distance,” according to Albert Einstein and two colleagues, was a direct
result of quantum mechanics if it failed to have more-classical underpinnings.
It so defied common sense that they refused to accept quantum mechanics as a
complete explanation for how physics really worked at the level of the very
small. The debate remained in the
realm of “thought experiments” until 1964, when Irish physicist John Bell,
working at the European Center for High Energy Physics in Geneva, described a
way to test the idea. Moreover, he concluded that
if one followed the details of Einstein’s argument to their logical
conclusion, quantum mechanics was more than incomplete, it was wrong. This
triggered an initial wave of experiments that demonstrated entanglement in the
1970s and 80s. In 1997, a team at the
University of Geneva conducted a particularly dramatic demonstration by
entangling packets of light called photons, then sending them in opposite
directions down fiber-optic lines to detectors nearly seven miles away. When they measured
properties of one photon, it had an instantaneous effect on the other. If the
interaction behaved in a classical way, a measurable amount of time would have
passed between measurement of one and the effect on the other. Polzik’s team is riding
what Dr Wootters calls a “new wave” of entanglement experiments, which has
emerged only in the mid-1990s and is driven by the quest to design and build
quantum computers. Cal Tech physicist Richard
Feynman is credited with being the first to propose the use of quantum
computing, particularly for studying quantum phenomena. Speed,
and more speed
But the idea got its biggest
boost in 1993, researchers say, when Peter Shor at AT&T Laboratories in
Florham Park, N.J., showed that a quantum computer could solve several types of
problems much faster than they could be solved on a conventional computer. Such problems range from
factoring large prime numbers, the key to breaking data-encryption codes, to the
“traveling salesman” problem, which tries to find the most efficient path
for people to take if they need to visit several customers in a given amount of
time. Quantum computers, Wootters
notes, require large assemblages of entangled particles to achieve the
data-crunching power required to solve these problems. Entanglement also holds
the key to quantum communication and quantum teleportation—ways of
transferring quantum information within and among quantum computers. The possibility of quantum
teleportation was first posited in 1993 by IBM researcher Charles Bennett and
colleagues. “Teleportation is a really
unfortunate term,” says University of Michigan physicist Christopher Monroe.
“It implies moving people from point A to point B,” when in fact it refers
to “creating a quantum state in one place that used to exist somewhere else”
with no intervening connection. In order to instantly
teleport those states, he continues, the sender and receiver must share
entangled resources, such as Polzik’s atomic clouds. In what Dr Monroe calls the
most notable teleportation experiment yet, three years ago a team of researchers
at the California Institute of Technology in Pasadena used quantum teleportation
to transfer photons over a three-foot distance. Unlike other experiments
that destroyed the transported photons as part of the process that confirmed
their arrival, the Cal Tech group devised a system to verify the photons’
arrival without destroying them. The next step, Monroe
continues, will be to teleport states of atoms or other particles of matter—a
feat he estimates is still 20 years away. *** *** *** |
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