Re: QUANTUM WEIRDNESS


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QUANTUM WEIRDNESS
The Gedanken Experimenter

http://www.sciam.com/article.cfm?chanID=sa006&articleID=ABBA5449-E7F2-99DF-3ACFAC15B16FEC60&colID=30


In putting teleportation, entanglement and other quantum oddities to
the test, physicist Anton Zeilinger hopes to find out just how unreal
quantum reality can get.

By JR Minkel


ANTON ZEILINGER -- QUANTUM QUESTIONER: Tests the foundations of
quantum physics and the reach of quirky phenomena such as
entanglement.

THE NUMBER 42: Like the answer to everything in The Hitchhiker's
Guide to the Galaxy, quantum mechanics needs a framework to make it
intelligible. Information may be the solution.

Physicist Anton Zeilinger may not understand quantum mechanics, but
he has not let that stand in his path. Besides paving the way for
ultrapowerful computers and unbreakable codes that run on quantum
effects, the 62-year-old Austrian has a gift for pushing the limits
of quantum strangeness in striking ways. Recently he observed the
delicate quantum link of entanglement in light flickered between two
of the Canary Islands, 144 kilometers apart. He dreams of bouncing
entangled light off of satellites in orbit.

Though better known to the world at large for such headline-grabbing
experiments, Zeilinger, who is based at the University of Vienna, has
gone to comparable lengths to test the underlying assumptions of
quantum mechanics itself. His results have left little hiding space
from the conclusion that quantum reality is utterly, inescapably
odd--so much so that 40 years after first encountering it as a
student, Zeilinger still gropes for what makes it tick. "I made what
I think was the right conclusion right away," he says, "that nobody
really understands it."

For almost 17 years Zeilinger's work has centered on tricks of
entangled light. Two particles are called entangled if they share the
same fuzzy quantum state, meaning neither of them begins with
definite properties such as location or polarization (which can be
thought of as a particle's spatial orientation). Measure the
polarization of one photon, and it randomly adopts a certain value,
say, horizontal or vertical. Oddly, the polarization of the other
photon will always match that of its partner. Zeilinger, whose group
invented a common tool for entangling polarization, likes to
illustrate the idea by imagining a pair of dice that always land on
matching numbers.

Equally mysterious, the act of measuring one photon's polarization
immediately forces the second photon to adopt a complementary value.
This change happens instantaneously, even if the photons are across
the galaxy. The light-speed limit obeyed by the rest of the world can
take a leap, for all that quantum physics cares.

Scientists have come to view entanglement as a tool for manipulating
information. A web of entangled photons might enable investigators to
run powerful quantum algorithms capable of breaking today's most
secure coded messages or simulating molecules for drug and materials
design. For six years Zeilinger pushed the record for most number of
photons entangled--three, then four (bumped to five in 2004, then
six, by a former researcher in his group). In 1997 Zeilinger first
demonstrated quantum teleportation: he entangled a photon with a
member of a second entangled pair, causing the first photon to
im­print its quantum state onto the other member. Teleportation could
keep signals fresh in quantum computers [see "Quantum Teleportation,"
by Anton Zeilinger; Scientific American, April 2000].

A few years later his group was one of three to encode secret
messages in strings of entangled photons, which eavesdroppers could
not intercept without garbling the message. He is not always the
first to achieve such a feat, but "he has a very good eye for an
elegant experiment and one that will convey the thing that he's
trying to convey," says quantum optics researcher Paul G. Kwiat of
the University of Illinois, a former member of Zeilinger's lab who is
now a collaborator.

"The only reason I do physics is be­cause I like fundamental
questions," Zeilinger says between bites of bagel with cream cheese
and honey. He had come to Denver for a physics meeting, where he
would tell assembled colleagues of his work beaming entangled photons
between La Palma and Tenerife in the Canary Islands--extending the
range of secret entangled messages by 10-fold. Broad-faced and
smiling, with oval glasses scrunched between his beard and a puff of
frizzy gray hair, he looks a little wolflike--ready to catch quantum
prey. "All I do is for the fun," he says.

Part of his fun is confirming the strangeness of quantum mechanics.
Quantum indeterminacy notoriously bothered Albert Einstein, who
called the theory incomplete. A particle should know where and what
it is, he believed, even if we do not, and it should certainly not
receive signals more quickly than at light speed.

Einstein's view remained a matter of interpretation and in the realm
of gedanken, or thought, experiments until 1964, when Irish physicist
John Bell proved that measurements of entangled particles could
distinguish quantum mechanics from Einstein's position, a mix of
locality (signals flow at light speed) and realism (particles possess
definite, albeit hidden, properties).

Light-based tests of Bell's theorem require two detectors to rapidly
switch the directions along which they measure the polarizations of
entangled pairs. Statistically, local realism dictates that the
polarizations can be linked, or correlated, only for a certain
percentage of measurements. In a classic 1982 Bell test that set the
standard for future attempts, French physicists upheld quantum
mechanics--and upended local realism--by observing a greater
percentage.

Zeilinger's first foray into entanglement was as a theorist, when, in
1989, he co-invented a nonstatistical version of Bell's theorem for
three entangled particles--called GHZ states, after the last names of
the discoverers (Daniel M. Greenberger of the City College of New
York, Michael A. Horne of Stonehill College in Easton, Mass., and
Zeilinger). The trio imagined three entangled photons each striking a
detector set to measure polarization in one of two directions, either
horizontal-vertical or twisted left or right. In principle, four
combinations of detector settings would set up a single measurement
capable of distinguishing quantum mechanics from local realism.

"It was the biggest advance in the whole business of the comparison
of quantum mechanics to local realistic theories since Bell's
original work," says physicist Anthony J. Leggett of the University
of Illinois. Realizing the GHZ experiment took Zeilinger until 2000.

The year before, he also closed a loop­hole in the 1982 French
experiment (other loopholes remain) by using two briskly ticking
atomic clocks to preclude any chance that the detectors were somehow
comparing notes sent at light speed.

A few months ago Zeilinger reported implementing a new kind of
statistical Bell test, devised by Leggett, that pits quantum
mechanics against a category of theories in which entangled photons
have real polarizations but exchange hidden particles that travel
faster than light. In principle, such faster-than-light theories
might have perfectly mimicked quantum strangeness and let realism go
unmolested. Not so, according to the experiment: the results could be
explained only by quantum unreality.

So what idea replaces realism? The situation calls to mind one of
Zeilinger's favorite books, the humorous novel The Hitchhiker's Guide
to the Galaxy, by Douglas Adams, in which a mighty computer crunches
the meaning of life, the universe and everything and spits out the
number 42. So its creators build a bigger computer to discover the
question. (An avid sailor, Zeilinger named his boat 42.)

If quantum indeterminacy is like the number 42, then what idea makes
it intelligible? Zeilinger's guess is information. Just like a bit
can be 0 or 1, a measured particle ends up either here or there. But
if a particle carries only that one bit of information, it will have
none left over to specify its location before the measurement.

Unlike Einstein, Zeilinger accepts that randomness is reality's
bedrock. Still, "I can't believe that quantum mechanics is the final
word," he says. "I have a feeling that if we get really deep insight
into why the world has quantum mechanics"--where the 42 comes
from--"we might go beyond. That's what I hope." Then, finally, would
come understanding.

Q: As in the song, "Where have all the flowers gone, long time passing?",
where have all the galaxies gone, long time passing?

A: The future.

GLB


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