Quantum Teleportation

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Posted by rdom on December 23, 1997 at 09:23:13:

On a Table, `Beam Me Up
Scotty'-Like Experiment
Using `Quantum Teleportation,' Researchers Make
State of Photon Vanish and Reappear

For the first time, scientists have demonstrated a form of
teleportation -- the perennial dream of science-fiction writers --
in a tabletop experiment.

The researchers from the University of Innsbruck in Austria,
who report their success in today's issue of the journal Nature,
caused something to vanish at one point and reappear
instantaneously a couple of yards away in the lab even though
there was no physical connection or form of communication
between the two locations.

The term "teleportation" conjures up images of objects
disintegrated in one place and reassembled in another. In this
case, what the Innsbruck team teleported was a physical
condition: the state of a photon (a particle of light) that was
destroyed in one place and simultaneously showed up in

They did not, however, transport anything massive, much less
something as ponderous as a "Star Trek" commander.
Although that is not expressly precluded by the laws of physics,
determining the precise state of every single subatomic particle
in a human body and sending instructions to copy them
elsewhere would require prohibitive amounts of data and
unimaginable feats of processing.

"Even for an object as small as a bacterium," said IBM fellow
Charles H. Bennett, one of six theorists who predicted the
teleportation effect four years ago, "it would be extremely hard
and would probably be more trouble than it was worth."

Unlike transfer of signals by radio or optical waves, there was
absolutely no kind of connection or communication between
the two locations. Instead, the information was carried by a
ghostly process called "quantum teleportation."

"In theory," said Innsbruck scientist Anton Zeilinger, "there is
no limit" to how far the process can send something. In a few
years, the technique might make possible hugely sophisticated
"quantum computers," new means of encrypting messages and
novel ways to store information about unstable entities such as
atoms that are just about to decay.

"It's a wonderful physical phenomenon," said Williams College
physicist William K. Wootters, who predicted the effect along
with Bennett, and now "this theoretical possibility is actually
within reach."

The landmark experiment relies on two peculiarities of quantum
mechanics, the often mystifying and counterintuitive rules that
govern the behavior of matter and energy on the smallest
scales. In those dimensions, physicists discovered early in this
century, objects such as subatomic particles do not have
specific, fixed characteristics at any given instant in time.

Instead, each particle exists in a sort of wavelike miasma of
superimposed probabilities that it will have a particular position
or momentum, or some other state. In fact, quantum mechanics
decrees, an individual particle does not actually have any
definite properties until it is measured. The act of measuring
somehow forces a particle or photon suddenly to collapse into
only one set of values, and destroys all the other possibilities.

A second, even weirder, peculiarity involves certain physical
processes that produce pairs of particles that must by nature
have opposite or complementary characteristics. For example,
if one particle of the pair spins clockwise, the other must spin
counterclockwise; if one photon is polarized in a certain plane,
the other is polarized the opposite way.

However, like any other quantum object, neither half of the
coordinated duos -- called "entangled pairs" -- has specific
properties before it is measured. This poses some outlandish
possibilities, as Albert Einstein and collaborators Boris
Podolsky and Nathan Rosen noted in a famous 1935 analysis.

Suppose, they reasoned, that an entangled pair of particles, A
and B, is created and each particle flies off into space in the
opposite direction. According to the probabilistic requirements
of quantum mechanics (which Einstein thoroughly distrusted,
arguing that God "does not play dice" with the universe),
neither particle has definite characteristics until it is measured.
Suppose further that one waits until the particles are millions of
miles apart, and then measures particle A. That act of
measurement forces A to assume one fixed set of properties
out of its myriad possibilities.

But because the other half of the entangled pair, B, has to have
the opposite properties, the act of measuring A instantaneously
"tells" B what to be. And if the particles are millions of miles
apart, that means that those instructions would somehow have
to travel between A and B far faster than the speed of light, an
outcome utterly forbidden by the laws of physics.

Einstein was rarely wrong. But numerous experiments have
shown that the effect he contemptuously dismissed as "spooky
action at a distance" is a fundamental aspect of nature.

And it is the principle behind the Innsbruck experiment.
Zeilinger and colleagues wanted to see if they could teleport
quantum information between a sender and a receiver; in this
case, two clusters of apparatus on an optical equipment bench.

The team created an entangled pair, and sent one photon ("A")
to the sender position and the other ("B") to the receiver
position a meter or two away. They then sent the receiver a
third photon ("C") whose specific polarization constituted the
information or "message" they wanted to transmit.

The sender equipment combined C and A and scrambled them
together into another entangled pair. Then it measured the pair,
destroying both photons in the process. The polarization of C
was already known, and A had to be the opposite of C.

But also, by definition, A had to be the opposite of B, the
photon that went to the receiver position. So if A was the
opposite of C, and B was the opposite of A, then B had to be
the same as C. That is, the polarization state of the C photon
should have been teleported accurately to the B photon -- even
though the two had never been in contact.

When the experimenters looked at the photon detector at the
receiver position, that is exactly what they found time after

"I would give a lot to know what Einstein would think about
this," Zeilinger said.

An Italian team recently achieved similar results, according to a
companion report in Nature. Such findings may hasten
development of a readable "quantum computer" in which
particles exist in several superimposed states simultaneously
and thus can perform various calculations simultaneously.

For example, encryption schemes use numbers so large that
finding their factors take conventional computers hours, days or
weeks. But a "quantum computer could factor a 200-digit
number in a few minutes, and a 500-digit number before
lunch," said physicist Benjamin Schumacher of Kenyon
College. In addition, Wootters said, teleportation might provide
a way to store information about fragile or unstable quantum
states in durable locations such as large, stable atoms.

"The realm of the possible is a bit bigger than we thought a few
years ago," Schumacher said. The field is moving so fast that
"I'm absolutely astonished about every 18 months."

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