Teleporting, the Quantum Way
Source: Space.com
October 12, 2000
You're under attack in an alien world. You're outgunned,
out-manned. It's time for a quick getaway -- it's time to teleport.
Of course, in the fictional Star Trek universe, all
you'd have to do is request an "immediate beam-out." In the real
world, as scientists currently understand it, your time would be
better spent dictating your last will and testament.
On the other hand, if you were a photon... an atom or a
molecule, maybe even a single-celled critter... there may be hope.
Spooky science
Building on a landmark paper first published in 1993
that examined methods of applying a phenomenon known as
"entanglement," teams of physicists at laboratories in Austria, Italy
and the United States have successfully "teleported" light-carrying
particles called
photons.
Strictly speaking, the teleportation process involves a
photon's individual quantum state, a key characteristic that defines
that particle's nature and behavior. Teleportation isn't exactly
faster-than-light, in the sense that an intermediate,
light-speed-or-below step is required.
One technique to create entanglement involves sending a
brief pulse of ultraviolet light through a specialized crystal that
splits a single, high-energy photon into two lower-energy photons.
Because the particles' polarization -- a mathematical property that
describes a photon's physical orientation -- is complementary, the
photons are said to be entangled.
In order to teleport, however, a third photon must also
be involved.
When the first of the two originally entangled
photons --- call it photon A --- is sent to the same location as an
un-entangled third photon --- call it photon C --- then A and C
subsequently become entangled. A special test, known as a Bell-state
measurement is then conducted, and photon C loses its original
quantum-state identity. Instantly, the second of the two originally
entangled photons --- call it photon B --- is itself transformed: B
becomes C. For all practical purposes, photon C has teleported to B's
location. And B is nowhere to be found.
Despite B's quantum-state destruction (because its
polarization is now that of C, for all intents and purposes, B is C
and no longer B), no cosmic rules have been broken. No information has
been sent faster than light because, for B to become C, information on
A's properties must be communicated via light-speed-observing devices
like radios, telephones or computers.
Additionally, there is no violation of the Heisenberg
uncertainty principle, which holds that no observer can know both the
position and speed of a basic particle (which would allow otherwise
impermissible, exact copying of matter down to the subatomic level).
Still, the strange fact that one particle can "know" the
state of another led Albert Einstein to describe entanglement as
"spooky action at a distance" when he and colleagues Boris Podolsky
and Nathan Rosen first described entanglement (called by insiders the
EPR effect) in the 1930s.
"It's as if two particles are in direct communication
with one another regardless of distance," says Williams Wootters,
professor of physics at Williams College and a co-author of the 1993
paper that inspired the quantum teleportation experiments.
"Entanglement seems to have nothing to do with separation.
Teleportation should work no matter how
far sender and receiver wander."
Are people next?
Teleporting a photon is one thing, but teleporting
people is quite another. A photon has a relatively simple structure.
By contrast, human beings are comprised of a mind-boggling array of
particles arranged in particular ways.
Not only would scientists need to exploit
as-yet-undiscovered means of exactly duplicating the quantum states of
all the particles in a human body, but fantastically powerful
computers would also be required for accurate reconstitution.
Even given enough raw materials from which to build a
teleport object, atom by atom, the time required for reconstruction
appears to be so ridiculously long, making any such effort
impractical.
"In principle, you can recreate anything anywhere, just
as long as you send information on the object luminally, you have the
raw materials and you're willing to destroy the original," said Hans
Christian von Baeyer, author of Taming The Atom and Chancellor
Professor of physics at the College of William and Mary.
"But it would take an unbelievable amount of data
processing. Even a coffee cup, without the coffee, would take many
times the age of the universe. Right now teleporting the information
that comprises a human being seems outrageously impossible."
In his book The Physics of Star Trek, physicist Lawrence
Krauss, Professor of Astronomy and chair of the Department of Physics
at Case Western Reserve University, describes the difficulties of
building a Trek-like transporter as nearly insurmountable.
In some 200 years, assuming continuing improvements to
computers, Krauss says we may have the requisite computational power
for teleportation. But there will be plenty of other problems that
appear to contemporary eyes to be insolvable.
"A single particle like a photon can tunnel through a
barrier, disappearing on one side and appearing on the other," Krauss
said. "We can't walk through walls. People are a complex, classical
system [of particles]. Some day we may figure out ways to make
classical systems behave quantum mechanically. But there's a big
difference in declaring something possible and then making it
practical. Transporting anything other than a particle is extremely
implausible."
The future of weird
What may be practical in the short term is the
application of quantum-teleportation techniques to information
processing and cryptography. In theory, quantum computing would be so
fast as to make contemporary supercomputing seem like slow-motion
stone carving. And quantum-mechanics-based encryption would, by its
very nature, be unbreakable.
"I think we'll be able to exploit quantum weirdness over
the next century," Krauss said. "Technology will allow us to see the
application of quantum mechanics on ever larger scales. As a result we
'll find devices and uses that people haven't thought of. It'll be new
and ingenious and unexpected."
Writing in Scientific American, Austrian physicist Anton
Zeilinger takes a bullish view on teleportation's potential, declaring
that ways soon many be found to teleport more complex systems without
violation of any physical laws. "The entanglement of molecules and
then their teleportation may be reasonably be expected within the next
decade," he wrote. "What happens beyond is anybody's guess."
Teleportation's ultimate payoff may be an enhanced
understanding of quantum mechanics. For physicists, the experimental
validation of theory may be satisfaction enough. But for society at
large, the benefit from quantum weirdness -- quantum science has thus
far led to semiconductors, lasers, CD players and all manner of things
digital --- may be substantial and tangible.
"All my career I've heard people say, 'Don't worry about
the deeper meaning of quantum mechanics. Just shut up and calculate,'"
von Baeyer said. "Now people are digging at the roots. People are
talking seriously about actual machines. We'll end up with a far
better understanding than we've ever had."
By James Schultz
Special to SPACE.com
http://www.space.com/businesstechnology/technology/quantum_teleportati
on_001012.html