Caltech physicists achieve first bona fide quantum teleportation
[just archiving the story..TLC]
PASADENA--Physicists at the California Institute of Technology,
joined by an international collaboration, have succeeded in the first true teleportation of a quantum state.
In the October 23 issue of the journal Science, Caltech physics professor H. Jeff Kimble and his colleagues write of their success in transporting a quantum state of light from one side of an optical bench to the other without it traversing any physical medium in between.
In this sense, quantum teleportation is similar to the far-fetched "transporter" technology used in the television series Star Trek. In place of the actual propagation of a light beam, teleportation makes use of a delicate quantum mechanical phenomenon known as "quantum entanglement," the quintessential ingredient in the emerging field of quantum information science.
"In our case the distance was only a meter, but the scheme would work just as well over larger distances," says Professor Samuel Braunstein, a coauthor from the University of Wales in Bangor, United Kingdom, who, with Kimble, conceived the scheme. "Our work is an important step toward the realization of networks for distributing quantum information--a kind of 'quantum Internet.'"
Teleportation of this kind was first proposed theoretically by IBM scientist Charles H. Bennett and colleagues in 1993. The Caltech experiment represents the first time quantum teleportation has actually been performed with a high degree of "fidelity." The fidelity describes how well a receiver, "Bob," can reproduce quantum states from a sender "Alice."
Although quantum teleportation was recently announced by two independent labs in Europe, neither experiment achieved a fidelity that unambiguously required the use of quantum entanglement between Alice and Bob.
"True quantum teleportation involves an unknown quantum state entering Alice's apparatus and a similar unknown state emerging from Bob's remote station," says Kimble. "Moreover, the similarity of input and output, as quantified by the fidelity, must exceed that which would be possible if Alice and Bob only communicated by classical means--for instance, by normal telephone wiring.
"Although there has been wonderful progress in the field, until now there has not been an actual demonstration of teleportation that meets these criteria."
In the experiment, the Caltech team generated exotic forms of light known as "squeezed vacua," which are split in such a way that Alice and Bob each receive a beam that is the quantum mechanical "twin" of the other. These EPR beams, named after the historic Einstein-Podolsky-Rosen (EPR) paradox of 1935, are among the strangest of the predictions of quantum mechanics. It was their theoretical possibility that led Einstein to reject the idea that quantum mechanics might be a fundamental physical law.
A trademark of quantum mechanics is that the very act of measurement limits the controllability of light in ways not observed in the macroscopic world: even the most delicate measurements can cause uncontrollable disturbances. Nevertheless, in certain circumstances, these restrictions can be exploited to do things that were unimaginable in classical physics.
Here, photons from the EPR beams delivered to Alice and Bob can share information that has no independent existence in either beam alone. Through this "entanglement," the act of measurement in one place can influence the quantum state of light in another.
Once Alice and Bob have received their spatially separate but entangled components of the EPR beams, Alice performs certain joint measurements on the light beam she wishes to teleport together with her half of the EPR "twins." This destroys the input beam, but she then sends her measurement outcomes to Bob via a "classical" communication channel. Bob uses this classical information to transform his component of the EPR beam into an output beam that closely mimics the input to Alice, resurrecting at a distance the original unknown quantum state.
A unique feature of Kimble's experiment is a third party called "Victor," who "verifies" various aspects of the protocol performed by Alice and Bob. It is Victor who generates and sends an input to Alice for teleportation, and who afterward inspects the output from Bob to judge its fidelity with the original input.
"The situation is akin to having a sort of 'quantum' telephone company managed by Alice and Bob," says Kimble. "Having opened an account with an agreed upon protocol, a customer (here Victor) utilizes the services of Alice and Bob unconditionally for the teleportation of quantum states without revealing these states to the company. Victor can further perform an independent assessment of the 'quality' of the service provided by Alice and Bob."
The experiment by the Kimble group shows that the strange "connections" between entities in the quantum realm can be gainfully employed for tasks that have no counterpart in the classical world known to our senses.
"Taking quantum teleportation from a purely theoretical concept to an actual experiment brings the quantum world a little closer to our everyday lives," says Christopher Fuchs, a Prize Postdoctoral Scholar at Caltech and a coauthor. "Since the earliest days of the theory, physicists have treated the quantum world as a great mystery. Maybe making it part of our everyday business is just what's been needed for making a little sense of it."
This demonstration of teleportation follows other work the Kimble group has done in recent years, including the first results showing that individual photons can strongly interact to form a quantum logic gate. Kimble's work suggests that the quantum nature of light may someday be exploited for building a quantum computer, a machine that would in certain applications have computational power vastly superior to that of present-day "classical" computers.
*****
By Kenneth Chang
ABCNEWS.com
It's not Star Trek. No “Beam me up, Scotty,” no
shimmering sparkles as someone materializes in the
transporter room on the U.S.S. Enterprise.
Instead, the apparatus sits on a table in a California
Institute of Technology lab. The signal travels about a yard,
along some coaxial cables. And all that's getting beamed
from one end of the table to the other is a bunch of photons.
But, scientists say it's teleportation, circa 1998.
Light Transport
What physicists at Caltech, Aarhus University in Denmark
and the University of Wales have accomplished is to take
something—a beam of light, in this case—and create a
replica some distance away.
“We claim this is the first
bona fide teleportation,” says
Caltech physics professor Jeff
Kimble, one of the
researchers. The advance
won't lead to Star Trek
technology, but could help with
sophisticated cryptography
and possibly ultra-powerful
“quantum computers.”
Kimble and his colleagues
report their findings in the Oct.
23 issue of the journal
Science.
Last December, two
research groups, one in Austria
and one in Rome, reported successful teleportation
experiments. The earlier experiments, however, were limited
to teleporting information about whether a photon was
polarized in the up or down position. Kimble's group extends
the theory and technique to work more broadly. It is the first
to verify that what goes into the transporter is the same thing
that comes out of it.
“I think it's a pretty important paper,” says Charles
Bennett, a researcher at IBM in Yorktown Heights, N.Y.,
and one of the authors of a 1993 article that worked out the
theoretical underpinnings of teleportation. “I was pretty sure
somebody would find one way or another to do it.”
Taking Advantage of Uncertainty
The experiment takes advantage of one spooky aspect of
quantum mechanics to circumvent one of its constraints.
The constraint is the Heisenberg Uncertainty Principle,
which states one can't precisely measure where something is
and how fast it's moving, at least not at the same time. In
essence, if you look too closely, you inevitably bump the
object you're looking at, and it's no longer where you
thought it was.
That would appear to be a problem if you're trying to
teleport something. If you can't precisely measure a photon
or an atom (or Capt. Kirk), how can you tell someone else
how to make an exact copy?
Surprisingly, the way to get around the Heisenberg
Uncertainty principle is to mess up the information so badly
that it's meaningless.
Quantum Psychic Twins
The magic comes from the fact that under certain, carefully
constructed circumstances, two particles or beams of light
become “entangled”—perturbations to one instantly affect the
other, even if they're separated far apart. “Entanglement
means if you tickle one the other one laughs,” Kimble says.
Think of them as psychic twins or quantum physics'
equivalent of encoder/decoder rings.
Albert Einstein once described entanglement as “spooky
action at a distance.”
Now teleportation becomes simple. The sender—who
physicists insist on always naming Alice—takes the original
item to be transported and combines it with her special
“entangled” encoder, producing what looks like gibberish.
Since it's gibberish to her, she hasn't disturbed the item's
underlying quantum mechanical state.
Alice then sends her gibberish to the recipient—who is
always named Bob—and he combines that gibberish with his
“entangled” decoder. “Bob puts them back together,” Kimble
says, “and out pops this same quantum state.”
Voila! Transport complete.
The Experimental Set-Up
In the tabletop experiment, the input is a beam of light.
The encoder/decoder rings are two specially prepared
entangled light beams. The input light is combined with the
entangled light and shined on a photodiode, which generates
an electrical current that's sent to the other end of the table.
There, the current is transformed back into light and
combined with the second entangled light beam.
Out comes the original light beam.
The researchers can tell it's the same beam of light,
because it has the same little imperfections as the original. In a
sense, what they've done is precisely reproduce random
noise.
Quantum Teleportation in Your Future
What's noise now could eventually become messages.
Scientists hope that quantum computers, which move
information about in this way rather than by wires and silicon
chips, will be infinitely faster and more powerful than
present-day computers.
“I believe that quantum information is going to be really
important for our society,” Kimble says. “Not in five years or
10 years, but if we look into the 100-year time frame it's
hard to imagine that advanced societies don't use quantum
information.”
And in principle, teleportation could be used to send
information to create replicas of objects, not just light beams.
Researchers are already looking to teleport atoms.
Could this mean the transporters of Star Trek could one
day be a reality?
“I don't think anybody knows the answer,” Kimble said.
“Let's don't teleport a person—let's teleport the smallest
bacterium. How much entanglement would we need to
teleport such a thing?”
Would such a teleported bacterium actually be the same
bacterium, or just a very good copy?
“Again,” Kimble says, “no one knows for sure.”
Reuters contributed to this report. |
