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 another.
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 time.
"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."