10/22/1998 07:00:00

Caltech physicists achieve first bona fide quantum teleportation

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 much 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.

Written by Robert Tindol