Inspecting teleported quantum information.

It's a stock scene in many science-fiction films and novels: A mysterious alien vanishes from one location, while a perfect replica shimmers into existence somewhere else. Science fiction has long relied on teleportation to provide this convenient shortcut. Now, researchers have uncovered a new consequence of quantum theory that makes it possible, in principle, to achieve "quantum teleportation" of information.

"It's a means by which you can take apart an unknown quantum state into classical information and purely quantum information, send them through two separate channels, put them back together, and get back the original quantum state," says Charles H. Bennett of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y.

Bennett and his collaborators describe their scheme in the March 29 PHYSICAL REVIEW LETTERS.

The notion of quantum teleportation hinges on the distinction between classical information (the kind conveyed by a newspaper or some other conventional medium) and quantum information (the kind represented by such characteristics as a microscopic particle's spin or a photon's polarization angle).

Classical information can be freely copied. It's not disturbed when observed, and it can't travel faster than the speed of light. In contrast, quantum information can't be observed without being disturbed, nor can it be copied reliably. "And it sometimes seems to propagate instantaneously," Bennett remarks.

The notorious Einstein-Podolsky-Rosen (EPR) effect stands as one of the more bizarre manifestations of this quantum waywardness (SN: 8/5/89, p.88). For example, suppose a single process within an atom simultaneously generates two photons that travel in opposite directions. According to quantum theory, neither photon has a particular polarization, or electric field orientation, until it's measured at a detector.

In fact, such a measurement transforms a photon's polarization from a range of possibilities into a specific, randomly chosen value. Surprisingly, measuring one photon's polarization causes the other photon of the EPR pair to acquire the opposite polarization at the same instant - even if the second photon is at the other end of the room or across the galaxy. "This is a phenomenon that cannot be explained by assuming that the two [photons] had [particular] polarizations at the moment they were created," Bennett notes.

Although this effect can't be harnessed to send controllable, faster-than-light messages, Bennett and his colleagues argue that it can be used to assist in the teleportation of information about a particle's quantum state (see diagram).

The sender, Alice, wants to convey to the receiver, Bob, a certain photon's unknown polarization. Instead of determining its polarization directly, and thereby disturbing it, she measures the relationship between the polarization angle of her mystery photon and that of a photon created in an EPR process. She then sends a message to Bob, using a conventional medium, to tell him that the two polarization angles are identical, are at right angles to each other, or have one of two other possible geometrical relationships.

Meanwhile, Bob has access to the second EPR photon. He can combine the classical information contained in Alice's message with the quantum information carried by his own EPR photon. This combination allows him to transform the quantum state of his EPR photon, which has never been anywhere near Alice's mystery photon, into an exact replica of the mystery photon's original quantum state. In effect, "Alice's measurement forces the other EPR particle to change in such a way that the classical information that comes out of her measurement enables someone else to produce a perfect copy of what went in," Bennett says.

However, although the EPR information travels instantly, the entire scheme still requires a finite amount of time. "It must be emphasized that our teleportation, unlike some science-fiction versions, defies no physical laws," the researchers say. "In particular, it cannot take place instantaneously...because it requires, among other things, sending a classical message from Alice to Bob."

Though of no practical value, this exercise in quantum logic helps elucidate the crucial differences between classical and quantum information, Bennett says.