Communicating with trits, not bits.

Digital communication involves transmitting information encoded as strings of 0s and 1s, or bits. Often, these bits are sent as electric pulses in wires, but the information can also be conveyed by particles that can exist in two different states. For example, the electric field of a photon may be polarized so that it vibrates either vertically or horizontally, enabling each particle in a stream of photons to carry one bit of information.

Theorists have pointed out that quantum mechanics actually allows the possibility of sending more than two bits per pair of two-state photons. "We report the first experimental realization of quantum communication," Harald Weinfurter and his coworkers claim in the June 17 Physical Review Letters. Weinfurter and his colleagues at the University of Innsbruck in Austria have demonstrated a technique that exploits quantum effects to send information encoded as 0s, 1s, and 2s. The researchers call these data units "trits." The scheme calls for pairs of two-state photons. Such photon pairs may both be vertically or horizontally polarized, or one photon may have a vertical polarization and the other a horizontal polarization. Quantum mechanics allows the alternative possibility of encoding information as different combinations, or superpositions, of the states of the two photons.

The researchers begin by splitting a beam of laser-generated ultraviolet photons into two new beams consisting of photons of half the original energy and double the wavelength. The photons of each pair created by the splitting have opposite polarizations but are quantum mechanically linked. One beam goes to the sender, and the other travels directly to the receiver. The sender encodes information by manipulating the polarization of the photons in one beam to select among different superpositions available to the photon pair.

These photons are then passed along to the receiver, where they interfere with the photons in the other, direct beam to create distinctive patterns. The receiver can decode the sender's message by noting which of three different detection possibilities each linked photon pair produces. Moreover, because the two photons can represent three states, they carry more information than the usual one bit per photon.

Using their apparatus, Weinfurter and his coworkers were able to transmit three letters of the alphabet, which normally require 24 bits, in just 15 trits.

"It's a remarkably clever piece of work," says Rolf Landauer of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y. However, it's not the only conceivable way to send more than one bit per photon, he notes (SN: 6/29/96, p. 404).