Light may be key to safeguarding secrets

By LIDIA WASOWICZ, UPI Senior Science Writer  |  Oct. 9, 2002 at 3:15 PM
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PALO ALTO, Calif., Oct. 9 (UPI) -- Advances in devices that emit the smallest possible amount of light may portend an era of guaranteed confidentiality in Digital Age communication, which until now has had about as much protection from prying eyes as a message on a post card.

Experiments in emitting one photon -- or packet -- of light at a time point to ways to thwart even the most determined hackers' attempts to eavesdrop on sensitive, private or other information not intended for public consumption, scientists said.

Communication signals carried by a single photon, as opposed to two or more, traveling through fiber-optic cables could provide an impregnable barrier against intruders bent on taking a look at what they are not meant to see, they said.

Because ordinary light sources cannot generate the extraordinarily dim, single-photon pulses reliably, researchers have set their sights on other alternatives, including quantum dots -- electron clusters so tiny, some 5,000 of them could stretch across a grain of sand. These molecule-sized "boxes" that trap and release electrons -- negatively charged elementary particles -- could serve as light-emitting diodes, or LEDs, that efficiently turn an electrical field into photons.

Working with quantum dots, a group of Stanford University researchers has taken an important step toward an ideal single-photon source, one that would produce exactly one particle of light in a "pure" state, said team leader Charles Santori.

"Past reports claiming to demonstrate single-photon sources have only demonstrated control over the number of photons emitted at a time," Santori told United Press International. "In our report, we demonstrate a device for which consecutively emitted photons are usually in the same quantum state."

The feat carries implications for the atom-scale field of quantum information, including potential application in quantum cryptography, said Santori, lead author of the report to be published in the Oct. 10 issue of the British journal Nature.

The conventional mode of transmitting classified information depends on a random generation of many numbers or keys, one of which will unlock the meaning of scrambled secrets. These codes can be cracked.

A message carried by a lone photon -- the smallest discrete quantity of light, called a quantum -- could detect intruders more easily than if two or more were involved, researchers said. Single-photon sources are fail-safe because under the quirky laws of quantum mechanics observing a light particle alters it, instantly alerting the sender and intended recipient of any attempt to break into their domain.

"If you have only one photon per pulse, you would immediately know that an eavesdropper had penetrated the system because the receiver at the opposite end could tell that the data had been disturbed," said W.E, Moerner, professor of chemistry and Harry S. Mosher Professor at Stanford, who was not involved in the current study. His team was the first to use lasers to get single molecules to emit single photons on demand at room temperature, an achievement reported last month in Nature.

In contrast, hackers can tap into messages safely using multiple photons by measuring one photon in a signal while allowing others to pass by undisturbed, thus eluding any detection of their snooping.

"Single-particle-emitting sources are essential for a truly secure form of quantum cryptography," said Andrew Shields, quantum information group leader at Toshiba Research Europe Limited in Cambridge, England. He led a team that invented the first single-photon source driven solely by electric current -- a new type of LED 10 times thinner than a human hair, reported earlier this year at a science meeting in Long Beach, Calif., and in the journal Science..

"Otherwise, if several photons spill out from a device at a time, the extra ones can be siphoned off by an eavesdropper, who could then intercept a message without being detected," Shields told UPI.

In the next 5 to 10 years, quantum information technology might be used to send messages over channels one photon at a time, Moerner predicted. Or it might to employ signals from a single photon to transmit an electronic "key" to decode encrypted messages.

"In one version of quantum cryptography, information is encoded on the states of single photons. People are interested in quantum cryptography because, under certain rules, security from eavesdropping is guaranteed by the laws of quantum mechanics," Santori told UPI.

"Previously demonstrated single-photon devices can be used for the simplest versions of quantum cryptography. However, most applications in quantum information require photons in a definite quantum state. For these applications, our device shows promise," he added.

"This is a significant experimental step forward in the very active fields of single-photon sources (for quantum cryptography) and quantum information processing," said Philippe Grangier of the Institut d'Optique in Orsay, France, who analyzed the work.

"It puts quantum cryptography closer to a 'real life' application," he told UPI.

As information pouring onto the evolving Internet increasingly takes the form of light pulses flowing through fiber-optic cables, techno-savvy busybodies conceivably could resort to beam splitters -- the optical version of a copying machine, which makes duplicates of the input beam -- to divert streams of light and access confidential material, scientists pointed out.

Such intrusion is not possible if a message is carried by a lone photon, however, thereby simplifying spotting snoops significantly.

"Single photons sources are also very important to make quantum cryptography more secure over long distances, i.e., when line losses become very large," Grangier said. "The type of source developed by Santori et al. is also useful for that purpose."

The researchers used laser pulses to produce single photons at extremely low temperatures -- near absolute zero. The crew created a system that allows passage of one photon -- or several, if desired -- in a controllable way.

Using split-second laser pulses, they triggered the emission of two photons, one in response to each pulse. Then they set the light particles on a collision course to observe the so-called "interference" reaction.

"Our experiment is also the first demonstration of 'two-photon interference' that uses photons generated independently, at different times. This is an effect where indistinguishable photons that collide at a partially reflecting mirror tend to 'bunch,' or exit together, in the same direction," Santori explained. "This effect results from the strange ... properties of identical particles in quantum mechanics."

Quantum cryptography does not require such a complex system, which is mandated in certain other applications related to quantum optics and quantum information, Grangier and Moerner said.

"(Because) quantum cryptography is now available commercially, single-photon sources may be useful immediately, provided that they are cheap, simple and reliable (although this is not completely the case yet)," Grangier told UPI. "Application to quantum computing is much further away."

The findings are a first step toward making molecular-based logic gates -- a basic building block of all computers -- for photon-based quantum computing that may be emerging in the next decade or so, scientists said.

Eventually, researchers hope to build entire memory chips smaller than bacteria for quantum computers, which potentially will pack millions or even billions of times as much processing power as the best of today's machines. A quantum computer's supreme abilities stem from certain quantum properties of atoms that allow them to work together as chunks of information, called quantum bits or qubits. They serve simultaneously as the device's processor and memory.

"Some of these applications, especially quantum computation, require extremely high performance from the single-photon source (as well as from other optical components), which our current device cannot yet provide," Santori said. "Some of the simpler applications may already be within reach, however."

Grangier concluded, "The NASDAQ will not recover due to single photons, but the crash will be over one day, and optical telecommunications remain fantastic technologies that have many years of development ahead of them."

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