Stopping cybersnoops with physics

Researchers are reporting encouraging progress in experiments to develop an unbreakable computer code that harnesses the unpredictable realm of quantum physics.

You'll probably never have to devise a quantum password to read your e-mail or get a quick $20 from your bank's cash machine: Encryption experts said a quantum code would be reserved to protect the deepest national security and industrial secrets from cyber-sneaks.

But such codes -- probably decades away -- would be vital if ultra-powerful quantum computers are developed that could decipher even the most complicated conventional math-based codes in a virtual heartbeat.

''If everyone could be sure that a quantum computer could never be built, then existing algorithm cryptography is good enough,'' said Charles Bennett, a quantum information research fellow at IBM's Thomas J. Watson Research Center in Yorktown Heights, N.Y.

''In a theoretical sense, quantum cryptography provides absolute security,'' said Bennett, who did not contribute to the latest studies.

Encryption is a lot like evolution. Since the Roman Empire, code-makers have been constructing ever-more complex formulae to keep information secret. Clever opponents don't rest until they find the new code's weakness and crack it. Then the cycle repeats itself.

Quantum codes rely on the principles of quantum mechanics, which allow matter and the information it carries to exist in several states simultaneously. Only the sender and the receiver would know the code and agree upon the quantum state in which they would use it.

Since quantum states are very fragile, an effort by an outsider to intercept the message and read it would automatically disturb the quantum state. Not only would the information be unreadable, it would alert the receiver that somebody was eavesdropping.

Several quantum experiments already have succeeded in delivering encoded information in weak flashes of photons, or light particles.

In the latest research published in the May issue of Physical Review Letters, scientists at Los Alamos National Laboratory and universities in Geneva and Vienna, Austria, manipulated the photons to improve their security and reliability.

Rather than transmit information within a stream of individual photons, they directed the light beam through a crystal, splitting each photon into an entangled pair of particles.

Photons generate an electric field, and the direction in which that field vibrates is called its polarity. Splitting the photons adds more randomness to the photons' polarity and more security to the message contained in them, the researchers report.

Both the sender and the receiver have equipment to capture photons with the agreed-upon polarities and build a key to interpret the message.

However, practical hurdles remain.

Some quantum experiments have dispatched encrypted photons along optical fibers measuring about 25 miles. That's far enough to send messages between government offices, bank branches or laboratories.

But the photons would wither or change quantum states across a nationwide fiber optic network or in a satellite relay, experts said.