No World War II spy thriller is complete without the scene in which the sexy secret agent steals an enemy codebook holding the secret keys used to encrypt messages. In the last half-century, technology for transmitting secret keys has advanced, but even the most sophisticated devices can't always prevent the key from being stolen. That may soon change. A new study presents a way to harness the weird laws of quantum physics to build a key-transmission system potentially as unbreakable as the laws of physics.
The famous Heisenberg Uncertainty Principle asserts that it is impossible to measure certain physical parameters, such as the position of a particle, without changing other parameters, such as its momentum. This property could be exploited to transmit cryptographic keys securely. The idea is that if Alice wants to send Bob a secret numerical key, she can encode the key's digits into the physical parameters of particles, which she then sends to Bob. If Eve tries to surreptitiously measure the particles, her measurements will change the state of the particles, betraying her presence.
Building devices based on this idea has been difficult: Standard schemes require Alice to encode each bit of information in a single photon, but it's hard to build detectors that accurately measure the photons on Bob's end. Now, Philippe Grangier of the Institute of Optics in Orsay, France, and colleagues have devised a way to encode the key in laser pulses consisting of hundreds of photons. In the 15 January issue of Nature, they report transmitting keys through an optical line at rates of 75,000 bits each second, more than enough for many applications. The researchers also showed that any eavesdropping carried out with today's technology--such as using a beam splitter to siphon off part of the laser beam--would be noticeable.
But they haven't yet proven that the system is secure against any possible attack allowed by the laws of physics, cautions John Preskill, a physicist at the California Institute of Technology in Pasadena. The team is currently working on such a proof, Grangier says. For now, the important thing is that the team's experiment proves that building such a system is feasible, says Richard Hughes, who studies quantum cryptography at Los Alamos National Laboratory in New Mexico. "It's a breakthrough proof-of-principle experiment," he says.