Take the time to break quantum encryption

The feat is theoretically possible thanks to a time-travel scenario permitted by Einstein’s general theory of relativity

Quantum physics offers James Bond and his ilk much more than a bit of solace—it permits quantum encryption, a completely spyproof way to send coded information. Eavesdropping by a third party can always be detected.

But now physicists have suggested that quantum codes may be breakable. The feat involves a trick that even Bond hasn’t mastered yet — time travel. By taking advantage of hidden paths to the past — routes that are predicted by some of Einstein’s equations — a nemesis could eavesdrop on a quantum-coded message without being detected.

Time travel, such as, say, through a “wormhole,” appears to make it possible to distinguish quantum information that usually can’t be distinguished. That ability would disrupt the absolute security of quantum encryption, theoretical physicist Todd Brun and collaborators report online in the quantum physics archive (arXiv:0811.1209v1).

“I believe it is a sound result that quantum cryptography would not work in this world,” comments Charles Bennett, who with Gilles Brassard developed the first quantum encryption protocol in 1984. “You might say it is a weakness of quantum cryptography — but if there were wormholes, people could go back in time and do worse feats of mischief than reading secret messages,” says Bennett, who was not involved in the new research.

Of course, governments and their agents have been passing secret messages since long before the arrival of quantum physics. Good old-fashioned encryption and quantum encryption alike rely on both the sender and recipient having a secret shared key to create or decipher the coded message. The key code might replace letters with numbers, or substitute shapes for words. As long as both parties are the only ones with the key to the code, “secret” messages can be sent in plain sight, yet remain secret.

Old-fashioned codes for sending messages between distant communicators are vulnerable because an eavesdropper might intercept the key. But in the quantum world, a key can be transmitted securely because quantum information is changed when it is looked at, alerting the sender and recipient when an eavesdropper is afoot.

For a quantum eavesdropper, “the only way not to be detected is to acquire no information because measurements disturb the state,” says Brun, of the University of Southern California in Los Angeles.

The inevitable detection of quantum-code eavesdropping results from the bewildering weirdness of interactions in the quantum world. A quantum particle (such as a photon of light) can exist in a fuzzy mixture of various states. But measuring the particle converts it to a particular “preferred state.” Imagine catching a bird in flight. In the quantum world, as long as it is in flight, the bird may be anything, from a pelican to a chickadee. But once it is caught, the bird becomes one bird only, and what it becomes is affected by the net you use to catch it.

Because measuring quantum information can affect it, an eavesdropper cannot escape detection. Say Alice sends Bob a chickadee that Eve intercepts. If Eve uses the wrong net, a big net for example, she won’t catch a chickadee, but a pelican. When Eve sends the intercepted bird along to Bob, eventually Bob and Alice will realize that Bob’s getting big birds when he should be catching small ones.

The new paper, though, mathematically describes how time travel could allow Eve to catch and send Alice’s bird in a way that subverts quantum encryption. Eve must send the bird through a wormhole in spacetime that creates a “closed timelike curve,” a path that allows time travel under the rules of Einstein’s general theory of relativity. Eve’s bird could interact with its younger self at the wormhole’s other end and become a clearly distinguishable bird.

This scenario can’t be ruled out, says Bennett, of IBM’s Watson Research Center in Yorktown Heights, New York. Like black holes, the existence of wormholes is valid on paper. (Picture traveling through regular time as walking along the surface of an apple—a “wormhole” goes through the apple, connecting you to the future or past.) The existence of wormholes “is not totally impossible, but it is pretty damn unlikely,” he says.

Even if wormholes did exist, they would probably be so small and unstable that they wouldn’t be around long enough to pass a message through. So quantum encryption seems safe for now. And while quantum encryption is a thriving field of research, it is in its early stages and most spies still use good old-fashioned methods, says Brun.

Of course, in both scenarios there is still a possibility that Alice or Bob is a double agent. This human frailty means that “the best schemes that people use involve guys with briefcases handcuffed to their wrists, even though none of that is perfectly secure” says Brun. “Unfortunately, science can only take you so far.”

More Stories from Science News on Physics