A way to crack quantum encryption: time-travel technique could break supposedly secure codes

Science News, Dec 6, 2008 by Rachel Ehrenberg

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. Any bad guy eavesdropping on Bond's messages to M could always be detected.

But now physicists suggest that quantum codes maybe breakable using a trick that even Bond hasn't mastered- time travel. By exploiting 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 alerting the senders.

Time travel, possibly 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, physicist Todd Brun of the University of Southern California in Los Angeles and colleagues report online November 7 (arxiv.org/abs/0811.1209).

"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.

Encryption relies on both sender and recipient having a secret shared key to create or decipher the coded message. 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.

Traditional 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 looked at, alerting sender and recipient when an eavesdropper is afoot.

For quantum eavesdroppers, "the only way not to be detected is to acquire no information because measurements disturb the state," Brun says.

Detection of quantum-code eavesdropping is possible because a quantum particle (say a photon) can exist in a fuzzy mixture of states. Measuring the particle converts it to a particular "preferred state." Imagine a bird in flight. In the quantum world, the flying bird may be anything from a pelican to a chickadee. But once it is caught, it becomes one bird--which kind depends on the net used to catch it.

Say Alice sends Bob a chickadee that Eve intercepts. If Eve uses a big net, she won't catch a chickadee, but a pelican. As Eve sends intercepted birds to Bob, eventually he and Alice will realize that Bob is getting big birds that should be small. The new paper shows how Eve could send Alice's bird through a spacetime wormhole (technically, a "closed time-like curve") that allows time travel. Eve's bird could interact with its earlier self at the wormhole's other end and become a distinguishable bird that fits the code.

This scenario can't be ruled out, says Bennett, of IBM's Watson Research Center in Yorktown Heights, N.Y. The existence of wormholes "is not totally impossible, but it is pretty damn unlikely, he says.