Professor Lo, lead scientist on the Toronto study, has cleverly used these problematic extra photons to dupe eavesdroppers. The light in his experiment is prepared in such a way that a small percentage of photons are decoys that contain no information at all about the key. “The eavesdropper has no idea which is the signal or which is the decoy,” Lo says. “In the end, Alice and Bob can compare, and Alice will announce [in a separate message that doesn’t need to be encrypted] which ones are the signals and which are the decoys.” The signal photons contain information about the key, but the eavesdropper doesn’t know which photons she measured.
The concept of using decoys in quantum cryptography was first proposed in 2003 by Won-Young Hwang, then at Northwestern University in Evanston, IL. In 2005, Lo says, his team mathematically proved that the technique could enhance security. In their most recent announcement, Lo and his team have shown for the first time that the decoy method can actually work in a real-world environment, using a modified off-the-shelf quantum cryptographic system and commercial fiber optics.
The major implication of their findings is that quantum cryptography should now be usable over greater distances. “Prior to decoy states, you couldn’t ramp up the signal to increase the distance,” says Jim Harrington, a researcher at Los Alamos National Laboratory, “because you would send out more than one photon.”
Lo and his team claim that the new technique can guarantee security over 15 kilometers of commercial fiber-optic lines. While this isn’t a distance record, Lo says small modifications to the setup could allow extremely secure transactions over 120 kilometers -– roughly the current upper limit claimed for commercial quantum encryption systems, such as those from id Quantique of Geneva, Switzerland, and MagiQ Technologies of New York City.