Optical Cryptography

His system, he said, starts with a laser that sends part of its beam into photo detectors which produce electrical signal that feed back to help power the laser. The resulting circuit behaves erratically -- something like the feedback you hear at a concert when the performer wanders too close to his stack of amps. Liu has found that if he picks his lasers carefully, he can set up two such nonlinear (chaotic) circuits whose feedback behavior is the same. Thus, if you have a message that needs to get from Albuquerque to Boston without being snooped on, you place a laser in each city. After the two lasers have been synchronized over an open channel, you add your message signal on top of the sending chaotic laser. And once the signal reaches Boston, you use the Boston laser to subtract off the chaos -- and to get the original message.
http://www.wired.com/news/technology/0,1282,50779,00.html