Very long lasers made of optical fibers offer a promising route to highly secure communications. Nothing beats quantum communication for absolute security, but the new method, which relies on classical rather than quantum physics, provides faster communication over long distances. It would also be feasible with existing hardware, in contrast to quantum communication that will require development of new, and probably expensive, components.
Physicists at Tel Aviv University and the California Institute of Technology propose spanning the distance between two people (call them Alice and Bob for convenience) who want to exchange a sensitive piece of information with an erbium-doped fiber. Erbium makes the fiber act like a laser, and the amount of power in the fiber laser depends on mirrors at their respective ends of the fiber laser.
To exchange information, imagine that Alice and Bob each have two types of mirrors on hand and that they agree to let one type of mirror represent the number 0 and the other type represent the number 1. To send a single bit of information, each of them place one of their mirrors at their end of the fiber. Because Alice knows which mirror she chooses, when she measures the power in the fiber laser she can determine which mirror Bob has chosen. Similarly, Bob can use the same reasoning to tell which mirror Alice has chosen.
An eavesdropper who is not allowed to see the mirrors but measures the power in the laser might be able to determine that one person chose mirror 0 and the other chose mirror 1, but she could not tell which person chose which mirror. As a result, she would not have enough information to determine what numbers Alice and Bob are exchanging.
A sensitive enough measurement of the light in the fiber could theoretically reveal the information that Alice and Bob are transmitting, but the authors show that including filters in the system and injecting random noise into the fiber would allow them to arbitrarily increase the technical challenges a would-be snooper faces in trying to eavesdrop. Unlike quantum communication, which is potentially absolutely secure, the fiber laser system could be designed to be just secure enough to ensure that communications are secret while keeping material costs down and long distance transmission speeds up.