Quantum Cryptography Cryptography, the science of secret communication, is becoming increasingly important with the growth of computer networks and electronic transactions. When personal, financial, military, or national security information is transferred from place to place, it becomes vulnerable to eavesdropping tactics that potentially may have catastrophic consequences. Such problems can be avoided by encrypting information to render it unintelligible to third parties.
Key material is therefore a valuable resource even though it conveys no information itself.
With conventional communications, which can be subject to passive monitoring by an eavesdropper, it is impossible to generate a certifiably secret key, and so cumbersome physical security measures for key generation, distribution, storage, and destruction after use are required. The security of QKD is based on fundamental principles of quantum physics. They possess a quantum channel and a public channel over which they can compare their numbers to distill a common subset. Alice proceeds by preparing a single photon with her laser source for each bit in her set of numbers in one of two possible ways, depending on whether her number is a 0 or a 1 . Each photon is then sent over the quantum channel.