This research involves the development of an experimental prototype of a quantum cryptographically protected communication system using two-photon entangled states of light, and a detailed theoretical study of the physical principles underlining the ultimate security of this technique. The exponentially growing flow of private and confidential data transmitted over telecommunication channels has brought with it the need for its protection to insure the security of sensitive information. Conventional encryption based on the mathematical complexity of factoring large numbers is still vulnerable to the intrusion of an unfriendly party in command of large computational power. Quantum cryptography was designed to address this issue by bringing to bear the power of the fundamental laws of quantum mechanics.
There are two major approaches in quantum crytography which, historically, appeared nearly simultaneously. One makes use of near single-photon states derived from coherent-state light. The other relies on the nonlocal character of two-photon entangled states generated in the nonlinear optical process of spontaneous parametric down conversion. A new scheme for quantum cryptography has been designed which overcome the limitations that have heretofore plagued the latter systems for use in cryptography, particularly the requirement for synchronous manipulation of two Mach-Zehnder interferometers that are well separated in space. This new approach is based on the use of a special polarization quantum intensity interferometer and utilized doubly entangled states generated by type-II spontaneous parametric down conversion.
