By combining some fundamental quantum mechanical principles of single photons with various aspects of information theory, quantum cryptography represents a fundamental shift in the basis for security from numerical complexity to the fundamental physical nature of the communications channel. As such, it promises the holy grail of data security: theoretically unbreakable encryption. Of course, implementing quantum cryptography in real broadband communications systems poses some unique challenges, including generating single photons, distilling random keys from the quantum key distribution process, and maintaining security at both the theoretical and practical level.