  BBN, Harvard, and Boston University are building the DARPA Quantum Network,
the world's first network that delivers end-to-end network security via
high-speed Quantum Key Distribution, and testing that Network against
sophisticated eavesdropping attacks. The first network link has been up and
steadily operational in our laboratory since December 2002. It provides a
Virtual Private Network between private enclaves, with user traffic protected
by a weak-coherent implementation of quantum cryptography. This prototype is
suitable for deployment in metro-size areas via standard telecom (dark) fiber.
In this paper, we introduce quantum cryptography, discuss its relation to
modern secure networks, and describe its unusual physical layer, its
specialized quantum cryptographic protocol suite (quite interesting in its own
right), and our extensions to IPsec to integrate it with quantum cryptography.
