  A team from BBN Technologies, Boston University, and Harvard University has
recently built and begun to operate the world's first Quantum Key Distribution
(QKD)network under DARPA sponsorship. The DARPA Quantum Network became fully
operational on October 23, 2003 in BBN's laboratories, and in June 2004 was
fielded through dark fiber under the streets of Cambridge, Mass., to link our
campuses with non-stop quantum cryptography, twenty-four hours per day. As of
December 2004, it consists of six nodes. Four are 5 MHz, BBN-built BB84 systems
designed for telecommunications fiber and inter-connected by a photonic switch.
Two are the electronics subsystems for a high speed free-space system designed
and built by NIST. This paper describes the motivation for our work, the
current status of the DARPA Quantum Network, its unique optical switching and
key relay protocols, and our future plans.
