  The Illinois Express Quantum Network (IEQNET) is a program to realize
metropolitan scale quantum networking over deployed optical fiber using
currently available technology. IEQNET consists of multiple sites that are
geographically dispersed in the Chicago metropolitan area. Each site has one or
more quantum nodes (Q-nodes) representing the communication parties in a
quantum network. Q-nodes generate or measure quantum signals such as entangled
photons and communicate the measurement results via standard, classical signals
and conventional networking processes. The entangled photons in IEQNET nodes
are generated at multiple wavelengths, and are selectively distributed to the
desired users via transparent optical switches. Here we describe the network
architecture of IEQNET, including the Internet-inspired layered hierarchy that
leverages software-defined networking (SDN) technology to perform traditional
wavelength routing and assignment between the Q-nodes. Specifically, SDN
decouples the control and data planes, with the control plane being entirely
implemented in the classical domain. We also discuss the IEQNET processes that
address issues associated with synchronization, calibration, network
monitoring, and scheduling. An important goal of IEQNET is to demonstrate the
extent to which the control plane classical signals can co-propagate with the
data plane quantum signals in the same fiber lines (quantum-classical signal
"coexistence"). This goal is furthered by the use of tunable narrow-band
optical filtering at the receivers and, at least in some cases, a wide
wavelength separation between the quantum and classical channels. We envision
IEQNET to aid in developing robust and practical quantum networks by
demonstrating metro-scale quantum communication tasks such as entanglement
distribution and quantum-state teleportation.
