Quantum Key Distribution (QKD) exploits the rules of quantum mechanics to generate and securely distribute a random
sequence of bits to two spatially separated clients. Typically a QKD system can support only a single pair of clients at a
time, and so a separate quantum link is required for every pair of users. We overcome this limitation with the design and
characterization of a multi-client entangled-photon QKD system with the capacity for up to 100 clients simultaneously.
The time-bin entangled QKD system includes a broadband down-conversion source with two unique features that enable
the multi-user capability. First, the photons are emitted across a very large portion of the telecom spectrum. Second, and
more importantly, the photons are strongly correlated in their energy degree of freedom. Using standard wavelength
division multiplexing (WDM) hardware, the photons can be routed to different parties on a quantum communication
network, while the strong spectral correlations ensure that each client is linked only to the client receiving the
conjugate wavelength. In this way, a single down-conversion source can support dozens of channels simultaneously--and
to the extent that the WDM hardware can send different spectral channels to different clients, the system can support
multiple client pairings. We will describe the design and characterization of the down-conversion source, as well as the
client stations, which must be tunable across the emission spectrum.