  Quantum key distribution (QKD) enables two parties to establish a secret key
over a potentially hostile channel by exchanging photonic quantum states,
relying on the fact that it is impossible for an eavesdropper to tap the
quantum channel without disturbing these photons in a way that can be detected
[1]. Here we introduce a large-alphabet QKD protocol that makes optimal use of
temporal and spectral correlations of entangled photons, reaching the maximum
number of inde- pendent basis states (the Schmidt number) and enabling
extremely high information content per photon together with an optimal rate of
secret key generation. This protocol, which we call 'Dense Wavelength Division
Multiplexed Quantum Key Distribution' (DWDM-QKD), derives its security by the
conjugate nature of the temporal and spectral entanglement of photon pairs
generated by spontaneous parametric down conversion. By using a combination of
spectral and temporal bases, we can adjust the protocol to be resource
efficient. We show that DWDM-QKD is well suited to approach the optimal key
generation rate using present-day sources, detectors, and DWDM opti- cal
networks from classical communications, as well as emerging optical
interconnect and photonic integrated chip (PIC) systems.
