  Quantum key distribution (QKD) and quantum communication enable the secure
exchange of information between remote parties. Currently, the
distributed-phase-reference (DPR) protocols, which are based on weak coherent
pulses, are among the most practical solutions for long-range QKD. During the
last 10 years, long-distance fiber-based DPR systems have been successfully
demonstrated, although fundamental obstacles such as intrinsic channel losses
limit their performance. Here, we introduce the first two-dimensional DPR-QKD
protocol in which information is encoded in the time and phase of weak coherent
pulses. The ability of extracting two bits of information per detection event,
enables a higher secret key rate in specific realistic network scenarios.
Moreover, despite the use of more dimensions, the proposed protocol remains
simple, practical, and fully integrable.
