Adaptive Pulse-Position Modulation for High-Dimensional Quantum Key Distribution

High-dimensional quantum key distribution (QKD) systems that exploit temporal correlation among entangled photons are of growing practical interest. In such systems, the observation time is typically partitioned into frames of fixed duration, with pulse-position modulation (PPM) coding used within each frame, via which a secret key is established between the parties. Such schemes can be very inefficient in their use of photons, since only a fraction of the frames can be used. As an alternative, we describe an efficient class of schemes with adaptive frame size whose performance can converge to the fundamental limit much more quickly. We analyze and compare the performances of both fixed and adaptive PPM schemes, taking into account photon transmission and detection losses. Further numerical results reveal the significant performance gain of adaptive PPM relative to fixed PPM.