  The binary (one-bit-per-photon) encoding that most existing quantum key
distribution (QKD) protocols employ puts a fundamental limit on their
achievable key rates, especially under high channel loss conditions associated
with long-distance fiber-optic or satellite-to-ground links. Inspired by the
pulse-position-modulation (PPM) approach to photon-starved classical
communications, we design and demonstrate the first PPM-QKD, whose security
against collective attacks is established through continuous-variable
entanglement measurements that also enable a novel decoy-state protocol
performed conveniently in post processing. We achieve a throughput of 8.0
Mbit/s (2.5 Mbit/s for loss equivalent to 25 km of fiber) and secret-key
capacity up to 4.0 bits per detected photon, thus demonstrating the significant
enhancement afforded by high-dimensional encoding. These results point to a new
avenue for realizing high-throughput satellite-based or long-haul fiber-optic
quantum communications beyond their photon-reception-rate limits.
