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.
@article{arxiv.1510.06126,
title = {Photon-efficient quantum cryptography with pulse-position modulation},
author = {Tian Zhong and Feihu Xu and Zheshen Zhang and Hongchao Zhou and Alessandro Restelli and Joshua C. Bienfang and Ligong Wang and Gregory W. Wornell and Jeffrey H. Shapiro and Franco N. C. Wong},
journal= {arXiv preprint arXiv:1510.06126},
year = {2015}
}