Floodlight quantum key distribution (FL-QKD) has realized a 1.3\,Gbit/s secret-key rate (SKR) over a 10-dB-loss channel against a frequency-domain collective attack [Quantum~Sci.~Technol. {\bf 3}, 025007 (2018)]. It achieved this remarkable SKR by means of binary phase-shift keying (BPSK) of multiple optical modes. Moreover, it did so with available technology, and without space-division or wavelength-division multiplexing. In this paper we explore whether replacing FL-QKD's BPSK modulation with a high-order encoding can further increase that protocol's SKR. First, we show that going to K-ary phase-shift keying with K=32 doubles---from 2.0 to 4.5\,Gbit/s---the theoretical prediction from [Phys.~Rev.~A {\bf 94}, 012322 (2016)] for FL-QKD's BPSK SKR on a 50-km-long fiber link. Second, we show that 2d×2d quadrature amplitude modulation (QAM) does not offer any SKR improvement beyond what its d=1 case---which is equivalent to quadrature phase-shift keying---provides.
@article{arxiv.1804.01147,
title = {High-order encoding schemes for floodlight quantum key distribution},
author = {Quntao Zhuang and Zheshen Zhang and Jeffrey H. Shapiro},
journal= {arXiv preprint arXiv:1804.01147},
year = {2018}
}