Toward Photon-Efficient Key Distribution over Optical Channels

TL;DR

This paper investigates high photon efficiency in optical quantum key distribution, deriving optimal rates and proposing practical schemes using coherent states and direct detection to maximize secret key bits per photon.

Contribution

It derives asymptotic expressions for optimal key generation rates in the photon-efficient regime and proposes practical schemes approaching these limits.

Findings

Derived asymptotic expressions for optimal rates

Proposed practical schemes using coherent states and direct detection

Schemes approach theoretical limits within certain approximations

Abstract

This work considers the distribution of a secret key over an optical (bosonic) channel in the regime of high photon efficiency, i.e., when the number of secret key bits generated per detected photon is high. While in principle the photon efficiency is unbounded, there is an inherent tradeoff between this efficiency and the key generation rate (with respect to the channel bandwidth). We derive asymptotic expressions for the optimal generation rates in the photon-efficient limit, and propose schemes that approach these limits up to certain approximations. The schemes are practical, in the sense that they use coherent or temporally-entangled optical states and direct photodetection, all of which are reasonably easy to realize in practice, in conjunction with off-the-shelf classical codes.