Entanglement distribution via photons over long distances enables many applications, including quantum key distribution (QKD), which provides unprecedented privacy. The inevitable degradation of entanglement through noise accumulated over long distances remains one of the key challenges in this area. Exploiting the potential of higher-dimensional entangled photons promises to address this challenge, but poses extreme demands on the experimental implementation. Here, we present an interstate free-space quantum link, distributing hyper-entanglement over 10.2km with flexible dimensionality of encoding by deploying a phase-stable non-local Franson interferometer. With this distribution of multidimensional energy-time entangled photons, we analyse the achievable key rate in a dimensionally-adaptive QKD protocol that can be optimized with respect to any environmental noise conditions. Our approach enables and emphasises the power of high-dimensional entanglement for quantum communication, yielding a positive asymptotic key rate well into the dawn of the day.
@article{arxiv.2204.07536,
title = {Non-local temporal interferometry for highly resilient free-space quantum communication},
author = {Lukas Bulla and Matej Pivoluska and Kristian Hjorth and Oskar Kohout and Jan Lang and Sebastian Ecker and Sebastian Philipp Neumann and Julius Bittermann and Robert Kindler and Marcus Huber and Martin Bohmann and Rupert Ursin},
journal= {arXiv preprint arXiv:2204.07536},
year = {2023}
}