High-Dimensional Intra-City Quantum Cryptography with Structured Photons

TL;DR

This paper demonstrates a high-dimensional quantum key distribution system using structured photons over a city-scale free-space link, achieving secure communication despite atmospheric turbulence.

Contribution

First experimental implementation of high-dimensional QKD in a real urban environment using structured photons combining spin and orbital angular momentum.

Findings

Achieved 11% quantum bit error rate with 0.65 bits per sifted photon in 4D protocol.

Secured communication with 5% error rate and 0.43 bits per sifted photon in 2D protocol.

Showed robustness of structured photons for intra-city quantum communication under turbulence.

Abstract

Quantum key distribution (QKD) promises information-theoretically secure communication, and is already on the verge of commercialization. Thus far, different QKD protocols have been proposed theoretically and implemented experimentally [1, 2]. The next step will be to implement high-dimensional protocols in order to improve noise resistance and increase the data rate [3-7]. Hitherto, no experimental verification of high-dimensional QKD in the single-photon regime has been conducted outside of the laboratory. Here, we report the realization of such a single-photon QKD system in a turbulent free-space link of 0.3 km over the city of Ottawa, taking advantage of both the spin and orbital angular momentum photonic degrees of freedom. This combination of optical angular momenta allows us to create a 4-dimensional state [8]; wherein, using a high-dimensional BB84 protocol [3, 4], a quantum bit error rate of 11\% was attained with a corresponding secret key rate of 0.65 bits per sifted photon. While an error rate of 5\% with a secret key rate of 0.43 bits per sifted photon is achieved for the case of 2-dimensional structured photons. Even through moderate turbulence without active wavefront correction, it is possible to securely transmit information carried by structured photons, opening the way for intra-city high-dimensional quantum communications under realistic conditions.