Optical RISs Improve the Secret Key Rate of Free-Space QKD in HAP-to-UAV Scenarios

TL;DR

This paper proposes using optical reconfigurable intelligent surfaces (ORISs) on rooftops to improve free-space quantum key distribution between high-altitude platforms and low-altitude platforms, accounting for atmospheric turbulence and platform fluctuations.

Contribution

It introduces an analytical channel model for HAP-to-LAP optical links with ORISs, enabling adaptive beam control to enhance secret key rates in challenging conditions.

Findings

Quadratic phase shifts improve SKR at high zenith angles or mild PE.

Linear phase shifts are better at low zenith angles or higher PE.

The model effectively mitigates beam broadening and pointing errors.

Abstract

Large optical reconfigurable intelligent surfaces (ORISs) are proposed for employment on building rooftops to facilitate free-space quantum key distribution (QKD) between highaltitude platforms (HAPs) and low-altitude platforms (LAPs). Due to practical constraints, the communication terminals can only be positioned beneath the LAPs, preventing direct upward links to HAPs. By deploying ORISs on rooftops to reflect the beam arriving from HAPs towards LAPs from below, reliable HAP-to-LAP links can be established. To accurately characterize the optical beam propagation, we develop an analytical channel model based on extended Huygens-Fresnel principles for representing both the atmospheric turbulence effects and the hovering fluctuations of LAPs. This model facilitates adaptive ORIS beam-width control through linear, quadratic, and focusing phase shifts, which are capable of effectively mitigating the detrimental effects of beam broadening and pointing errors (PE). Consequently, the information-theoretic bound of the secret key rate and the security performance of a decoy-state QKD protocol are analyzed. Our findings demonstrate that quadratic phase shifts enhance the SKR at high HAP-ORIS zenith angles or mild PE conditions by narrowing the beam to optimal sizes. By contrast, linear phase shifts are advantageous at low HAP-ORIS zenith angles or moderate-to-high PE by diverging the beam to mitigate LAP fluctuations.