CV Quantum Communications with Angular Rejection Filtering: Modeling and Security Analysis

Continuous-variable quantum key distribution (CVQKD) over free-space optical links is a promising approach for secure communication, but its performance is limited by turbulence, pointing errors, and angular leakage that can be exploited by an eavesdropper. To mitigate this, we consider an angular rejection filter that defines a safe-zone at the receiver and blocks signals from outside the desired cone. A system and channel model is developed including turbulence, misalignment, and safe-zone effects, and information theoretic metrics are derived to evaluate security. Simulation results show that the safe zone significantly reduces information leakage and that careful tuning of beam waist, angular threshold, and aperture size is essential for maximizing the secret key rate. Larger apertures improve performance but increase receiver size, while longer links require sub 100 urad alignment accuracy. These results highlight safe-zone enforcement and parameter optimization as effective strategies for practical and secure CV-QKD.

💡 Research Summary

This paper addresses a critical vulnerability in free‑space continuous‑variable quantum key distribution (CV‑QKD): the possibility that an eavesdropper (Eve) can collect optical power that arrives at the receiver from angles outside the intended line‑of‑sight cone. To mitigate this, the authors propose installing an Angular Rejection Filter (ARF) behind the receiving lens, thereby defining a “safe‑zone” with a half‑angle θ_safe (or an equivalent radius r_safe at the receiver plane). Any light that falls outside this cone is absorbed by a surrounding ring and is assumed to be fully intercepted by Eve, who is otherwise ideal (no turbulence, no loss).

The paper builds a comprehensive system and channel model that simultaneously captures three impairments: (1) atmospheric turbulence, modeled by a Gamma‑Gamma distribution with shaping parameters α and β; (2) random pointing errors, modeled as a two‑dimensional Gaussian angular deviation with variance σ_θ², which translates into a lateral offset r_e = Z_L·√(θ_ex²+θ_ey²) at the receiver plane; and (3) the safe‑zone effect, expressed through the collection efficiency of the legitimate receiver (Bob) η_B and the leakage efficiency to Eve η_E. Bob’s overall transmissivity is written as

 η_B = η_sys · h · A₀ · exp