Orthogonal Frequency Division Multiplexing Continuous Variable Terahertz Quantum Key Distribution

TL;DR

This paper introduces a high-throughput CVQKD protocol using OFDM in the terahertz band, enhancing spectral efficiency and enabling secure quantum communication over terrestrial and space distances with practical on-chip sources.

Contribution

It presents a novel OFDM-based CVQKD protocol tailored for THz frequencies, including security analysis, simulation results, and practical implementation considerations with superconducting Josephson junction sources.

Findings

Secret key rates up to 72 bits per channel use in open-air conditions.

Terrestrial links limited to 4.5 meters due to atmospheric absorption.

Inter-satellite links can exceed 100 km with minimal losses.

Abstract

We propose a novel continuous-variable quantum key distribution (CVQKD) protocol that employs orthogonal frequency-division multiplexing (OFDM) in the terahertz (THz) band to enable high-throughput and secure quantum communication. By encoding quantum information across multiple subcarriers, the protocol enhances spectral efficiency and mitigates channel dispersion and atmospheric attenuation. We present a comprehensive security analysis under collective Gaussian attacks, considering both terrestrial free-space channels, accounting for humidity-induced absorption, and inter-satellite links, incorporating realistic intermodulation noise. Simulations show secret key rates (SKR) reaching ~72 bits per channel use in open-air conditions. While intermodulation noise imposes trade-offs, optimised modulation variance enables resilience and secure communication range. The maximum terrestrial quantum link extends up to 4.5 m due to atmospheric THz absorption, whereas inter-satellite links can support secure communication over distances exceeding 100 km, owing to minimal propagation channel losses in space. We evaluate the practical implementation of our protocol using recently developed on-chip coherent THz sources based on superconducting Josephson junctions. These compact, voltage-tunable emitters produce wideband coherent radiation, making them ideal candidates for integration in scalable quantum networks. By incorporating their characteristics into our simulations, we assess secure key generation under various environmental conditions. Our results show secure communication over distances up to 3 m in open air, and up to 26 km in cryogenic or vacuum environments. This work advances the prospect of compact, high-capacity CVQKD systems for both terrestrial and space-based THz quantum communication.