# Quantum-Secure Coherent Optical Networking for Advanced Infrastructures in Industry 4.0
## 1 Introduction
Therefore, this paper is guided by a research question that addresses the heart of this fragmentation: \"How can coherent optical networks that rely on OTU4 framing and MACsec be enhanced to safeguard against emerging quantum threats while maintaining the throughput and latency requirements of existing infrastructure?\"  
This question captures the dual necessity of optical-layer protection and quantum key distribution in coherent environments. It targets the management plane vulnerabilities that arise in bridging router control planes with advanced optical modules and focuses on bridging the gap between classical cryptography, optical encryption, and emerging quantum technologies. We implemented a qualitative security analysis that systematically probes each interface, protocol boundary, and cryptographic layer. This analysis systematically followsrecognized risk assessment and critical infrastructure protection frameworks [13; 14], ensuring the comprehensive identification of both classical and quantum vulnerabilities. This vulnerability analysis underpins this study's creation and validation of a post-quantum security framework that merges spectral phase masking with multi-homodyne coherent detection at the physical layer and QKD key distribution for key management. By unifying these approaches in a single design, our solution delivers robust cryptographic strength, forward secrecy, and backward compatibility in coherent optical networks operating at 400 Gbps and above. This research makes specific contributions through an original vulnerability analysis of coherent infrastructures and a novel design for post-quantum security. First, it systematically identifies overlooked weaknesses in bridge architectures that coordinate the router control plane with coherent modules. This includes close scrutiny of the management interfaces where protocol conversion and synchronization occur, as these seemingly routine operations can introduce timing discrepancies. Second, it classifies vulnerabilities in OTU4 framing, where overhead bytes and frame alignment can be manipulated to create covert channels or disrupt signal integrity. Third, it proposes a new approach that incorporates optical phase mask encoding with multi-homodyne coherent detection and quantum key distribution for holistic security. The theoretical findings indicate that such a layered strategy resists both contemporary classical attacks and future quantum-based decryption methods, particularly in the context of coherent optical transport at data rates of 400 Gbps and above. We conclude that a concerted security strategy, one that bridges the physical encryption aspects of spectral phase encoding with post-quantum key distribution, can substantially mitigate vulnerabilities exposed at the interface between traditional data networking protocols and coherent optical platforms. This analysis provides a blueprint for researchers, network operators, and hardware vendors seeking to future-proof optical transport networks. The structure of this manuscript proceeds as follows. First, a review of related work addresses classical encryption vulnerabilities, the integration of MACsec over OTU4, and the latest developments in quantum-resistant protocols. By cataloging vulnerabilities within and across architectural boundaries in Section 4, this research provides a robust empirical foundation for the comparative and integrative frameworks that follow. Section 5 reviews existing optical encryption approaches in the context of these vulnerabilities, while Section 6 presents a cohesive post-quantum security framework designed to mitigate the weaknesses documented here. A discussion evaluates the significance of these findings, their limitations, and potential avenues for more advanced or large-scale implementations.