Hybrid Quantum Security: Integrating QKD and PQC in Brownfield Optical Networks

The future security of digital communications will increasingly rely on the integration of Quantum Key Distribution (QKD) and Post-Quantum Cryptography (PQC), indicating a necessary convergence rather than a competitive relationship between these two technologies. Following the publication of NIST FIPS 203, 204 and 205, the industry now possesses standardised algorithmic tools for migrating away from RSA and elliptic-curve cryptography (ECC). At the same time, for critical infrastructure facing “harvest-now, decrypt-later” adversaries, purely software-based measures provide only computational, not information-theoretic, security. We argue that the optimal solution is a hybrid architecture, in which PQC provides authentication and cryptographic agility across heterogeneous endpoints, while QKD supplies an additional physical-layer shield for high-value links. The central engineering challenge is brownfield coexistence: deploying QKD over existing lit telecom fibers that carry high-power classical traffic. We review three spectral coexistence strategies to mitigate Spontaneous Raman Scattering (SpRS) – O-band separation, dense C-band DWDM integration with guard bands, and an “inverted” spectrum approach where data uses the O-band and QKD uses the C-band – and we discuss emerging physical media such as hollow-core and multi-core fibers. At the protocol layer, we discuss hybrid key encapsulation and the role of crypto-agile gateways in combining keys from classical, PQC and QKD sources [1]. Finally, we survey industrial pilots (e.g., SK Telecom, EuroQCI / OPENQKD) and outline open challenges in cost, standardisation and operations. The goal is to provide practitioners with a realistic, engineering-oriented overview of how PQC and QKD can be combined in brownfield optical networks.