Partially coherent optical chip enables physical-layer public-key encryption

Public-key encryption is essential for secure communications, eliminating the need for pre-shared keys. However, traditional schemes such as RSA (Rivest-Shamir-Adleman) and elliptic curve cryptography rely on computational complexity, making them increasingly susceptible to advances in computing power and algorithms. Physical-layer encryption, which leverages the intrinsic properties of physical systems, offers a promising alternative with security rooted in physics. Despite progress in this field, public-key encryption at the optical layer remains largely unexplored. Here, we propose a novel optical public-key encryption scheme based on partially coherent light sources. The cryptographic keys are encoded in the incoherent optical transmission matrix of an on-chip Mach-Zehnder interferometer mesh, providing high complexity and resilience to computational attacks. We experimentally demonstrate encrypted image transmission over 40 km of optical fiber with high decryption fidelity and achieve a 10 Gbit/s optical encryption rate using a lithium niobate photonic chip. This represents the first implementation of public-key encryption at the physical optical layer. The approach offers key advantages in security, cost, energy efficiency, and compatibility with commercial optical communication systems. By integrating public-key encryption into photonic hardware, this work opens a new direction for secure and high-speed optical communications in next-generation networks.