Optical chip pioneers physical-layer public-key encryption with partial coherence

In the information age, our digital lives, from online payments to private communications, depend on a powerful technology known as the "public-key cryptosystem." This can be envisioned as a "digital safe" with two distinct keys: a public key for anyone to encrypt information, and a private key, held only by the recipient, for decryption. The security of algorithms like RSA is based on classical mathematical problems, such as factoring a large integer into its two prime constituents, a task that is practically impossible for conventional computers. This "computational complexity" has long served as a robust firewall for our digital world.

However, this firewall is facing a disruptive threat from the rise of quantum computing. Unlike classical computers, quantum machines can run special algorithms like Shor's algorithm, which could theoretically break the mathematical problems underlying current public-key encryption in a remarkably short time. Once a practical quantum computer is built, it could render our current encryption obsolete, exposing global finance, national defense, and personal privacy to unprecedented risks. This "post-quantum" crisis is the primary driver for a global search for next-generation encryption technologies. A promising frontier is "physical-layer security," which seeks to build security based on the fundamental physical laws governing information carriers like photons, rather than on computational complexity. Optical solutions are particularly attractive due to their vast bandwidth and low power consumption. However, most previous optical encryption techniques have required pre-shared keys between communicating parties, limiting their flexibility in open networks. Therefore, developing a true public-key encryption system at the physical optical layer, one that operates without pre-shared keys has remained a key unsolved challenge in the field.

To address this challenge, the research team at Huazhong University of Science and Technology has successfully implemented the first complete public-key encryption system at the physical optical layer. The core innovation lies in the novel integration of two physical principles on a single photonic chip: the intrinsic randomness of "partially coherent light" and the "reciprocity" of optical systems. In this scheme, partially coherent light acts as a "natural encryptor." While a conventional laser's phase is orderly and predictable like a marching band, the phase of partially coherent light is inherently chaotic and random, like a bustling crowd. When information is encoded onto this "random light," its critical phase information is naturally concealed by this physical randomness. An eavesdropper can only intercept an indecipherable, random optical field.

At the same time, by leveraging the reciprocity principle the fact that light can travel a path in reverse the legitimate recipient holding the private key (the specific optical chip) can deterministically cancel out this random effect and accurately decrypt the message. This provides the authorized user with a key to "tame" the randomness, achieving secure asymmetric encryption. The team validated the scheme through key experiments: they successfully transmitted encrypted images over 40 kilometers of optical fiber with high decryption fidelity and separately demonstrated an encryption rate of 10 Gbit/s using standard on-off keying (OOK) signals. These results prove the system’s feasibility for long-distance communication and its potential for high-speed applications.

This technology can provide "quantum-safe" communication for critical sectors like finance and defense. Compared to other advanced technologies, it is more cost-effective and compatible with existing fiber networks, paving the way for the widespread adoption of high-level physical-layer security. Future work includes heterogeneously integrating functional modules onto a single chip to improve performance and using higher-dimensional systems, such as multi-core fibers, to create a larger key space for even greater security.

This work was published in Opto-Electronic Advances under the title "Partially coherent optical chip enables physical-layer public-key encryption".

About The Authors:

Professor Jianji Dong's team has long been engaged in research in the fields of integrated optics and optical computing. They have made significant progress in areas such as optical nonlinear activation functions, optical deep neural network chips, monolithically integrated optical Ising machine chips, monolithically integrated optical recurrent computing accelerator chips, optical programmable logic arrays, and the training and inference of optical computing chips. In the last five years, the team has published more than 200 papers in renowned optical journals including eLight, Nature Communications, Advanced Photonics, Light Science & Applications, and Optica. The team leads projects such as the National Key R&D Program, the National Science Fund for Distinguished Young Scholars, and the National Science Fund for Excellent Young Scholars. Professor Dong has received the National Excellent Doctoral Dissertation Award, the first prize of the Hubei Provincial Natural Science Award twice, and the second prize of the National Teaching Achievement Award (Graduate Level) once. He serves as the Executive Editor-in-Chief for the journal Frontier of Optoelectronics, a committee member of the National Intelligent Computing Standardization Committee, and a standing committee member of the Fiber Optics and Integrated Optics Committee of the Chinese Optical Society. He also initiated and organized the optoelectronic computing series for the Photonics Open Course.

Read the full article here: http://dx.doi.org/10.29026/oea.2025.250098

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