STL and C-DOT successfully demonstrate quantum key distribution over multi-core fibre

STL, in collaboration with the Centre for Development of Telematics (C-DOT), India’s telecom R&D centre under the Department of Telecommunications (DoT), Ministry of Communications, has successfully demonstrated India’s first Quantum Key Distribution (QKD) transmission over a 100 km, four-core Multi-Core Fibre (MCF) network. This marks a notable development in the efforts to build quantum-secure, future-oriented communication infrastructure in the country.

STL has designed, developed, and deployed a Multi-Core Fibre cable as part of an end-to-end solution for both aerial and underground networks. This includes cabling, connectivity, installation, and deployment. The MCF technology was implemented within the DoT-sponsored Advanced Optical Communications (AOC) Test Bed at IIT Madras and complies with Essential Requirements (ER) set by the Telecommunication Engineering Centre (TEC). STL is reportedly the first company globally to deploy MCF in both underground and aerial infrastructure and is actively involved in contributing to the standardisation of fibre, cable, and related test and measurement technologies.

Multi-Core Fibre (MCF) technology enables data transmission across multiple cores within a single optical fibre, offering potential benefits in reducing physical infrastructure and associated costs. Traditionally, QKD systems require a separate, dedicated ‘dark fibre’ to transmit quantum signals. The use of MCF allows quantum signals and classical data traffic to be transmitted simultaneously but separately, using distinct cores in the same fibre, thus maintaining signal integrity.

Dr Rajkumar Upadhyay, CEO of C-DOT, commented,"This initiative is a key milestone for India’s telecom ecosystem. We are pleased to collaborate with partners like STL in advancing India’s fibre infrastructure. This demonstration shows the practical viability of integrated quantum-classical networks using next-generation optical fibres and represents a significant cost-saving opportunity for QKD deployment."

In this demonstration, quantum signals were transmitted through one core of the fibre while the remaining three cores carried high-speed user data. The QKD link remained stable and error-free over a distance of more than 100 km, even as large volumes of classical data were transmitted in parallel. This successful test suggests that quantum-secured communication can coexist with high-capacity data transmission over shared infrastructure, offering a potential pathway to secure 5G/6G rollouts without requiring additional fibre.

The integration of QKD with MCF addresses two main challenges in quantum communication: the requirement for separate dark fibre and the need to accommodate increasing volumes of data. This approach could support cost-effective deployment of secure communication systems at scale.

Rahul Puri, CEO of STL’s Optical Networking Business, stated,"This collaborative effort with C-DOT demonstrates how optical innovations can support secure communication systems. Integrating QKD with our Multi-Core Fibre technology highlights the potential for developing future-ready digital infrastructure through joint public and private initiatives."

Dr Rajkumar Upadhayay, CEO of C-DOT, spoke with Voice & Data about the role this initiative will play in India’s telecom ecosystem, he said, "Quantum computers are expected to arrive soon, and with their emergence, current encryption methods will become obsolete or vulnerable. While quantum computing offers significant benefits in areas such as healthcare, drug discovery, financial services, modelling, and simulation, it also presents serious challenges. Notably, the computational power of quantum machines could break existing encryption standards.

As a result, we must adopt new mechanisms to protect encrypted data. Quantum communication, a relatively new field, addresses this challenge by developing methods and systems capable of maintaining secure encryption, even in the era of quantum computing."

He futher explained, "Quantum encryption generally follows two branches, one based on physics and the other on mathematics. In the physics-based approach, quantum key distribution (QKD) is used, where encryption keys are generated using photons with extremely low power. Transmitting these photons requires a separate ‘dark’ fibre, dedicated solely to carrying the quantum channel, which increases the cost and complexity of deployment. This requirement has been a major limitation to wider adoption.

However, the innovation introduced by STL changes this landscape significantly. Traditionally, a single optical fibre contains just one core, but STL has developed a multi-core fibre technology. In the fibre we tested, there are four cores: three cores are used for transmitting regular data, while the fourth core is reserved for transmitting the quantum channel,used to generate encryption keys.

This means quantum communication and regular data transmission can now take place within the same physical fibre, eliminating the need for separate infrastructure. This advancement will substantially reduce costs and make the deployment of quantum key distribution systems far more practical and scalable."