C-DOT and STL demonstrate India’s first QKD transmission using multi-core fibre

In a notable advancement towards the development of quantum-secured communication networks in India, the Centre for Development of Telematics (C-DOT) and Sterlite Technologies Limited (STL) have successfully completed the country’s first Quantum Key Distribution (QKD) transmission using a four-core Multi-Core Fibre (MCF).

By enabling data transfer across multiple cores within a single fibre, Multi-Core Fibre (MCF) technology presents an effective solution that significantly reduces infrastructure costs and physical space requirements. According to the Ministry of Communications, MCF allows for the physical separation of quantum and classical signals into distinct cores within a single fibre. In the context of Quantum Key Distribution (QKD), which typically requires a dedicated dark fibre for the quantum channel, this capability enables the simultaneous transmission of QKD and high-capacity data traffic over the same fibre without compromising the integrity of the quantum signal. This approach results in notable savings on fibre costs, the Ministry announced on Thursday, 19 April 2025.

What is Multi-Core Fibre (MCF)? 

Multi-Core Fibre (MCF) is a highly developed form of optical fibre that holds multiple cores, usually two or more, within one cladding. In contrast to traditional single-core optical fibres, which carry data through one central core, MCF enables data to be simultaneously carried across several physically distinct pathways contained within the same fibre strand.

Every core in an MCF is a standalone optical channel, and it allows for greater data capacity, increased efficiency, and decreased infrastructure needs. By bonding multiple cores in a single fibre, MCF can dramatically reduce physical space and cost of installing multiple single-core fibres, making it a strong contender for high-density, high-capacity communication network applications.

In next-generation emerging technologies like Quantum Key Distribution (QKD), MCF brings the additional benefit of physical decoupling of quantum and classical signals over a single fibre, ensuring the security of quantum transmission while allowing standard high-speed data traffic.

MCF is central to enabling the creation of future-proof optical networks, supporting use cases from high-speed internet and data centres through secure communications to advanced telecom infrastructure.

What is Quantum Key Distribution (QKD)?

Quantum Key Distribution (QKD) is a technique of secure encryption key exchange between two parties based on the principles of quantum mechanics. In contrast to conventional encryption methods based on intricate mathematical algorithms, QKD makes use of the intrinsic properties of quantum particles, typically photons, to provide confidentiality to the key exchange process.

What QKD actually does is enable two parties to acquire a shared, secret cryptographic key, which can be used to encrypt and decrypt messages. The advantage of QKD is that it can warn the parties of any eavesdropping attempt. Due to the quantum principle referred to as the observer effect, any measurement or intercept of the quantum key will alter its state, instantly warning the communicating parties of the presence of a third party. QKD itself doesn't convey data but does provide secure transfer of encryption keys. This is why it finds very specific application in high-security-data applications like government communications, banking, defense, and critical infrastructure.

With more frequent cyber attacks and the weaknesses of conventional encryption being more and more apparent, particularly in light of possible future quantum computers, QKD is also being considered a building-block technology for quantum-proof communications. 

In the recent demonstration, quantum signals for QKD were transmitted through one core, while the remaining three cores in the same fibre carried high-speed user data. A stable QKD link was successfully established over a distance exceeding 100 km, demonstrating the robustness and reliability of quantum communication even in the presence of high-capacity classical data traffic, according to the Centre for Development of Telematics (C-DOT).

C-DOT also claims to have developed and deployed industry-grade QKD systems, which have received Technology Approval from the Telecommunication Engineering Centre (TEC), a body under the Department of Telecommunications (DoT).

"This initiative marks a significant milestone for India’s telecom ecosystem. It clearly demonstrates the feasibility of integrated quantum-classical networks over next-generation optical fibres, substantially reducing the cost of QKD deployments," said Dr Rajkumar Upadhyay, CEO of C-DOT.

Rahul Puri, CEO of STL’s Optical Networking Business, commented,"This collaborative breakthrough with C-DOT highlights India’s increasing capabilities in developing next-generation digital infrastructure."