India teams complete 5.56 km free-space quantum link

QNu Labs, BISAG-N and IIT Gandhinagar have completed a 5.56 km free-space quantum key distribution field trial in India, which they describe as the country's first free-space QKD link at the 5 km scale.

The trial linked BISAG-N and IIT Gandhinagar over open atmosphere and combined quantum key distribution with BISAG-N's Vedic Kavach platform, which uses post-quantum cryptography. It also included end-to-end integration with a browser and web server using keys generated through the quantum link.

Quantum key distribution, or QKD, is designed to share encryption keys using quantum principles, with the aim of making interception detectable. In this case, the link operated over free space rather than fibre, using an optical channel between the two sites.

According to the organisations, the field trial achieved a quantum bit error rate, or QBER, below 5% and a secure key rate of 230 to 260 bits per second. At that rate, the system generated about one new 256-bit key every second.

A test message was encrypted, transmitted and decrypted during the demonstration. The organisations said the 256-bit keys were delivered through the ETSI GS QKD 014 interface to support the application test.

How it worked

QNu Labs said its Pointing, Acquisition and Tracking system maintained the 1550 nm quantum channel during the trial. The setup used coarse pointing through a gimbal and fine steering at a 100 Hz tracking bandwidth to keep the two terminals aligned.

According to the organisations, the system achieved coarse pointing accuracy of 5 microradians and fine pointing accuracy of up to 0.1 microradians. They said this kept the quantum channel aligned despite atmospheric conditions and movement along the optical path.

The trial used the DPS-Decoy QKD protocol. The broader architecture combined a hardware layer for quantum key distribution with a software layer based on post-quantum cryptography and quantum random number generation.

The organisations said the design allows the application layer to remain protected even if the physical quantum channel is temporarily unavailable. This reflects one of the central challenges in moving quantum communications from controlled research settings into operational networks.

Research link-up

The project brought together a private-sector quantum security company, a government-backed scientific body and an academic institution. BISAG-N operates under India's Ministry of Electronics and Information Technology, while IIT Gandhinagar provided the field environment for the experiment.

Sunil Gupta, Co-founder and Chief Executive Officer of QNu Labs, said, "Quantum security will become meaningful at scale only when technologies can move beyond controlled laboratory environments and operate across real communication infrastructure. This field trial shows that QKD, supported by precise optical pointing and tracking, can be deployed over a multi-kilometre free-space link and integrated with a post-quantum security layer. The successful 5.56 km demonstration is an important step towards developing longer-distance quantum-secure networks and exploring architectures that can eventually support multiple nodes and satellite-based quantum communication."

The test adds to a growing body of work seeking to combine QKD with post-quantum cryptography rather than treat the two as competing approaches. In practice, many researchers and security specialists see hybrid designs as a way to balance physical-layer security with software-based resilience.

Cmde Manish Tripathi (Retd.), In-charge, ISTF, IIT-G, said, "The successful demonstration of a 5.56 km free-space QKD link provides a valuable field environment for advancing practical quantum communication technologies. The collaboration brings together academic research, indigenous technology and real-world experimentation, and demonstrates how these capabilities can be integrated to address the emerging requirements of secure communications."

For BISAG-N, the demonstration also focused on application-level integration. The organisation's Vedic Kavach browser and web server were used as the software environment for the secure communication test.

Dr Vinay Thakur, DG, BISAG-N, said, "The integration of Vedic Kavach with QNu Labs' QKD technology demonstrates the potential of combining post-quantum cryptography with quantum key distribution as complementary layers of security. The successful end-to-end application test is an important step towards developing practical quantum-resilient communication architectures that can evolve with emerging security requirements."

Free-space quantum links are often discussed in the context of secure communications between fixed sites, mobile platforms and, in the longer term, satellite systems. The 5.56 km result does not resolve the engineering challenges involved in scaling such systems, but it provides a field result that the organisations say can support further work on longer links and multi-node networks.

The reported performance, including a QBER below 5% and a secure key rate of 230 to 260 bps, is significant because stable error rates and key generation are central to whether a QKD link can support practical encryption workflows.