India Tests a 5.56 km Free-Space Quantum Key Distribution Link in Gandhinagar
An Indian startup, QNu Labs, worked with BISAG-N (Bhaskaracharya National Institute for Space Applications and Geo-informatics) and IIT Gandhinagar to test a free-space Quantum Key Distribution (QKD) link. "Free-space" means the quantum signal travelled through open air, not through an optical fibre cable.
The link covered about 5.56 km between the BISAG-N campus and IIT Gandhinagar in Gujarat. The field trial was carried out on the night of 27-28 September 2026.
A tracking system called Pointing, Acquisition and Tracking (PAT) kept the light beam locked between the two sites. The Quantum Bit Error Rate (QBER) stayed below 5%, and secret keys were produced at about 230-260 bits per second.
The keys were fed into Vedic Kavach, BISAG-N's software platform built on post-quantum cryptography. Test messages were then locked (encrypted) and unlocked (decrypted) end to end.
The setup combines two layers: QNu Labs' hardware QKD device (called Armos) and BISAG-N's software-based post-quantum encryption. If the open-air quantum channel is briefly blocked, the software layer can still keep messages safe.
The trial is a step towards longer links and, later, satellite-based quantum communication, which is one of the goals of India's National Quantum Mission.
The Quantum Key Distribution (QKD): Unbreakable Keys from Physics
Quantum Key Distribution (QKD) is a way for two people to create a shared secret key using single particles of light, called photons. The key is then used to lock and unlock messages. Its safety comes from a basic rule of quantum physics: you cannot measure or copy a quantum state without disturbing it. So if a spy tries to listen in, the errors in the key rise, and the two users know the line is not safe.
Key Details
- The first QKD method, BB84, was proposed by Charles Bennett and Gilles Brassard in 1984.
- QKD security rests on the no-cloning theorem: an unknown quantum state cannot be copied perfectly.
- Quantum Bit Error Rate (QBER) is the share of key bits that come out wrong. For BB84, a secure key is not possible if the QBER goes above roughly 11%. The Gandhinagar link stayed below 5%.
- QKD can run over optical fibre or through free space (open air or space). Free-space links suit places where laying fibre is hard and are the base for satellite QKD.
- Earlier Indian free-space trials: ISRO's Space Applications Centre over 300 m (March 2021), and DRDO with IIT Delhi using entanglement over more than 1 km (June 2025, QBER below 7%).
- QKD only shares the key. The actual message is still locked with a normal encryption method using that key.
The Gandhinagar trial used QKD through 5.56 km of open air, kept the error rate well under the danger level, and produced fresh secret keys every second. This extends India's free-space QKD distance well beyond the earlier 300 m and 1 km trials.
Post-Quantum Cryptography (PQC): The Parallel Approach to Quantum-Safe Security
Post-Quantum Cryptography (PQC) means new mathematical locks (encryption methods) that even a powerful future quantum computer should not be able to break. Unlike QKD, PQC needs no special quantum hardware. It is ordinary software that runs on today's computers, phones and networks. It is called "parallel" because countries use it side by side with QKD to become quantum-safe.
The Gandhinagar trial joined both approaches. QKD made the keys, and Vedic Kavach (a PQC platform) used them to encrypt test messages. This "hybrid" design means the system stays protected even if the open-air quantum link drops for a while.
National Quantum Mission (NQM)
The National Quantum Mission (NQM) is the Government of India's eight-year programme to build quantum technologies inside the country. It covers quantum computers, quantum communication, quantum sensors and quantum materials. The Department of Science and Technology (DST) runs it, and it works through hubs at leading institutes.
Free-space QKD over several kilometres is a building block for the NQM's goal of satellite-based quantum communication over 2,000 km. The Gandhinagar trial shows a startup, a government institute and an IIT working together, which is the kind of ecosystem the mission aims to grow.
- Link distance: about 5.56 km, between BISAG-N and IIT Gandhinagar (Gujarat)
- Trial date: night of 27-28 September 2026
- QBER: below 5%; secure key rate: 230-260 bits per second
- Partners: QNu Labs (Bengaluru startup, founded 2016), BISAG-N (autonomous society under the Ministry of Electronics and Information Technology), IIT Gandhinagar
- QKD hardware: Armos (QNu Labs); software layer: Vedic Kavach (BISAG-N, post-quantum cryptography)
- Earlier trials: ISRO 300 m free-space QKD (2021); DRDO-IIT Delhi entanglement-based free-space link over 1 km (2025)
- BB84 QBER security limit: about 11%
- National Quantum Mission: approved 19 April 2023, ₹6,003.65 crore, 2023-24 to 2030-31