India's First Free-Space Quantum Key Distribution Link: Secret Keys Sent 5.56 km Through Open Air
India carried out its first field test of a free-space quantum key distribution (QKD) link over 5.56 km. "Free-space" means the signal travelled through open air as a beam of light, not through an optical fibre cable.
The link connected the Bhaskaracharya National Institute for Space Applications and Geo-informatics (BISAG-N) and IIT Gandhinagar, both in Gandhinagar, Gujarat. The trial was run on 27 and 28 September 2026 by the Bengaluru-based quantum start-up QNu Labs, with BISAG-N and IIT Gandhinagar.
The system kept a steady quantum bit error rate (QBER) below 5% and produced secret keys at 230 to 260 bits per second. (QBER is the share of key bits that arrive wrong. A low QBER means no one is likely to be listening in.)
A Pointing, Acquisition and Tracking (PAT) system kept the narrow light beam locked on target between the two sites, even when air movement or building sway tried to push it off.
The quantum keys were fed into BISAG-N's Vedic Kavach platform, a software system that uses post-quantum cryptography and quantum random number generation. Test messages were encrypted and decrypted end to end.
The two layers work together: QKD gives keys guaranteed by physics, and post-quantum software keeps data safe even if the light link is briefly down. The demonstration is seen as a step towards city-wide and satellite-based quantum-secure networks under the National Quantum Mission.
The Quantum Key Distribution (QKD): Unbreakable Keys from Physics
Quantum Key Distribution (QKD) is a way for two people to share a secret key using single particles of light, called photons. The key is then used to lock (encrypt) and unlock (decrypt) messages. Its strength comes from a rule of quantum physics: you cannot measure a quantum particle without disturbing it. So if a spy tries to read the key on the way, the errors give the spy away.
Key Details
- The first QKD method, BB84, was proposed by Charles Bennett and Gilles Brassard in 1984. It was presented at a conference held in Bangalore (now Bengaluru), India.
- QKD rests on the no-cloning theorem: an unknown quantum state cannot be copied perfectly. A spy cannot quietly make a copy of each photon.
- Senders and receivers watch the quantum bit error rate (QBER). For BB84, a secure key is not possible if the QBER goes above about 11%. The Gandhinagar link stayed below 5%.
- QKD can run through optical fibre or through free space (open air, or between a satellite and the ground). Free space avoids digging for cables and suits difficult terrain, mobile sites and future satellites.
- Earlier Indian free-space tests: ISRO's Space Applications Centre sent quantum keys 300 m between two buildings on 19 March 2021; DRDO and IIT Delhi showed entanglement-based free-space QKD over more than 1 km in June 2025 (about 240 bits per second, QBER below 7%).
- China's Micius satellite, launched in 2016, was the world's first quantum science satellite and showed satellite-to-ground QKD.
The BISAG-N to IIT Gandhinagar trial is QKD in action over open air. At 5.56 km it is the longest free-space QKD link shown in India so far, more than five times the 2025 DRDO-IIT Delhi distance. The low error rate shows that no one could have been secretly reading the key.
National Quantum Mission (NQM)
The National Quantum Mission is the Government of India's eight-year programme to build quantum technologies inside the country. It covers four areas: quantum computing, quantum communication, quantum sensing and metrology, and quantum materials and devices. It is run by the Department of Science and Technology (DST). Its aim is to make India one of the leading countries in this field and to protect India's communications from future quantum computers.
The 5.56 km free-space link is a building block for the NQM's 2,000 km satellite-based and inter-city QKD goals. Free-space links are exactly what a satellite-to-ground connection needs. QNu Labs had also worked on a secure quantum communication network under the mission earlier in 2026.
Post-Quantum Cryptography (PQC): Encryption Built to Survive Quantum Computers
Post-quantum cryptography (PQC) means new methods of locking digital data that even a powerful future quantum computer cannot break. Unlike QKD, PQC does not need any special physics hardware. It is just new mathematics, written as software, that runs on today's ordinary phones, computers and servers. Its job is to replace the older locks that quantum computers are expected to crack.
In the Gandhinagar trial, the quantum keys were plugged into BISAG-N's Vedic Kavach, a post-quantum cryptography platform. This shows the "two-layer" model India's roadmap talks about: QKD for physics-based key sharing, and PQC software that keeps messages safe even when the light link is briefly unavailable.
- Distance of the free-space QKD link: 5.56 km, between BISAG-N and IIT Gandhinagar (Gandhinagar, Gujarat)
- Field trial dates: 27 and 28 September 2026; carried out by QNu Labs (Bengaluru)
- Quantum bit error rate: stable below 5%; secure key rate: 230 to 260 bits per second
- QKD hardware: QNu Labs' Armos; beam kept aligned by a Pointing, Acquisition and Tracking (PAT) system
- Keys integrated with Vedic Kavach, BISAG-N's post-quantum cryptography platform with quantum random number generation
- Earlier Indian free-space QKD: ISRO SAC, 300 m (March 2021); DRDO and IIT Delhi, over 1 km (June 2025)
- BB84, the first QKD protocol (1984), was presented at a conference in Bangalore
- National Quantum Mission: approved 19 April 2023; ₹6,003.65 crore; 2023-24 to 2030-31; target of 2,000 km satellite-based and inter-city QKD