Indian scientists have demonstrated a quantum-secure communications link through open air across more than five times the distance covered in a similar test last year.
In the latest demo, on the night of September 27-28, scientists connected the Bhaskaracharya National Institute for Space Applications and Geo-informatics (BISAG-N) and IIT-Gandhinagar. QNu Labs from Bengaluru supplied the equipment.
The feat has been described as India’s first free-space quantum-key distribution (QKD) link at a distance of 5.56 km. QKD is a technology to make communications more secure. Two parties generate a shared encryption key by transmitting information encoded in particles in photons.
If an eavesdropper attempts to intercept these particles and measure their quantum states, the act will produce errors the two parties will be able to detect.
Since these errors cannot be avoided, QKD allows parties to share encryption keys securely, using quantum physics.
In 2025, scientists at IIT-Delhi and the Defence Research & Development Organisation had demonstrated a QKD link through just over a kilometre of free air. Four years prior, the Indian Space Research Organisation (ISRO) had demonstrated a QKD link over 300 m.
In most instances where QKD is used on land, operators transmit the photons through fibre-optic cables. In open-air tests like the ones at IIT-Delhi and IIT-Gandhinagar, the particles travel through the atmosphere between two points with a direct line of sight. So the transmitter and the receiver need to be perfectly aligned, and scientists have to adjust the light signal passing between them for atmospheric turbulence and light pollution.
In the IIT-Gandhinagar demo, a Ministry of Electronics and IT statement reported the team had achieved a quantum bit error rate — the fraction of quantum bits received incorrectly — of under 5%, considered desirable.
The setup was able to generate the information required for secure keys at 230-260 bits per second. When scientists fed this information into a cryptographic platform at BISAG-N, they were able to encrypt and decrypt messages.
In the same demo, scientists also used post-quantum cryptography (PQC), a security paradigm in which information is safe from ‘hacking attempts’ made using both conventional and quantum computers. Combining QKD and PKC provides two layers of protection.
The open-air tests are the stepping stones to deploying QKD links between satellites orbiting the Earth and stations on the ground.
China launched the world’s first QKD satellite, ‘Micius’, in 2016. It orbits the Earth at around 500 km. Another Chinese satellite called Jinan-1, launched in 2022, was shown to perform QKD in real time with portable ground stations and generate up to 1 million bits per overhead pass during tests.
The National Quantum Mission in India has set a target to achieve satellite-based secure quantum communications between ground stations separated by up to 2,000 km and a 2,000-km inter-city QKD network over fibre-optic cables.
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