India’s First 5.56 km Free-Space Quantum Key Distribution Link

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Key takeaways

India successfully demonstrated its first 5.56 km free-space Quantum Key Distribution (QKD) link between BISAG-N and IIT Gandhinagar in collaboration with QNu Labs.
The trial kept the Quantum Bit Error Rate (QBER) below 5% using advanced Pointing, Acquisition, and Tracking (PAT) technology.
The system uses a hybrid security model that combines QNu Labs hardware with BISAG-N software alongside Post-Quantum Cryptography (PQC).
This breakthrough supports the goals of India’s National Quantum Mission (NQM) to develop long-range secure quantum communication networks and satellite links.

India recently completed a successful test of its first 5.56 km Free-Space Quantum Key Distribution (QKD) link in the field.
The trial created a secure communication channel between BISAG-N (Bhaskaracharya National Institute for Space Applications and Geo-informatics) and IIT Gandhinagar .
The project was carried out jointly with QNu Labs , an Indian deep-tech startup.

Quantum Key Distribution (QKD) is a secure method that uses the quantum properties of photons to share secret cryptographic keys between two distant locations.
It does not send the actual message through quantum states . Instead, QKD makes a secure encryption key that traditional systems use to lock and unlock the real data.
Free-Space QKD sends these encryption keys through laser beams in open air , removing the need for physical fibre-optic cables.
QKD works based on Heisenberg’s Uncertainty Principle and the No-Cloning Theorem.
According to Heisenberg’s Uncertainty Principle , measuring a quantum system changes it. If a spy tries to intercept the photons, the quantum state collapses and immediately alerts the users of a breach.
The No-Cloning Theorem states that creating a perfect copy of an unknown quantum state is physically impossible , which stops hackers from secretly copying the keys.
QKD does not send the real message. It only creates a secure cryptographic key to protect the data.
Any attempt to read the quantum states disturbs them and causes visible errors. This lets users catch eavesdropping and throw away the compromised key.
Unlike fibre-based setups, free-space QKD needs a direct line-of-sight between the transmitter and receiver. It requires careful alignment, and weather conditions like atmospheric turbulence or bright daylight can weaken the optical signal.

This recent 5.56 km trial is a huge step up from past short tests, such as the 300m test back in 2021.
The Quantum Bit Error Rate (QBER) measures the share of incorrect quantum bits received. A lower QBER means a safer channel, and during this 5.56 km trial, the QBER stayed below 5%.
The trial used QNu Labs' advanced Pointing, Acquisition, and Tracking (PAT) system to keep the laser beam aligned across the open air over the entire distance.
The setup used a Two-Layered Security Architecture combining hardware from QNu Labs (Armos) and software from BISAG-N (Vedic Kavach).
This hybrid setup mixed QKD with Post-Quantum Cryptography (PQC), which uses encryption algorithms built to stay safe against future quantum computers.
If bad weather temporarily breaks the physical atmospheric link, the mathematical PQC software keeps protecting the data.
Free-space terrestrial QKD is an important stepping stone toward satellite-based quantum communication because both send quantum signals through the atmosphere between line-of-sight optical terminals.
As quantum computers grow strong enough to break normal encryption, this technology provides a quantum-safe shield for defence, critical infrastructure, and financial networks against future cyber attacks.
India’s National Quantum Mission (NQM) aims to build satellite-based secure quantum communication between ground stations up to 2,000 km apart, alongside a 2,000 km inter-city QKD network using optical fibre.

Fibre-based QKD uses fibre-optic cables for transmission, while free-space QKD uses open air, the atmosphere, or space.
Fibre-based systems usually cover only a few hundred kilometres without trusted repeaters due to signal loss, whereas free-space systems can cover thousands of kilometres, especially using satellites.
Fibre-based connections are more stable and protected from weather, while free-space links face disruptions from fog, rain, turbulence, and daylight.
Fibre setups use fixed connections without active tracking, but free-space setups need precise Line-of-Sight (LoS) along with active pointing and tracking systems.
Earlier under the NQM , the startup QNu Labs also tested India’s first 500 km QKD network using current optical-fibre infrastructure.
Globally, China launched the world’s first quantum-communication satellite, Micius, in 2016 from low-Earth orbit at roughly 500 km, and later launched Jinan-1 in 2022.