India has achieved a significant milestone in quantum-secure communications by successfully testing a 5.56-kilometre open-air Quantum Key Distribution (QKD) link between BISAG-N and IIT Gandhinagar, India Today reported.

The field trial was conducted on the night of 27-28 September by QNu Labs in collaboration with BISAG-N and IIT-Gandhinagar. The demonstration transmitted quantum-encoded light across a 5.56-kilometre atmospheric path, proving that secure quantum communications can operate reliably over several kilometres in real-world conditions.

The system maintained a Quantum Bit Error Rate (QBER) below 5 per cent, a key benchmark for secure quantum communications. It also generated secret encryption keys at a rate of 230-260 bits per second, effectively producing enough material for approximately one new AES-256 encryption key every second.

A critical element of the experiment was QNu Labs' Pointing, Acquisition and Tracking (PAT) system, which continuously maintained precise alignment of the narrow optical beam throughout the entire 5.56-kilometre link despite atmospheric disturbances.

The generated quantum keys were integrated with BISAG-N's Vedic Kavach platform, enabling successful encryption and decryption of test messages. This created a dual-layer security architecture combining quantum key distribution with post-quantum cryptography, ensuring communications remain protected even if the physical quantum channel becomes temporarily unavailable because of cloud cover, atmospheric turbulence, dust, noise or alignment issues.

The achievement is particularly important because much of today's digital security infrastructure depends on cryptographic systems such as RSA and ECC, which could eventually be vulnerable to sufficiently powerful quantum computers.

Security experts are increasingly concerned about "harvest now, decrypt later" attacks, in which adversaries collect encrypted information today with the intention of deciphering it once advanced quantum computing capabilities become available.

QKD addresses this challenge by using the fundamental principles of quantum mechanics to generate and distribute encryption keys. Any attempt to intercept or measure the quantum signal alters its state, providing an immediate indication of potential eavesdropping.

However, quantum signals degrade over long distances when transmitted through optical fibre, limiting the practicality of fibre-only quantum networks across a country the size of India.

This is where free-space and satellite-based quantum communications become strategically important. By transmitting quantum signals through the atmosphere rather than relying exclusively on fibre networks, far greater distances may eventually be covered.

The 5.56-kilometre demonstration therefore serves as a valuable testing ground for technologies required in future space-based quantum networks. Atmospheric transmission allows researchers to evaluate challenges such as turbulence, dust, background noise and precision tracking before attempting similar operations in orbit.

ISRO is already progressing towards this objective. Its Quantum Technology Demonstration in Space (QuTDS) payload has been built, tested and delivered for an upcoming technology demonstration mission. The payload includes optical systems capable of accurately pointing towards target stations and transmitting quantum signals.

The next major initiative is SAQTI, or Secure Applications using Quantum and Optical Technologies, being pursued under the National Quantum Mission. The project aims to demonstrate secure quantum communications between satellites and ground stations, as well as between satellites themselves.

The latest accomplishment represents the newest stage in India's steadily advancing quantum communications programme. The country progressed from a 300-metre QKD demonstration in 2021 to longer-distance trials in subsequent years, culminating in the current 5.56-kilometre free-space achievement.

Additional developments under the National Quantum Mission include larger quantum-secure network demonstrations spanning hundreds of kilometres, reflecting India's broader ambition to establish indigenous quantum-resilient communication infrastructure and prepare for future satellite-based quantum networks.

With the successful validation of core free-space technologies, India's next major objective is expected to be extending quantum-secure communications from terrestrial links to orbital platforms, laying the groundwork for a future quantum-secure space communication network.

Agencies