India has sent quantum encryption keys across 5.56 km of open air without fibre-optic cables. The experiment worked, the messages were encrypted and decrypted, and the researchers now have their sights set on a much bigger challenge: quantum-secure networks that could eventually reach satellites.
India’s efforts to build a quantum-resistant security architecture came one step closer to reality recently. A little over a year ago, we reported on DRDO and IIT researchers making moves to get India into the quantum warfare space, looking at the possibility that this technology will one day render traditional techniques of eavesdropping on military communications obsolete.
Today, the initiative took another step in a different direction. Researchers from deep-tech company QNu Labs along with Bhaskaracharya National Institute for Space Applications and Geo-informatics (BISAG-N) and IIT Gandhinagar have demonstrated quantum key distribution over a distance of 5.56km. This night-long experiment, performed on 27-28 September, was made public by the Ministry of Electronics and Information Technology on 3 October.
Quantum Keys, No Cables
What actually went across those 5.56km? They used quantum key distribution (QKD), a method that enables two parties to produce and share a secret cryptographic key using quantum signals. In this case, the keys were sent via an optical link in free space over the distance between BISAG-N and IIT Gandhinagar.
The benefit becomes clear when you consider locations where laying down fibre-optic lines would be challenging, costly or simply unfeasible. Free-space optical communication would be useful for linking installations separated by difficult landscapes or bodies of water and will ultimately facilitate communication between earth stations and satellites.
QKD also has an interesting security feature: any attempt to measure or eavesdrop on the quantum signals disturbs them, leading to observable changes in certain circumstances. This makes the technology desirable for securing sensitive transmissions, especially since some experts believe quantum computers in the future will be able to crack popular encryption schemes currently in use.
The Numbers Behind the 5.56 km Test
As per the ministry’s press release, the quantum bit error rate of the channel remained below 5% and the keys were generated at a rate of 230-260 bits per second. QNu Labs utilised a pointing, acquisition and tracking (PAT) system to keep the optical signal aligned throughout the duration of the transmission.
Once generated, the keys were combined with BISAG-N’s Vedic Kavach framework that implements post-quantum cryptography and quantum random number generation.
The teams managed to encrypt, transmit and decrypt test messages using the combined setup. While the speed at which keys are generated seems small compared to traditional networks, QKD isn’t intended to serve as a replacement for the internet’s data-carrying capabilities; rather, it supplies only the cryptographic keys.
Preparing for the Day Encryption Breaks
This timing is crucial, because the security implications of quantum technology aren’t just theoretical anymore. While large-scale, fault-tolerant quantum computers capable of breaking common public-key cryptography in real-world attacks do not yet exist, governments and security experts are already planning for their eventual arrival, including the threat of attackers harvesting encrypted data today and decrypting it later.
Quantum key distribution can help in this regard by changing the way encryption keys are distributed, while post-quantum cryptography focuses on mathematical algorithms resistant to quantum computer attacks.
In the Gandhinagar experiment, the team used both quantum key distribution and post-quantum cryptography, rather than relying solely on one or the other. While a successful field test is a significant achievement, building a reliable network requires successfully navigating the aforementioned issues repeatedly, over various ranges and under different conditions.
Space, The Final Frontier
The longer-term goal here is space. QNu Labs said the 5.56-km test “lays the foundation” for extended-range quantum-secure networks and space-based quantum communications. Eventually, satellites might be able to establish connections between far-flung ground stations, without needing to lay fibre-optic cable between every pair of nodes. They could allow quantum communication between ground stations separated by thousands of miles.
The Indian government has already invested heavily in quantum research, funding a National Quantum Mission aimed at developing a range of quantum technologies from computing to communications to sensing.
A terrestrial free-space link isn’t the same as a satellite quantum network, of course. The test demonstrated a quantum link between two points on Earth, not a link between Earth and space. Satellite communications pose their own set of engineering challenges, such as signal attenuation, pointing accuracy and atmospheric effects near the receiver. However, being able to transmit quantum signals over several kilometres of free space is a necessary prerequisite to eventually establishing a satellite quantum network.
In conclusion, India’s most recent quantum achievement isn’t about creating a theoretically unbreakable communications system, but about getting a complex technology to function outside the laboratory environment. Future questions will center on how well the technology performs under varying conditions, its maximum range, and the ability to connect multiple links into a wider network.
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