IIT-Gandhinagar Team Achieves India's Longest Free-Space Quantum Link

Quantum Key Distribution (QKD) is part of India's broader push to secure communications against the long-term threat posed by future quantum computers. Classical encryption systems such as RSA rely on mathematical problems that ordinary computers find hard to solve, but a sufficiently powerful quantum computer could break them. This is often called the 'harvest now, decrypt later' threat, meaning data encrypted today could be stored and decrypted once quantum computers mature. India's response is the National Quantum Mission, approved for 2023-2031, which covers four verticals: quantum computing, quantum communication, quantum sensing and quantum materials. It aims to build quantum computers with 50-1000 physical qubits and set up four Thematic Hubs (T-Hubs) at leading institutions.

A qubit (quantum bit) is the basic unit of quantum information, different from a classical bit. A classical bit is either 0 or 1, but a qubit can exist in a superposition of both states at once (meaning it holds a combination of both values until measured). This same quantum behaviour underlies QKD: instead of encoding a key in classical bits, QKD encodes it in the quantum states of individual photons, such as their polarization. Any attempt to intercept or measure these photons disturbs their quantum state, which is what makes eavesdropping detectable.

India's free-space QKD trials have progressed in stages. ISRO earlier demonstrated a QKD link over 300 metres. IIT-Delhi and the Defence Research and Development Organisation (DRDO) later achieved a free-space QKD link of just over 1 km. The latest demonstration by IIT-Gandhinagar, BISAG-N and QNu Labs extends this to 5.56 km, more than five times the previous open-air distance, and is described as India's first free-space QKD link at this scale.