India Demonstrates Its First 5.56 km Free-Space Quantum Key Distribution Link

Quantum Key Distribution (QKD) is a way for two people far apart to create a shared secret key, a long string of 0s and 1s, using single particles of light called photons. They then use this key to lock (encrypt) and unlock (decrypt) their messages. What makes QKD special is that if anyone tries to spy on the key while it is being made, the laws of physics leave a mark, and the two users find out. So its safety comes from physics, not from how hard a maths puzzle is.

Why do we need it? Today, almost everything online, from UPI payments to army messages, is protected by encryption. Common systems like RSA are safe only because some maths problems, like breaking a huge number into its prime factors, would take normal computers thousands of years. In 1994, Peter Shor showed that a large enough quantum computer could solve these problems quickly (Shor's algorithm). Such computers do not exist yet. But spies may already be copying and storing encrypted data today, hoping to unlock it later. This is called "harvest now, decrypt later". QKD protects against this because its key cannot be cracked later by any computer, however fast.

Where did it come from? The ideas behind QKD came step by step:

• 1982: The no-cloning theorem was proved. It says you cannot make a perfect copy of an unknown quantum state. This is the heart of QKD's security.
• 1984: Charles Bennett and Gilles Brassard proposed the first QKD method, called BB84 (their initials and the year).
• 1991: Artur Ekert proposed E91, a method based on quantum entanglement.
• 2016: China launched the Micius satellite (QUESS mission) in August. In 2017, it shared entangled photons between two ground stations about 1,200 km apart and helped secure a video call between Beijing and Vienna.
• 2022: The Nobel Prize in Physics went to Alain Aspect, John Clauser and Anton Zeilinger for experiments with entangled photons that laid the base for quantum information science.

How does QKD work? (The BB84 idea, step by step) The two users are usually called Alice (sender) and Bob (receiver). A spy is called Eve.

1. Alice sends Bob a stream of single photons. Each photon carries one bit (0 or 1) through its polarisation, the direction in which the light wave vibrates. She picks, at random, one of two "settings" (called bases) for each photon: straight (up-down/sideways) or slanted (diagonal).
2. Bob does not know Alice's settings. He measures each photon using his own randomly chosen setting.
3. Later, on a normal public channel, Alice and Bob tell each other only which settings they used, not the bit values. They keep the bits where their settings matched and throw the rest away. What remains is the sifted key.
4. They compare a small sample of the key in public. If Eve had measured the photons on the way, the act of measuring would have disturbed them. This shows up as extra errors.
5. If the error rate is low enough, they correct the few errors and shrink the key a little more so Eve knows nothing about it (privacy amplification). Now they share a secret key.

A simple picture: imagine a soap bubble. You cannot touch it to check its shape without changing or bursting it. In the same way, Eve cannot "look" at a photon without changing it, and she cannot photocopy it (no-cloning).

The error rate is the alarm bell. The share of bits that differ between Alice and Bob is the Quantum Bit Error Rate (QBER). Some errors always come from noise in equipment and air. For BB84, a secure key can be made only if QBER stays below about 11%. Above that, users must assume a spy is listening and throw the key away. A QBER below 5%, as in the Gujarat trial, gives a comfortable safety margin.

Fibre vs free-space QKD:

• Fibre QKD sends photons through optical cables. It is stable, but the glass slowly absorbs photons. Beyond about 100 km or so, too few survive. Long networks therefore need trusted nodes, guarded relay stations where the key is passed on.
• Free-space QKD sends photons through open air using telescopes. It needs a clear line of sight and suffers from fog, rain, daylight and air turbulence. So tests are usually done at night, and the telescopes need a Pointing, Acquisition and Tracking (PAT) system to stay aimed. Its big advantage: in space there is no air, so satellite QKD can join places thousands of kilometres apart.

India's milestones in QKD:

• 19 March 2021: ISRO's Space Applications Centre (SAC), Ahmedabad, showed free-space quantum communication over 300 metres between two buildings, using a NavIC receiver for time matching.
• 2022: DRDO and IIT Delhi showed a QKD link of more than 100 km between Prayagraj and Vindhyachal (Uttar Pradesh) over existing commercial optical fibre.
• 2025: IIT Delhi and DRDO showed entanglement-based free-space QKD over more than 1 km on the IIT Delhi campus, with about 240 bits per second and an error rate below 7%.
• September 2026: QNu Labs, BISAG-N and IIT Gandhinagar showed free-space QKD over 5.56 km.

Commonly confused concepts:

• QKD vs Post-Quantum Cryptography (PQC): QKD is hardware that uses physics (photons) to share a key. PQC is software: new maths-based methods believed to be too hard even for quantum computers. In August 2024, the US standards body NIST published the first three PQC standards (FIPS 203, 204 and 205). The two are often used together, as in the Gujarat trial.
• Quantum communication vs quantum computing: Quantum communication moves information safely (QKD is its main use today). Quantum computing does calculations using qubits.
• QKD vs quantum teleportation: QKD only creates a shared key; the actual message still travels by normal channels. Teleportation transfers a quantum state using entanglement, not physical matter, and it cannot send information faster than light.
• Prepare-and-measure (BB84) vs entanglement-based (E91): In BB84, Alice prepares photons and Bob measures them. In E91, a source creates pairs of entangled photons and sends one to each user.

Issues, criticism and the way forward:

• Distance and speed: Key rates are low (hundreds of bits per second over a few km). Fibre loss and weather limit range.
• Real devices are not perfect: QKD is secure in theory, but flaws in real lasers and detectors have been exploited in lab attacks. Methods like decoy states and careful testing are used to close these gaps.
• Cost and trusted nodes: Equipment is costly, and trusted relay nodes become weak points.
• Authentication: QKD still needs a way to confirm Alice is really talking to Bob, which uses classical methods.
• Different views: Some security agencies, such as the US National Security Agency, prefer PQC over QKD for now because of these limits. Supporters say QKD gives long-term safety that maths cannot promise.
• Way forward: Satellite QKD, quantum repeaters and quantum memories (to extend range without trusted nodes), Indian standards and testing labs, and hybrid systems combining QKD with PQC.

Concepts to Know:

• Photon: The smallest possible packet of light. QKD sends information one photon at a time.
• Encryption key: A secret string of bits used to scramble a message so only someone with the same key can read it.
• Polarisation: The direction in which a light wave vibrates, like up-down or sideways. It can be used to carry a 0 or a 1.
• Quantum entanglement: A link between two particles so that measuring one instantly tells you about the other, however far apart they are.
• Qubit: The basic unit of quantum information. Unlike a normal bit, it can be in a mix of 0 and 1 until measured.

Key Details

• No-cloning theorem: 1982; BB84: Bennett and Brassard, 1984; E91: Artur Ekert, 1991
• Shor's algorithm: 1994 (threat to RSA-type encryption)
• BB84 secure only if QBER is below about 11%
• China's Micius satellite: launched August 2016; 1,200 km entanglement distribution in 2017
• Nobel Prize in Physics 2022: Aspect, Clauser, Zeilinger (entangled photons)
• NIST PQC standards: FIPS 203, 204, 205 (August 2024)
• India: SAC/ISRO 300 m free-space (March 2021); DRDO–IIT Delhi 100+ km fibre, Prayagraj–Vindhyachal (2022); IIT Delhi 1 km+ entanglement-based free-space (2025); 5.56 km free-space, Gujarat (September 2026)

The National Quantum Mission (NQM) is the Government of India's eight-year programme to build quantum technologies in India. These are machines that use the strange rules of the tiny world of atoms and light. The mission covers four areas: quantum computers, quantum communication, quantum sensors and the special materials needed to build them. It was approved by the Union Cabinet on 19 April 2023 with a budget of ₹6,003.65 crore for 2023-24 to 2030-31, and is run by the Department of Science and Technology (DST).