BharatNotes What is Quantum Key Distribution?
Quantum Key Distribution (QKD) lets two parties — conventionally called Alice and Bob — establish a shared secret key over an insecure channel with security guaranteed by the laws of quantum physics rather than by computational hardness. The first and most widely used protocol, BB84, was proposed by Charles Bennett and Gilles Brassard in 1984. Alice encodes random bits in the polarisation of single photons using two randomly chosen "bases"; Bob measures each photon in a randomly chosen basis. They later compare bases over a public channel and keep only the bits where bases matched, forming the raw key.
How the Physics Guarantees Security
Two quantum principles make QKD tamper-evident:
- No-cloning theorem — an unknown quantum state cannot be copied perfectly, so an eavesdropper (Eve) cannot duplicate photons to read them undetected.
- Measurement disturbance — measuring a photon in the wrong basis irreversibly alters it. Eve, forced to guess the basis, introduces detectable errors.
Alice and Bob estimate the Quantum Bit Error Rate (QBER); if it exceeds a threshold, they assume interception and discard the key. This is the crucial difference from classical cryptography: security is rooted in physics, not in the assumption that certain mathematics is hard to compute.
Why It Matters
| Aspect | QKD | Classical / Post-Quantum Crypto |
|---|---|---|
| Basis of security | Laws of physics | Mathematical complexity |
| Threat from quantum computers | Immune | RSA/ECC breakable by Shor's algorithm; PQC resists it via new maths |
| Eavesdropping | Detectable | Often undetectable |
| Infrastructure | Needs special optical/photonic hardware | Software-upgradable |
QKD and Post-Quantum Cryptography (PQC) are complementary, not identical — PQC is software-based encryption resistant to quantum attack (NIST finalised the first PQC standards FIPS 203, 204 and 205 on 13 August 2024), whereas QKD is a hardware-based key-exchange method.
Current Status in India
- DRDO–IIT Delhi: Demonstrated a QKD link between Prayagraj and Vindhyachal, Uttar Pradesh, over a distance of more than 100 km on commercial-grade optical fibre — announced via PIB on 23 February 2022, the first such demonstration in the country.
- ISRO / Physical Research Laboratory (PRL): Demonstrated free-space (atmospheric) QKD over 300 metres at Ahmedabad, using an indigenous NavIC receiver for time synchronisation and a gimbal mechanism for optical alignment, paving the way for satellite-based QKD.
- National Quantum Mission (NQM): Approved by the Union Cabinet on 19 April 2023 with an outlay of ₹6,003.65 crore for 2023-24 to 2030-31. Its deliverables include satellite-based secure quantum communication over a 2,000 km range within India and inter-city QKD over 2,000 km.
UPSC Angle
Focus on the agency, distance and venue of each Indian demonstration, the QKD-versus-PQC distinction, and the strategic rationale: protecting banking, defence and critical information infrastructure from "harvest-now, decrypt-later" attacks once large quantum computers arrive. Link it to the NQM and to cyber-security debates in GS3.
