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Science & TechFree till Sep 9

Quantum Key Distribution: Secure Communication

July 19, 2026

TOPIC CLASSIFICATION

Subject: Science & Technology — Quantum Technologies — Secure Communication
Sub-topic: Quantum Key Distribution (QKD) — BB84 Protocol, E91 Protocol, Decoherence, Quantum Entanglement, No-Cloning Theorem, Heisenberg Uncertainty Principle, QKD vs Classical Cryptography, QKD Implementations (Fiber-based, Free-Space, Satellite-based), Micius Satellite (China), India's QKD Efforts — ISRO/RRI (300 km QKD), DRDO QKD, MeitY Quantum Communication Lab, National Quantum Mission (2023-2031)
Mains GS Paper-III: Science & Technology — developments in quantum technologies and their applications in security and communication.

EXAMINER REASONING

QKD represents the frontier of secure communication — theoretically unbreakable encryption based on quantum mechanics. Prelims tests: BB84 protocol (Charles Bennett, Gilles Brassard — 1984), quantum bits (qubits), no-cloning theorem, Heisenberg uncertainty principle (measurement disturbs the system), photon polarisation, entanglement-based QKD (E91), decoy state protocol, Micius satellite (China), India's QKD milestones (300 km by ISRO/RRI, 100 km by DRDO). Mains demands: (a) why QKD is needed — Shor's algorithm breaks RSA-2048 encryption — quantum computers pose existential threat to classical cryptography — QKD provides information-theoretic security, (b) BB84 protocol — Alice sends photons in random polarisation bases — Bob measures in random bases — sifting — error correction — privacy amplification — eavesdropper detection (intercept-resend increases error rate >11% triggers abort), (c) entanglement-based QKD — E91 protocol (Ekert, 1991) — uses Bell's inequality to detect eavesdropping, (d) practical challenges — photon loss in optical fibre (exponential attenuation — 0.2 dB/km), trusted relay nodes needed for long distances, quantum repeaters still experimental, (e) satellite QKD — Micius (China, 2016) demonstrated QKD over 1200 km (space to ground) — overcomes fibre attenuation — enables global QKD network, (f) India's progress — ISRO/RRI QKD over 300 km (2023) — DRDO QKD over 100 km (2022) — National Quantum Mission (₹6,003 crore) targets quantum communications testbed (2026), (g) post-quantum cryptography (PQC) — alternative to QKD — new cryptographic algorithms resistant to quantum computers — NIST standards (2024). The examiner's favourite framing is: "Can quantum key distribution provide truly unbreakable security? What are the practical challenges?"

Core Concept

QKD Protocol Fundamentals

Protocol / Concept

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Detail
BB84 Protocol (1984)Alice sends single photons in 4 polarisation states (0°, 45°, 90°, 135°) in 2 bases (+ and ×) — Bob measures in random bases — ~50% match — sifted key — error correction — privacy amplification
E91 Protocol (Ekert, 1991)Uses entangled photon pairs — Alice and Bob each measure their photon — Bell's inequality test guarantees security — any eavesdropping destroys entanglement
Decoy State ProtocolPractical solution to multi-photon pulses — reduces vulnerability to photon-number-splitting (PNS) attacks
No-Cloning TheoremUnknown quantum states cannot be perfectly cloned — fundamental quantum limit — makes eavesdropping detectable
Heisenberg Uncertainty PrincipleMeasuring a quantum system disturbs it — eavesdropper cannot intercept without introducing detectable errors
QubitQuantum bit — can be 0, 1, or superposition of both — in QKD: photon polarisation, phase, or time-bin encodes the qubit
Key RateLimited by photon source, detector efficiency, fibre attenuation, dark counts — typical secure key rate: kbps over km — drops exponentially with distance
Maximum DistanceFibre: ~500 km with trusted relays — Satellite: 1200+ km (demonstrated by Micius)

QKD vs Classical Cryptography

AspectClassical CryptographyQuantum Key Distribution
Security basisMathematical complexity (factoring, discrete log)Laws of quantum mechanics
Threat from quantum computersRSA, ECC, Diffie-Hellman broken by Shor's algorithm (1994)Immune — security is information-theoretic
Eavesdropper detectionCannot detect — relies on assumed computational limitsAny interception introduces detectable errors
Key distributionPublic key infrastructure (PKI) — trusted third partiesDirect quantum channel — no trusted third party needed
Practical maturityMature — deployed everywhereStill emerging — limited distance, specialised hardware
ComplementarityQKD + classical encryption (hybrid) — QKD distributes key, classical algorithms use it for bulk encryption

Key Facts

FactDetail
BB84 proposed1984 by Charles Bennett and Gilles Brassard
First QKD demonstrationIBM (1989) — over 32 cm in open air
Micius satelliteChina (2016) — world's first quantum communications satellite — QKD over 1200 km
China's quantum networkBeijing-Shanghai QKD backbone (2000+ km) — 32 trusted relay nodes — operational since 2017
ISRO/RRI QKD2023 — QKD over 300 km optical fibre in Kashmir — national record
DRDO QKD2022 — QKD over 100 km in optical fibre
India's National Quantum Mission₹6,003 crore (2023-2031) — quantum communications testbed by 2026
Quantum repeatersStill experimental — would enable QKD over unlimited distances without trusted relays
NIST PQC standards2024 — 3 algorithms standardised (CRYSTALS-Kyber, CRYSTALS-Dilithium, SPHINCS+)
QKD marketProjected $2B+ by 2030 — driven by banking, defence, government

PYQ Table

YearQuestionType
2023"What is Quantum Key Distribution? How does it differ from classical cryptography?"Mains
2022"Explain the BB84 protocol for quantum key distribution."Mains
2021The no-cloning theorem is fundamental to which emerging technology?Prelims
2020"How can quantum technologies enhance national security?"Mains
2019Micius is associated with which field?Prelims

Statement Elimination Guide

StatementTruth ValueWhy?
"QKD provides information-theoretic security"TrueSecurity is based on laws of quantum mechanics — not mathematical assumptions — fundamentally unbreakable
"Quantum key distribution can be used to encrypt bulk data"FalseQKD is used for key distribution only — the distributed key is then used with classical symmetric encryption (like AES) for bulk data
"The no-cloning theorem prevents quantum states from being perfectly copied"TrueThis is a fundamental quantum principle — any attempt to copy an unknown quantum state will introduce errors — this is the basis of QKD security
"India has launched a quantum communications satellite"FalseIndia has not yet launched a quantum communications satellite — ISRO has demonstrated QKD over 300 km optical fibre — satellite QKD is in the planning stage
"Shor's algorithm can break AES encryption"FalseShor's algorithm breaks RSA and ECC (public key) — AES symmetric encryption is only affected by Grover's algorithm (quadratic speedup — can be mitigated by doubling key size)

Current Affairs Hook

2023-26: India's National Quantum Mission (NQM) — approved by Cabinet (April 2023) — ₹6,003 crore budget — 4 hubs (quantum computing, communication, sensing, materials) — target: quantum communications testbed by 2026 — quantum computer with 50-100 qubits by 2028. ISRO/RRI QKD demonstration (2023) — 300 km optical fibre link — world-class achievement. MeitY — establishing a National Quantum Communication Network linking 20+ institutions — trial phase. NIST PQC standards (August 2024) — FR-256 Kyber, ML-KEM, SLH-DSA — US government transition to PQC by 2035. China's quantum advances — Micius successor (Jinan-1, 2022) — quantum satellite network — ground-to-aircraft QKD — China leading in satellite QKD. QKD integration in banking — RBI exploring QKD for inter-bank secure transactions — pilot with IDRBT (Hyderabad). DRDO — QKD for military communications — tri-services quantum communications testbed. ITU-R — standardisation of satellite QKD frequency bands (2024).

Interlinkages

  • → Cybersecurity (GS-III): QKD as solution to quantum threat to encryption — complements classical security
  • → Defence (GS-III): Military communications — DRDO QKD for secure tri-service networks
  • → Space (GS-III): Satellite QKD — global secure network — ISRO's role in quantum communications
  • → Banking (GS-III): RBI/IDRBT — quantum-safe banking — financial security
  • → IT Act (GS-II): Encryption laws — traceability vs unbreakable QKD — surveillance implications
  • → National Security (GS-III): Quantum as strategic technology — India-China competition in quantum
  • → Telecom (GS-III): Fibre optic QKD — BharatNet integration for secure rural communications

Common Mistakes

MistakeCorrection
"QKD is a form of quantum computing"QKD is a quantum communication protocol — it uses quantum effects to distribute cryptographic keys — it does not perform computation
"QKD can be used to send secret messages directly"QKD distributes keys (random bits) — not messages — the key is then used with classical encryption (e.g., AES) to encrypt messages
"Quantum computers will make all existing encryption obsolete"Only public-key cryptography (RSA, ECC) is vulnerable to Shor's algorithm — symmetric encryption (AES) is only partially affected (Grover's algorithm — doubling key size provides security)
"QKD is a mature, widely deployed technology"QKD is still emerging — limited to niche use cases (banking, defence, government) due to distance limitations, cost, and hardware requirements
"India leads the world in satellite QKD"China leads with the Micius satellite (2016) and subsequent operational quantum network — India's QKD is at the laboratory and demonstration stage

Revision Snapshot

Quantum Key Distribution (QKD)
├── Physics Basis:
│   ├── No-Cloning Theorem — cannot copy unknown quantum states
│   ├── Heisenberg Uncertainty — measurement disturbs the system
│   └── Entanglement — Bell's inequality for E91 protocol
├── BB84 Protocol:
│   ├── Alice → photons in 4 polarisation states (2 bases)
│   ├── Bob → random measurements → sifting → key distillation
│   └── Eavesdropper detected >11% QBER → abort
├── Implementation:
│   ├── Fibre: ~500 km max — 0.2 dB/m attenuation — trusted relays
│   ├── Satellite: 1200 km (Micius) — global QKD possible
│   └── Quantum repeaters (experimental) — unlimited distance
├── India's Progress:
│   ├── ISRO/RRI: 300 km fibre QKD (2023)
│   ├── DRDO: 100 km QKD (2022)
│   └── National Quantum Mission (₹6,003 Cr — 2023-31)
└── Timeline: QKD → Quantum Internet → Global QKD Network

Source Notes

  • National Quantum Mission — DST (2023)
  • ISRO/RRI — QKD over 300 km — Technical Report (2023)
  • DRDO — QKD Demonstration Report (2022)
  • Bennett & Brassard — BB84 Protocol (1984)
  • Ekert — E91 Protocol (1991)
  • NIST — Post-Quantum Cryptography Standards (2024)
  • Micius Satellite — Pan et al. (Nature, 2017)
  • MeitY — Quantum Communications Roadmap (2024)
  • IDRBT — Quantum-Safe Banking Report (2024)
  • ITU-R — QKD Standards (2024)