Quantum Cryptography Post Tenebras Lux! Grégoire Ribordy Changer Logo  A Quantum Leap for Cryptography Logo en bas ?

Outline Introduction: Cryptography Quantum Information Processing Quantum cryptography protocole Practical system Applications and outlook

Cryptography Eve Bob Alice Key transmission Document Document Secure communication Bob Key Key Decryption Encryption Alice Key transmission Logo en bas: sans slogan; slogan à droite Key generation

Secret key cryptography Encryption and decryption key identical Problem: Key exchange The longer the key, the higher the security One-time pad

Public key cryptography Encryption key  Decryption key (public) (private) One way function Key distribution problem solved? Caution: vulnerable Mathematical progress: Security is based on mathematical assumptions Technological progress: Computers become more powerful 13  31 = 403

Computational complexity Complexity of a computer program… P(n) = n + n2 + n3+…+nk E(n) = exp (n) = n + n2 + n3 + n4 … + n In cryptography

Another look at Moore’s Law Trend of computer chip development  hit a « quantum wall » Potential of quantum physics not yet exploited in industrial applications

Quantum Physics and Cryptography Public key cryptography cracking Eve Document Document Secure communication Bob Key Key Decryption Encryption Alice Key transmission Logo en bas: sans slogan; slogan à droite Key generation

Quantum Random Number Generator Physical randomness source Commercially available Applications Cryptography Numerical simulations Statistics Concurrence: logiciel QRNG (actuellement – 2e gén – application: SSL – contact avec fabriquant cartes crypto)

Quantum Physics and Cryptography Public key cryptography cracking Eve Document Document Secure communication Bob Key Key Decryption Encryption Alice Key transmission Logo en bas: sans slogan; slogan à droite Key generation

Classical vs quantum communications Communication System Secure channel over dedicated optical fiber Absolute security guaranteed by the laws of quantum physics "0" "1" Fragile ! Mentionner Heisenberg Limitation de la distance

Quantum communications Transmitting information with a single-photon Light Polarization Linear States = "0" = "1"

Eavesdropping (1) A single-photon constitutes an elementary quantum system It cannot be split Semi-transparent mirror 50%

Eavesdropping (2) Communication interception Use quantum physics to force spy to introduce errors in the communication Alice Bob "0" "0" Eve

Polarization measurement Using polarizing filters to measure polarization states and and probabilistic modification But and ? Heisenberg’s Uncertainty Relations = "0" = "0" Base 1 Base 2 = "1" = "1"

Quantum Cryptography Protocole BB84 A better name: Quantum Key Distribution

Key Distillation (ideal case) Transmission Qubits Alice Bob Quantum channel Sifted key Reconciliation Basis QBER estimate 0 : no eavesdropping Reveals rather than prevents eavesdropping A better name: quantum key distribution QBER = > 0 : eavesdropping

Key Distillation (realistic case) Transmission Qubits Alice Bob Quantum channel (losses) Raw key Public channel Reconciliation Basis Sifted key QBER estimate correction Error amplification Privacy Key Key

Implementing the quantum channel Necessary components Channel Single-Photon Source Single-Photon Detector

Quantum Cryptography System Collaboration: id Quantique – UniGe Pilot tests in 2003

Field tests Optical fibers Distance: 67 km Genève – Lausanne

Deployment Computer network A Computer network B Optical Fiber (classical channel) Optical Fiber (quantum channel) QKD Hardware QKD Hardware Traffic Network A to B Encrypted traffic Encryption Main features Encryption Transparent High-bit rate (1 Gbit/s) Remote monitoring Automated key management Classical channel Decryption Encrypted traffic Traffic Network B to A Quantum channel Key exchange

Applications Advantages Constraints Automated key management Long term security Constraints Optical fiber Distance < 100 km High-security applications in a metropolitan area network Financial sector, e-government Storage, disaster recovery

Extending the distance Secure relays Improved components Photon counting detectors Photonic crystal fibers: 0.2 dB/km  0.02 dB/km Quantum repeater Free-space links to satellites

Quantum Repeater Quantum Teleportation Rudimentary quantum repeater Quantum version of a fax Recently at Unige: teleportation of a photon over 2km Rudimentary quantum repeater 01100111010101000110010

Free-space QKD Satellites = secure relay Experiments over horizontal links 23.4 km in the German Alps Tokyo Genève

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