Quantum Cryptography

1. NTNU FoU Norsk kryptoseminar, 17-18. oktober 2002. NTNU, Trondheim Quantum Cryptography Vadim Makarov and Dag R. Hjelme Institutt for fysikalsk elektronikk NTNU www.vad1.com/qcr/

2. Classical vs. quantum information • Classical information Perfect copy Unchanged original • Quantum information Imperfect copy Broken original

3. Qubit: polarization stateof a single photon Measure? 50% Measure? 50%

4. What is the problem with classical cryptography? • Secret key cryptography • Requires secure channel for key distribution • In principle every classical channel can be monitored passively • Security is mostly based on complicated non-proven algorithms • Public key cryptography • Security is based on non-proven mathematical assumptions (e.g. difficulty of factoring large numbers) • We DO know how to factorize in polynomial time! Shor’s algorithm for quantum computers. Just wait until one is built. • Breakthrough renders messages insecure retroactively

5. The holy grail: One-time pad • The only cipher mathematically proven • Requires massive amounts of key material m m c k k

6. Alice Bob Message Message Open (insecure) channel Decoder Encoder Encoded message Key Secure channel Key distribution • Secret key cryptography requires secure channel for key distribution. • Quantum cryptography distributes the key by transmitting quantum states in open channel.

7. NTNU Quantum key distribution Bob Alice Diagonal detector basis Diagonal polarization filters Horizontal-vertical detector basis Horizontal-vertical polarization filters Light source Alice’s bit sequence 1 0 1 1 0 0 1 1 0 0 1 1 1 0 Bob’s detection basis Bob’s measurement 1 0 0 1 0 0 1 1 0 0 0 1 0 0 Retained bit sequence 1 – – 1 0 0 – 1 0 0 – 1 – 0 Image reprinted from article: W. Tittel, G. Ribordy, and N. Gisin, "Quantum cryptography," Physics World, March 1998

8. 50% 50% 50% 50% 50% 50% Sender Eavesdropper Receiver Eavesdropping with wrong reference system

9. Sender (Alice) Receiver (Bob) Transmission line L L 2 D f 1 Source 0 2 S S f 1 2 D 1 1 Interferometric QKD channel  1 = 0 or 90 - "1" Reference systems:  2 = 0  2 = 90  1 = 180 or 270 - "0"

10. Implementation: interferometer structure Alice Variable Ratio PM Coupler Polarization Combiner Variable Delay Line Phase Modulator 1 Polarizer Laser PM fiber Attenuator 1300 nm (or 1550 nm) Pulse Rate = 10 MHz Alice's PC Public Communication Channel Line Standard SM fiber Eve's Territory Bob Bob's PC Phase Modulator 2 Polarization Controller PM Coupler 50/50 APD '0' Polarization Combiner Polarizing Splitter '1' PM fiber

11. Photo 1. Alice (uncovered, no thermoisolation installed)

12. Photo 2. Bob (uncovered, no thermoisolation installed)

13. Gate Pulse Generator -VAPD tgate Bias VE VB Transmission Lines, Z=50 T=1/(gate pulse rate) Vbias C = CAPD APD Inside Cryostat t Differential Amplifier Epitaxx APD Single-photon detector:APD in Geiger mode tgate down to 1ns gate pulse rate = 20 MHz

14. Recovery from errors • Individual attacks: 15% • All theoretically possible attacks: 11% Eve’s information Bob’s information QBER limit:

15. Maximum link distance, km 70 1550 nm 30 20 1300 nm 5 850 nm 0 0 5E-5 Few % Detector noise level (dark count probability) Distance limitation

16. 1 1 2 3 Alice Bob Components of security 1. Conventional security 2. Security against quantum attacks 3. Security against Trojan horse attacks - ones that don’t deal with quantum states, but use loopholes in optical scheme

17. Practical security: large pulse attack Alice Phase Modulator Attenuator Alice's PC Line Eve’s Equipment - interrogating Alice’s phase modulator with powerful external pulses (can give Eve bit values directly)

18. Received OTDR pulse Eavesdropping experiment Alice 4% reflection Phase Modulator Laser Vmod Eve L1 OTDR Out Variable attenuator In L2 Fine length adjustment to get L1 =L2 0 4.1 8.2 Vmod, V

19. Photo 3. Artem Vakhitov tunes up Eve’s setup

20. Re-keying satellites/Global key distribution network 1.9 km 10 km 23.4 km

21. Quantum key distribution in network • Multi-user key distribution Bob 1 Passive splitter Bob 2 Alice Bob 3 • Multiplexing with telecom traffic 1300 nm 28 km Bob Alice WDM WDM Data transmitter Data receiver 1550 nm 1.2 Gbit/s

22. Entangled photon pairs 1560nm Entangled Photon Pairs Nonlinear Crystal Pump Pulses 780nm To Bob Random state prepared passively Passive Measurement Alice

23. A A A B B B C C C Advanced multi-party protocols:Secret sharing and splitting

24. Commercial status • id Quantique (Geneva) first commercially available quantum key distribution system: • MagiQ Technologies (Boston) • EQUIS project (Heriot-Watt University and Corning; UK) compact integration into standard PCs • + several research groups, telecom/ electronics companies