Vadim Makarov vad1/qcr/

1. Quantum CryptographyKvantekryptering Lecture in "Fiberkomponenter" course, November 13, 2003 Vadim Makarov 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. in RSA cipher, 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 (key of same length as message, used only once) 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. 20 GHz travelling-wave phase modulator, 1300 nm(manufacturer: Uniphase)

14. Results of electrical breakdown in the waveguide(microphotograph)

15. Real-time phase trackingin the interferometer • Due to thermal drift, interferometer needs automatic phase adjustment every few seconds. • Phase adjustment takes 0.3 s and requires no additional components, e.g. no variable attenuator.

16. Photo 4. Bob (left) and Alice (right), thermoisolation partially installed

17. 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

18. Timing resolution of photon detection Photon counts due to 75 ps laser pulse FWHM  200 ps Dark counts within 5ns gate

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

20. 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

21. 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

22. 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)

23. Typical values of reflection coefficients for different fiber-optic components (courtesy Opto-Electronics, Inc.)

24. 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

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

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

27. Quantum key distribution infiber-optic 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

28. Commercial status Two small companies trying to sell QKD systems: • id Quantique (Geneva)2002 • MagiQ Technologies (Boston)November 2003 • + several telecom/ electronics companies, research groups keep close eye on commercializing but don't develop a production version yet. Navajo

29. Faglærere: Dag R. Hjelme, Astrid Dyrseth DiplomaThesisAvailable!