Quantum Cryptography Kvantekryptering

Transcription

1 Lecture in "Fiberkomponenter" course, November 13, 2003 NTNU Quantum Cryptography Kvantekryptering Vadim Makarov

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

3 Qubit: polarization state of a single photon Measure? Measure? 50% 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 NTNU The only cipher mathematically proven Requires massive amounts of key material (key of same length as message, used only once) m k c m k

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

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

8 NTNU Sender 50% 50% 50% 50% 50% 50% Eavesdropper Receiver "0" "1" "1" "0" Sender "0" "0" "1" "0" "1" "1" "0" "1" "0" "1" "0" "1" Tyvlytter Resultat av måling Mottaker Referanse Eavesdropping with wrong reference system

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

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

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

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

13 NTNU 20 GHz travelling-wave phase modulator, 1300 nm (manufacturer: Uniphase)

14 Results of electrical breakdown in the waveguide (microphotograph)

15 Real-time phase tracking in 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 Single-photon detector: APD in Geiger mode Gate Pulse Generator Bias -V APD t gate Transmission Lines, Z=50Ω V B V E C = C APD APD Inside Cryostat V bias T=1/(gate pulse rate) Differential Amplifier t t gate down to 1ns gate pulse rate = 20 MHz Epitaxx APD

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 Eve s information Bob s information QBER limit: Individual attacks: 15% All theoretically possible attacks: 11%(?)

20 Maximum link distance, km 70 Distance limitation λ, nm Fiber attenuation, db/km Detectors Si, room temperature Ge, -196 C InGaAs, 60 C nm 1300 nm nm 0 0 5E-5 Few % Detector noise level (dark count probability)

21 1 Components of security Alice Bob 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 NTNU Alice Phase Modulator Alice's PC Attenuator 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 Eavesdropping experiment Alice NTNU 4% reflection Phase Modulator Laser V mod Eve Out L 1 OTDR In Variable attenuator Received OTDR pulse L 2 Fine length adjustment to get L 1 = L V mod, 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 in NTNU fiber-optic network Multi-user key distribution Bob 1 Passive splitter Alice Bob 2 Multiplexing with telecom traffic Bob nm Alice WDM 28 km WDM Bob Data transmitter 1550 nm 1.2 Gbit/s Data receiver

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

29 Faglærere: Dag R. Hjelme, Astrid Dyrseth NTNU Diploma Thesis Available!