Recent develpoment in Quantum Communication

1. Recent develpoment inQuantum Communication 周志隆 physics dept. CYCU choucl@cycu.edu.tw

2. Why do people study QIS • computations effectively performed simultaneously (quantum parallelism) • classically intractable problems may become feasible quantum-mechanically • unbreakable shared codes • teleportation

3. Computation QIS Modern electronics Communication

4. Quantum V.S. Classical (1) • In principle, classical states can befaithfully distinguished from each other.

5. Quantum V.S. Classical (2) • Quantum states are non-orthogonal in general. Quantum states cannotbe identified faithfully.

6. =a | 0 > + b | 1 > qubit Quantum V.S. Classical (3) • Quantum states can besuperposed states. The principle of superposition

7. 可能狀態 古典的 | 0 0 > | 0 1 > | 1 0 > | 1 1 > Entangled states： Quantum V.S. Classical (4) • Quantum states can benon-local.

8. Not possible Quantum no-cloning theorem • unknown quantum states cannot be copied due to linearity of QM.

9. Do we need Quantum Communication? (1) Quantum computer can break some of the best public key cryptosystems. (2)Quantum key distributionprovides provably secure distribution of private information. (3)Quantum cryptography is technically viable and affordable.

10. Quantum communication network? However, QC.. • does not provide a complete solution for all cryptographic purposes. authentication rapid delivery of keys robustness distance and location independence resistance to traffic analysis

11. Quantum Communcation Protocols

12. 竊聽者 Bennett & Brassard BB84 protocol 利用光的 2 個偏振方向，代表 “0” 與“1”

13. Bit value “1” BB84 Alice 隨機選擇光的極化 Bob 隨機選擇測量方向 Bob 公告測量方向 Alice 告訴Bob哪些測量 方向選對了。這些方向當作 共同的加解密金匙！

14. Bit survival rates: 50% Key-breaking prob. BB84--example 1 0 0 0

15. Bit values “0” Bennett, PRL 68,3121(1992) B92 protocol Alice prepares a random classical bit“a”, and sends Bob the following quantum state depending on the “a” value.

16. “0”bits (Z basis) (X basis) B92 protocol Bob also prepares a random classical bita’, and measures his quantum bit with one of the following bases according to the value of a’. a’=0 a’=1

17. Bit survival rates:25% Key-breaking prob. for Alice for Bob B92 protocol—example Bob anouces it publicly bases

18. Quantum nature of signals • Signals are non-orthogonal states. • Eavesdropper cannot clone signals • Eavesdropping will inevitably incur disturbance to the signal. qubits. cannot be distinguished with 100% confidence

19. Alice Bob Ekert PRL 67, 661(1991) Ekert scheme (EPR protocol) Entangled pairs of qubits are prepared as EPR states

20. EPR protocol • Randomly select a subset of EPR pairs Test violation of Bell’s inequality The fidelity of the remaining EPR pairs is then inferred from the test. • Alice and Bob obtain correlated classical bit strings------ secret keys

21. Other protocols • Variations of BB84 • Two-state protocols • Six-state protocol Respect the symmetry of the qubit state space Reduce Eve’s info gains

22. Free space Optical fiber QC experiments… Photon source Photon counter Quantum channel

23. InGaAs/InP APD Silicon-base APD < 0.3 dB/km Telecommunication optical fiber l ~ 1300, 1550 nm commercial photon counter low absorption Quantum channels • Single-mode fibers • Free-space links l ~ 800 nm

24. low-loss window Free space links beam-pointing is difficult for moving targets l ~ 770 nm line-of-sight communication Transmission in free space ~ < 10 km

25. Free space links • High transmission window at l ~ 770 nm  compatible w. commercial silicon APD photon-counter • Atmosphere is weakly dispersive and nonbirefirngent at the wavelengths. plain polarization coding is possible. • Energy transmitted spread out in space- higher loss during transmission. • Background lights can couple into the receiver - higher error rate. • It depends on weather conditions.

26. Swiss communication group & idQuantique in Europe MagiQ technology in USA Cambridge research laboratory of Toshiba Research Europe QC via optical fiber ~ 100 km

27. Photon sources • Faint laser pulses • Entangled photon pairs byparametric down conversion or Franson’s method Single-photon Fock states are difficult to realized experimently. coherent states with an untra-low mean photon number semiconductor laser & attenuators

28. Producing entangled photons • Parametric down conversion. • Franson’s method.(using unbalanced Mach-Zender interferometer)

29. Parametric down conversion • PDC is a process that a pump photon in a nonlinear crystal has a small probability of splitting into two photons of lower frequency. Type II: two converted photons have orthogonal polarizations

30. PDC beta-BaB2O4

31. Most pulses are empty decrease in bit rate Faint laser pulses Prob. of finding n photons Pulse contains more than 1 photon mean photon numberm ~ 0.1

32. Photon pairs by PDC • Very inefficient ----- it takes1010 pump photons to create a pair in a given mode. • The system can be made compact and handy. 40 cm × 45 cm ×15 cm

33. Single-photon detectors • Avalanche photodiodes, photomultipliers, Josephson junctions, quantum dots, MODFET • Most experiments used APD’s Silicon APD’s l ~ 800 nm Ge APD’s l ~ 1300 nm InGaAs/InP APD’s l ~ 1500 nm • Some group design photon-detecting device for QC experiment. Toshiba team at Cambridge use MODFET

34. Exp. with faint laser pulses • Polarization coding • Phase coding

35. Polarization coding emits classical pulse Bennett, Bessette, etc.1992 Beamsplitter: base choice 30 cm the pulse attenuated by the filters BB84 four-state protocol

36. Polarization coding Drawbacks: (1) Polarization transformation induced by long optical fibers is unstable.  require active alignment of bases. (2) No polarization-maintaining fibers actually maintain polarization.

37. Nyon Geneva Polarization coding Swisscom used optical fibers for QC experiments between Geneva and Nyon , 1996 Transmission distance 23km

38. long+short counts Phase coding time PM fB Bob PM fA Alice LD APD’s Double Mach-Zender implementation ~ 100 km

39. Destructive interfere Phase coding(BB84) Interference

40. Exp with photon pairs • Polarization entanglement • Energy-time entanglement

41. dis/advantage w. photon pairs

42. BBO pumped by argon laser analyzer: simple and efficient Polarization entanglement Both BB84 & Ekert’s scheme were realized with distances less than 1 km.

43. Energy-time-entangled photon pairs yr 2001 BB84 protocol , yr 2001. KNbO3 crystal

44. Active groups worldwide

45. Quantum cryptography in Taiwan? • We can do it, since we have (1) physicistswho know quantum information theory well. (2) experienced researchers who have good knowledge on quantum optics. (3) well-established laboratories of quantum optics. and most of the exp. components are commercial and easy to get.

46. Possiblefirst QC lab in Taiwan 中央大學：徐子民教授，欒丕綱、陳彥宏助理教授, 中原大學：周志隆助理教授 NCTS:徐立義博士 中華電信：蔡一鳴博士 以及 願意投入QC 研究的學者、學生…..

47. The End

48. Key distillation • Classical error correction • Privacy amplification raw key distilled key

49. Producing entangled photons • Parametric down conversion. • Franson’s method.(using unbalanced Mach-Zender interferometer)

50. Parametric down conversion • PDC is a process that a pump photon in a nonlinear crystal has a small probability of splitting into two photons of lower frequency. Type II: two converted photons have orthogonal polarizations