Quantum Communication

1. With 1 photon: Q cryptography With 2 photons: Q crypto, Bell tests, qutrits, plasmons With 3 photons: Q teleportation With 4 photons: entanglement swapping News from the industry forehead Quantum Communication Nicolas Gisin Hugo Zbinden, Ivan Marcikic, Hugues de Riedmatten, Sylvain Fasel, Jeroen van Houwelingen, Rob Thew Group of Applied Physics, University of Geneva

2. Q communication in optical fibres Two problems : Losses and decoherence. How to minimize them ? • The transmission depends on the wavelength - Lower attenuation : 1310 nm (0.3 dB/km) and 1550 nm (0.2dB/km) (telecom wavelengths) • Decoherence due to birefringence : Polarization Mode Dispersion photons at telecom wavelength  Time-bin codingwith

3. Alice Bob 1 1 0 0 j f D 0 n h D 1 switch switch varia ble coupler variable coupler Time-bin qubits • qubit : • any qubit state can be created and measured in any basis Projective measurement State preparation W. Tittel & G. Weihs, Quant. Inf. Comput.1, Number 2, 3 (2001)

4. Time-bin entanglement 0 A 2 km optical fibers 1 A f Variable coupler Laser PDC + f + 1 B 0 • Photon pair creation in a non-linear crystal • Parametric down-conversion (PDC) • Energy and momentum conservation V net =96% After 2x2km of optical fibers B Robust against decoherence in optical fibers ls=1310nm lp=710nm li=1550nm a Detectors & Coincidence b R. Thew et al.,Phys. Rev. A 66, 062304 (2002)

5. 1-photon:Q crypto CDC IF Number of events (normailzed) P(1)=60% P(2)=0.02% g(2)(0)=0.0012 34 KHz Asynchronous heralded single-photon source (quant-ph/0408136) 31 km Results: (PRA 63,012309, 2001 and S. Fasel et al., EJPD 30, 143, 2004) Time between a click at detector A and a click at detector B [# trigger signals] Alice Bob 2-

6. 2-photons: Bell test over 50 km = short path = long path Time arrival on A1 Time arrival on B1 Type I NLC Creating photons @ 1.3 & 1.55 mm deterministic sepation with WDM coupler Bob Alice

7. Stabilisation of the interferometers • Idea: verify from time to time the phase Every 100 s the phase is brought back to a given value

8. Bell test over 50 km Violation of Bell inequalities by more than 15s Marcikic et al., PRL, in press, quant-ph/0404124 • With phase control we can choose four different settings a = 0°or 90°andb = -45° or 45° • Violation of Bell inequalities:

9. 2-photons: Qutrit Entanglement

10. Bell Violation with qutrits I(lhv) = 2 < I(2) = 2.829 < I(3) = 2.872 I = 2.784 +/- 0.023

11. 2-photons: Plasmon assisted entanglement transfer In collaboration with Prof. Erni, Zürich  a short lived phenomenon like a plasmon can be coherently excited at two times that differ by much more than its lifetime. At a macroscopic level this would lead to a “Schrödinger cat” living at two epochs that differ by much more than a cat’s lifetime.

12. 3 photons: Q teleportation & Q relays  2 bits U Bell Ä y EPR Classical channel Charlie Alice Bob 2 km 2 km 2 km BSM EPR source y y J. D. Franson et al, PRA 66,052307,2002 Collins et al., quant-ph/0311101

13. Q repeaters & relays Bell measurement . . * * * * entanglement entanglement ?? entanglement J. D. Franson et al, PRA 66,052307,2002; D. Collins et al., quant-ph/0311101 REPEATER RELAY Bell measurement . entanglement QND measurement + Q memory H. Briegel, W. Dür, J. I. Cirac and P. Zoller, Phys. Rev. Lett. 81, 5932 (1998) entanglement

14. Experimental setup Bob Charlie fs laser @ 710 nm Alice:creation of qubits to be teleported 55 m Alice creation of entangled qubits m m 3 . 1 1 . Charlie:the Bell measurement 3 m m Bob:analysis of the teleported qubit, 55 m from Charlie 2 km of optical fiber r e s coincidence electronics a l s f I. Marcikic et al., Nature, 421, 509-513, 2003 & InGaAs Ge InGaAs BS 2km 2km 2km 1 . 5 m m m WDM m 5 . 1 WDM RG RG LBO LBO sync out H. de Riedmaten et al.,PRL 92, 047904-1/4, 2004

15. results Equatorial states Mean Fidelity = 78 ± 3% mean fidelity: Fpoles=77.5 ± 3 % = 77 ± 3% North & south poles 77.5 ±2.5 % » 67 % (no entanglement) Raw visibility : Vraw= 55 ±5 % = 77.5 ± 2.5 %

16. Experimental setup Bob Charlie fs laser @ 710 nm Alice:creation of qubits to be teleported 55 m Alice creation of entangled qubits m m 3 . 1 1 . Charlie:the Bell measurement 3 m m Bob:analysis of the teleported qubit, 55 m from Charlie On the same spool ! 2 km of optical fiber r e s coincidence electronics a l s f See Halder et al quant-ph/0408092 & InGaAs Ge InGaAs BS 2km 2km 2km 1 . 5 m m m WDM m 5 . 1 WDM RG RG LBO LBO sync out H. de Riedmaten et al.,PRL 92, 047904-1/4, 2004

17. Entangled photons that never interacted Bell state measurement 4-photons: Entanglement swapping EPR source EPR source

18. Actively stabilized interferometers Sources of time-bin entangled photons Bell state measurement Entanglement analysis The experiment 1km 1km d

19. Four Bell states involved in the experiment ! In the experiment :Partial Bell state measurement Entanglement swapping

20. Superposition basis: results V = (80 ±4) % F  90 % 78 hours of measurement !

21. Results: computational basis Mean Fidelity =

22. Yesterday, September 29, id Quantique, DeckPoint and the University of Geneva officially inaugurated the first data archive site secured with Quantum Cryptography. News from the industry forehead http://www.swissquantum.deckpoint.ch/embargo/index_en.php

23. Data archiving network secured by Quantum Crypto  10 km http://www.swissquantum.deckpoint.ch/embargo/index_en.php

24. Quantis: Quantum random numbers on demand (www.randomnumbers.info) 4 Mbit/s per module, up to 4 modules on one PC card www. idquantique.com 1 light source 1 beam splitter 2 photon counters in a few cm3 (4x5x1 cm3) !!!

25. Few « qubit » Applications Coherent Q measurement of the degree of polarization: DOP=1 symbole plein: polarimètre DOP=0.74 (-) y symbole vide: projection sur DOP=0.34 1.0 0.9 0.8 0.7 0.6 DOP 0.5 0.4 0.3 Photon-counting OTDR 0.2 0.1 0.0 0 20 40 60 80 100 120 temps(s) J.Lightwave Tech, 2, 390, 2004 PRL 91, 167902, 2003

26. Conclusions Where are the applications? Next September 29, id Quantique, DeskPoint and the University of Geneva will officially inaugurate the first data archive site secured with Quantum Cryptography.

27. Low pump High pump NV Quantum Dot  /  1550 nm / 7nm 650 nm / >50 nm 919 nm / 0.7 nm NT 34 kHz 803 kHz 5.3 MHz 76 MHz P1 [%] 60 60 2.2 8.3 P2 [%] 0.02 0.3 0.002 0.04 g(2)(0) [%] 0.12 1.4 7 2 Single Photon Sources Beveratos et. al., PRL 89, 187901 (2002) Vuckovic et. al., APL 82, 3596 (2003)Fasel et. al. in preparation

28. Size of the classical communication One proton in one cm3 at a temperature of 300 K:  bits bits 1020 protons in one cm3 at a temperature of 300 K  1020 x 155  1022 bits To be compared to today’s optical fiber communication in labs: 1 Tbyte x 1024 WDW channels x 1000 fibers  1019 bits/sec.   1 hour !!

29. f s A l A variable coupler non-linear crystal l B B s Extensions to entanglement in higher dimensions: - qutrits: R. Thew et al, quant-ph/0307122 - up to dimesnion 20: H. Deriedmatten et al, quant-ph/0309058 depending on coupling ratio and phase f, maximally and non-maximally entangled states can be created R. Thew et al, PRA 66, 062304, 2002 entangled time-bin qubit

30. Bell state measurement H. Weinfurter, Europhysics Letters 25, 559-564 (1994) H. de Riedmatten et al., Phys. Rev. A67, 022301 (2003) B A 2 1