Quantum Communication with Atomic Ensembles

Transcription

1 Quantum Communication with Atomic Ensembles Julien Laurat C.W. Chou, H. Deng, K.S. Choi, H. de Riedmatten, D. Felinto, H.J. Kimble Caltech Quantum Optics FRISNO 2007, February 12, 2007

2 Entanglement : Paradox to Applications A B Product states Entanglement : Non classical correlations between distant systems Entangled states.... EPR correlations EPR correlations. Cryptography. Classical communication Teleportation

3 At Long Distances Problem : Decoherence Result : Entanglement decays exponentially with the distance Solution : Repeater Caltech. Les Houches Fibre Optique Telecom Repeater A quantum state can t be amplified

4 At Long Distances Problem : Decoherence Result : Entanglement decays exponentially with the distance Solution : Repeater Caltech. Les Houches. Quantum Repeater. Goal : Connect with a fidelity close to 1 in a not too long time

5 Quantum Repeaters : Principles 1) Divide into segments and. generate.. entanglement... L 0 L 0 L 0 L F< F~1 2) Purify the entanglement Fidelity close to 1, long distance But time exponentially large with the distance Entanglement (often) and purification (always) are probabilistic : each step ends at different times ) Connect the pairs

6 Quantum Repeaters : Principles 1) Divide into segments and. generate.. entanglement... L 0 L 0 L 0 L F< F~1 2) Purify the entanglement ) Connect the pairs Fidelity close to 1, long distance But time exponentially large with the distance Entanglement (often) and purification (always) are probabilistic : each step ends at different times. «Scalability» : requires the storage of the entanglement : Quantum Memories

7 Quantum Network Quantum node generate, process, store quantum information Requirement : Atom-light interface CavityQED Quantum channel transport / distribute quantum entanglement Ideal system but difficult to scale up. Single atom Atomic Ensembles Quantum networks enabled by distributed entanglement Continuous Variables Single Excitation

8 «DLCZ»

9 Outline «DLCZ building block» : writing, reading, memory time Entanglement between two ensembles Real-time Conditional Control of Quantum Simultaneous storage of two excitations in Memories two remote ensembles Polarization entanglement between two nodes

10 «Building Block» (DLCZ) Large ensemble of atoms Witha Λ-type level configuration Duan, Lukin, Cirac and Zoller, Long-distance quantum communication with atomic ensembles and linear optics, Nature 414, 413 (2001)

11 Creating a Single Atomic Excitation Nonclassical correlations between field 1 and the ensemble Write Field 1 : the excitation probability Write Field 1 Collective atomic state

12 Retrieving the Single Excitation Nonclassical correlations between field 1 and the ensemble Field 2 Read read Read Field 2 Nonclassical correlations between fields 1 and 2

13 Experimental Setup Counter-propagating and off-axis configuration H Field 2 Read V Write H Field 1 V Si APD 30 ns, Very weak 200 µm

14 Conditional Field-2? Field 2 Read q c Suppression of the two-photon component Retrieval efficiency of the stored excitation Multi-excitations Coherent state limit Plateau : Single excitation Sub-Poissonian q c ~ 50% α = 0.7 ± 0.3% Background noise J. Laurat et al., Efficient retrieval of a single excitation stored in an atomic ensemble, Opt. Express 14, 6912 (2006)

15 Storage Time Writing Reading Write Field 1 Field 2 Read Programmable Delay Around 10µs H. De Riedmatten, J. Laurat, C.W. Chou, E.W. Schomburg, D. Felinto H.J. Kimble, Direct measurement of decoherence for entanglement between a photon and a stored excitation, PRL 97, (2006) D. Felinto et al., Phys. Rev. A 72, (2005)

16 Outline «DLCZ building block» : writing, reading, memory time Entanglement between two ensembles Simultaneous storage of two excitations in two remote ensembles Polarization entanglement between two nodes

17 Entanglement between Two Ensembles Atoms entangled Light 50/50 Beam splitter Light Atoms entangled

18 Entanglement between Two Ensembles 1 photon detected 1 atom transferred 50/50 Beam splitter

19 How to Verify the Entanglement? L 2 L Tomography ρ L, R entangled? atoms L ρ 2,2 L R Map matter state to field state R atoms R 2 R où 2 L 2 L Coherence d Individual statistics p ij 2 R 2 R Concurrence / C > 0 Entanglement of formation E > 0 W. K. Wootters, Phys. Rev. Lett. 80, 2245(1998)

20 Experimental Density Matrix Populations 2 L Coherence 2 L 2 R 2 R D1c D1b <1, suppression of 2-photon events relative to single-excitation events

21 Outline «DLCZ building block» : writing, reading, memory time Entanglement between two ensembles Real-Time Conditional Control of Quantum Simultaneous storage of two excitations in Memories two remote ensembles Polarization entanglement between two nodes

22 Simultaneous Storage of Single Excitations in Two Remote Memories Field 1 L Write Write R Field 1 Repumper Repumper p 11 : Probability of both ensembles are prepared with one excitation, heralded by the two field-1 clicks. 44-fold increase in p 11! (N=23, 12µs)

23 First Application : HOM L R Two independent sources of single photons Field 2 Field 2 λ/2 BS V=0.77±0.06 (Integrated data) 28-fold increase in p 1122! (N=23, 12µs) D. Felinto, C.W. Chou, J. Laurat, H. de Riedmatten, H. Kimble, Conditional control of the quantum states of remote atomic memories for Q. networking, Nature Physics 2, 844 (2006)

24 Outline «DLCZ building block» : writing, reading, memory time Entanglement between two ensembles Simultaneous storage of two excitations in two remote ensembles Polarization entanglement between two nodes

25 How Having one Click on Each Side? Node L 3 m Entangled! Node R ϕ L 2 LU 2 RU ϕ R D La BS 2 LD LU Entangled! RU 2 RD BS D Ra D Lb LD RD D Rb Effective state giving one click on each side

26 Polarization Entanglement Node L 3 m Node R 2 L 2 LU 2 RU 2 R 2 LD LU LD RU RD 2 RD Effective state giving one click on each side

27 Experimental Setup Write PBS Repumper LU LU Read BS R BS W RU LD RD LD D 2LV BS 1 D 2RV D 2LH D 2RH λ/2 λ/4 Compensator D 1Va D 1Vb Beam displacer D 1Ha D 1Hb

28 Experimental Setup Interferometers Entangling the (U, D) Pairs Write PBS Repumper BS W LU Read BS R RU LD RD D 2LV BS 1 D 2RV D 2LH D 2RH λ/2 λ/4 Compensator D 1Va D 1Vb Beam displacer D 1Ha D 1Hb

29 Results : Bell Violation Large violation : quantum key distribution with security at minimum against individual attacks Duration that the first entanged pair is stored before retrieval C.W. Chou, J. Laurat, H. Deng, K.S. Choi, H. de Riematten, D. Felinto, H.J. Kimble, Functional Quantum Nodes for Entanglement Distribution over a Scalable Quantum Networks, arxiv\quant-ph

30 2 nodes separated by 3m 2 ensembles per node Asynchronous preparation (memory) of 2 parallel entangled pairs Polarization coding Polarization entanglement distribution, violating Bell, in a scalable fashion

31 In a Nutshell Q. Repeaters, DLCZ et Building Block Photon pair : α<1% Efficient retrieval : 50% Memory time ~ 10 µs Write Writing Field 1 Reading Field 2 Read Entanglement Heralded Without postselection, C~0.1 Conditional Control Asynchronous preparation Polarisation Entanglement 2 nodes, 4 ensembles Bell violation Node L 3m Node R 2 L 2 R LU LD RU RD

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