Quantum Memory with Atomic Ensembles

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1 Lecture Note 5 Quantum Memory with Atomic Ensembles

2 Difficulties in Long-distance Quantum Communication Problems leads Solutions Absorption (exponentially) Decoherence Photon loss Degrading entanglement quality Entanglement Swapping Entanglement Purification Synchronization of independent lasers

3 Drawbacks of The former QC Experiments Drawback Feasible solution Probabilistic entangled photon source Quantum memory cost of resource cost of resource

4 Novel Solution with Atomic Ensembles! Storage of light in atomic ensembles [C. Liu et al., Nature 409, 490 (2001);] [D. F. Phillips et al., Phys. Rev. Lett. 86, 783 (2001)] motivate Long-distance quantum communication with atomic ensembles and linear optics [L.-M. Duan et al., Nature 414, 413 (2001)]

5 Three level atoms: medium of quantum memory a> b>

6 DLCZ protocol Optically dense Atomic Ensemble: N atoms with Lambda System 1 b = 1 N b i a 0 i

7 Atomic Ensemble: Magneto-Optical Trap Step 1: State preparation Cold 87 Rb- Atoms in MOT: Number: >10 8 Density: ~ /cm 3 Temperature :~ 100 μk Optical Density: ~3 Size: ~ 3 mm

8 Basic Experimental Sequence Momentum conservation Step 2: Anti-Stokes Photon Storage time T Step 3: Stokes Photon

9 Non-classical photon pair Generation Cross-correlation is used to show quantum correlation between the single-photon pair

10 Quantum Memory: Experimental results Cross correlation g (2) AS,S of anti-stokes and Stokes photon VS the detected probability of anti-stokes photon p AS g (2) AS,S > 2 => nonclassical light Lifetime measurement of the quantum memory. Due to the dephasing of the collective spin state, the life time is determined to be 13 s

11 A Conditional Single Photon Source Enhance probabilistic process by application of multiple Write pulses => Read-out becomes deterministic

12 Deterministic and Storable Single-photon Source Clean Write

13 Single photon quality single photon quality is determined by anti-correlation

14 Deterministic and Storable Single-photon Source Anti-correlation a of the single photon VS anti-stokes photon production rate p 1. Anti-correlation a of the single photon VS storage time dt. [S. Chen et al., Phys. Rev. Lett ]

15 2/p 2 => 2/p

16 Time Sequence Cleaning Write Cleaning & Write Pulse Cleaning Write Read Pulse On Click in Both Successful Write process

17 Efficient Generation of Entanglement Predicts S=2.3 S=2.37±0.07>2 The probability of generation of entanglement is enhanced by 2 order with the help of feedback circuit. [Z.-S. Yuan et al PRL , (2007)]

18 Problem in DLCZ -- Mach-Zehnder-type interference needed Entanglement in DLCZ: ab = 0 a,1 b ± 1 a,0 b Require: time jitter at a subfemtosecond level over a time scale of up to hour. Realistic: fs for transferring a timing signal over 32 km for averaging times of 1 second [PRL 99, (2007)] Experimentally Forbidden!! [Z.-B. Chen et al., Phys. Rev. A (2007)]

19 Robust Quantum Repeater --Hong-Ou-Mandel-type interference is used [B. Zhao et al., Phys. Rev. Lett (2007)]

20 Atom-Photon Entanglement H = 0 B U 1 B D V = 1 B U 0 B D ( ) = 1 ( 2 H V + e i 1 V H ) 1 2 H V + ( AS S ei ) V AS H S

21 Indistinguishability In distinguishable Photons 2-photon interference V = 1, = Predicts V=87% V=(82±3)% V=(80±1)% Stokes:[Z.-S. Yuan et al PRL , (2007)] anti-stokes:[t. Chaneliere et al PRL ]]

22 Phase Locking

23 Quality of the Entanglement MOT Lock beam MOT on MOT off Phase stability after lock Short term fluctuation: /30 Long term drift: cancelled Entanglemant signal to noise ratio, excitation rate of 3

24 Teleport a photonic qubit to atomic qubit

25 Memory-built-in Teleportation Fidelities and Storage [Y.-A. Chen et. al, Nature Physics 4, 103 (2008)]

26 A Novel Entanglement

27 A novel entanglement Momentum conservation Entangled state = 1 2 H R + ei 1 ( V L ) Entanglement verify = 1 ( 2 H H + ( AS S ei ) V AS V ) S

28 Characterization of the novel Atom-Photon entanglement source Visibility of the entanglement entanglement storage [S. Chen et. al, Phys. Rev. Lett 99, (2007)]

29 Entangling two Remote Atomic Qubits

30 Swapping result Bell inequality in 500 ns S=2.26±0.07

31 Entanglement generation via 300 m optical channel W = ( 0.33 ± 0.02) < 0 F = Tr + ( + ) I,II = 0.83 ± 0.02 [Z.-S. Yuan et al., Nature, under review (2008); preprint available at

32 Extending the Lifetime by Clock State Dephasing: residual magnetic field Solution: magnetic field insensitive state Clock state F = 2 87 Rb D1 line F= 2 clock states long coherence times 2s! F= 1 m f = Physikalisches Insitut

33 Extending the Lifetime by Clock State Lifetime limited by loss of atoms Even longer lifetime requires colder atoms c = (1.0 ± 0.1)ms [B. Zhao et al., Nature Physics, under review (2008) Physikalisches Insitut

35 1. Long lifetime high retrieve efficiency quantum memory m f =-1,F=1> & m f =1,F=2> 3.23G Prevent atom motion Trap atoms in photonic band gap hollow core fiber Trap atoms in optical lattices Physikalisches Insitut

36 2. long-distance quantum teleportation of atomic qubits To Verify Physikalisches Insitut

37 3. Quantum computation & quantum simulation Efficient and deterministic generation of single photons & entanglement via feedback circuit Generation of cluster state One-way quantum computing Quantum simulation Physikalisches Insitut

38 4. satellite-based quantum communication Quantum teleportation Classical channel BSM Quantum channel Quantum memory Telescope UT A-P Entanglement Physikalisches Insitut

39 Photons> + Atoms> Powerful Quantum Superposition Brilliant Future in Quantum Communication!

Quantum Networks with Atomic Ensembles

Quantum Networks with Atomic Ensembles Daniel Felinto* dfelinto@df.ufpe.br C.W. Chou, H. Deng, K.S. Choi, H. de Riedmatten, J. Laurat, S. van Enk, H.J. Kimble Caltech Quantum Optics *Presently at Departamento