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Guilhem Dubois Supervisor: Jakob Reichel Atomchips group, Laboratoire Kastler Brossel, ENS Paris Preparation, manipulation and detection of single atoms.

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Presentation on theme: "Guilhem Dubois Supervisor: Jakob Reichel Atomchips group, Laboratoire Kastler Brossel, ENS Paris Preparation, manipulation and detection of single atoms."— Presentation transcript:

1 Guilhem Dubois Supervisor: Jakob Reichel Atomchips group, Laboratoire Kastler Brossel, ENS Paris Preparation, manipulation and detection of single atoms on a chip

2 Single atoms : remarkable features Well-controlled system! Testbed for Quantum Mechanics Qubit candidate? Cooling & trapping a b T coh > 10s

3 Outline Introduction: experiments with single atoms Cavity QED and single atom detection Experimental setup Detection of waveguided atoms Preparation and detection of trapped single atoms Detection with minimum backaction Quantum Zeno effect

4 Single atoms toolbox 1.Preparation 2.Interaction 3.Detection

5 Single atoms toolbox 1.Preparation 2.Interaction with … 3.Detection light fields (in free space, in a cavity) atom-photon entanglement [Volz et al. PRL 96 (2006)] non-classical states of light - Fock states [Deleglise Nature 455 (2008)] - polarisation-entangled photons [Wilk Science 317 (2007)] another single atom (atom-atom entanglement) controlled collisions [Mandel et al. Nature 425 (2003)] Rydberg blockade [Gaëtan et al. Nat. Phys. 5 (2009)]

6 Single atoms toolbox 1.Preparation : constraints deterministic specific internal state e.g. clock states specific motional state e.g. trap ground state 2.Interaction 3.Detection

7 Single atoms toolbox 1.Preparation : feedback deterministic specific internal state e.g. clock states specific motional state e.g. trap ground state 2.Interaction 3.Detection : here atom counting minimum backaction (spontaneous emission) How can we achieve that ?

8 Outline Introduction Cavity QED and single atom detection Experimental setup Detection of waveguided atoms Preparation and detection of trapped single atoms Detection with minimum backaction Quantum Zeno effect

9 Atom-cavity system Strong coupling regime : g >> small mode volume good quality mirrors e b optical cavity atom coupling g

10 Cavity QED experiments single atom - single photon interaction Evidence of field quantisation & photon counter Brune et al. PRL 76 (1996) Quantum light sources Hijlkema PhD thesis (2007) Detection of single atoms Oettl et al. PRL 95 (2005)

11 Resonant Jaynes-Cummings spectrum g,1 b,0 e,0 energy b,1 b,0 +,1 energy coupling g -,1 splitting 2g Interaction single atom - single photon visible! e b

12 Principle of single atom detection in a cavity 1.Optimum measurement rate 1 measurement = 1 photon 2.With losses L : ¡ signal = L £ ¡ inc

13 Detection with minimum backaction? Backaction characterized by sp For a free space detector: factor C !

14 Outline Introduction Cavity QED and single atom detection Experimental setup Detection of waveguided atoms Preparation and detection of trapped single atoms Detection with minimum backaction Quantum Zeno effect

15 AutoCADs view Integrated atom chip-cavity system

16 Atom chip basics 1cm Applications: - BEC - precise transport and positioning - atomic clocks and interferometers - single atom manipulation? Magnetic traps: - versatility - strong confinement close to the surface

17 Miniaturized Fabry-Perot cavity

18 finesse F = 38000 coupling g /2 = 160 MHz cavity decay / 2 = 50 MHz atomic decay / 2 = 3 MHz cooperativity C = g 2 /2 = 85 Cavity QED Strong coupling regime! - tunable - small mode volume w 0 =4 m ; d=39 m - integrated 150 m from chip surface

19 Outline Introduction Cavity QED and single atom detection Experimental setup Detection of waveguided atoms Preparation and detection of trapped single atoms Detection with minimum backaction Quantum Zeno effect

20 Detection of waveguided atoms Principle LASER APD Atomic waveguide Detection zone a BEC … the easiest way to put SINGLE atoms in the cavity

21 Detection of waveguided atoms Reference with no atoms

22 Detection of waveguided atoms Single run with atoms

23 Detection of waveguided atoms Experiment Threshold these are single atoms !!!

24 Outline Introduction Cavity QED and single atom detection Experimental setup Detection of waveguided atoms Preparation and detection of trapped single atoms Detection with minimum backaction Quantum Zeno effect

25 Trapping & detecting the atoms in the cavity mode Transfer magnetic trap Optical dipole trap @ 830nm Experiments with BEC : see Colombe et al. Nature 450 (2007)

26 Positioning the BEC in the cavity input fibre output fibre Y Dipole trap @ 830nm BEC in magnetic trap N ~ a few 1000s Probe light @ 780nm Initial cloud size ~1 m single-site loading possible.

27 Vacuum Rabi Splitting with collective enhancement Laser detuning Δ L-A [GHz] Y. Colombe, T. Steinmetz, G. Dubois, F. Linke, D. Hunger and J.Reichel Nature 450 (2007) How to get to the single atom regime?

28 From the BEC to just a single atom Problem: Evaporation down to N=1 not possible. Solution: Extract a single F=2 atom from a reservoir of F=1 atoms – and detect it. F'=0,1,2,3 Cavity tuned to F=2 -> F=3 transition F=2 F=1 Reservoir (N~10) Weak MW pulse (@6.8 GHz) ~2% transfer probability/atom

29 Usual strategy to obtain trapped single atoms First trapped cavity QED experiments (Caltech, Garching) Problem: the atom is hot - cooling required (Raman sideband cooling, cavity cooling) Possible improvement: optical conveyor belt (Bonn, Zurich) We do differently! We aim at direct preparation in the trap ground state Analogy with our scheme : position internal state. dip ! Wait and trap scheme:

30 Preparation and detection iterative sequence time F=2 F=1 1000 ~10 mw Detection mw Detection Etc … Reservoir preparation F=3

31 0 or 1 atom in F=2? n APD ~ 25n APD < 1

32 Analysis of detection pulses successful transfers (~10%) unsuccessful transfers (~90%) Transfer efficiency 10% Relative transmission 1.4% =0.35 =25 threshold after ~10 pulses Reliable preparation

33 Lifetime of the atoms during detection or ?? single run

34 Lifetime of the atoms during detection Average lifetime 1.2 ms Limited by depumping to F=1 or ?? Fit Fidelity=99.7% + QND measurement stat. limit depump limit

35 Outline Introduction Cavity QED and single atom detection Experimental setup Detection of waveguided atoms Preparation and detection of trapped single atoms Detection with minimum backaction Quantum Zeno effect

36 How can we measure spontaneous emission? Zeeman random walk: But not visible in lifetime !

37 Measurement and preparation of a specific Zeeman state (F=2;m F =0) B Measurement of m F

38 Diffusion in the Zeeman manifold Fit

39 Detection figure of merit : backaction Better than a perfect free space detection ! Possible to prepare a single atom without changing the motional state !

40 Detection without perturbation ? with L ~ 0.1 : C ~ 20 expected value C ~ 85 ??? What is the real measurement rate of the system? for a lossless observer ¡ m = ¡ inc = C ¡ sp can we check that ???

41 Outline Introduction Cavity QED and single atom detection Experimental setup Detection of waveguided atoms Preparation and detection of trapped single atoms Detection with minimum backaction Quantum Zeno effect

42 Quantum Zeno Effect m = Coherence decay rate between a and b mw Cavity & atomic excited state F=2;m F =0 F=1;m F =0 m = Photon input rate ~ 20 £ Spontaneous emission rate b a

43 Summary Preparation of trapped single atoms starting from a BEC: preparation in a specific Zeeman state qubit clock states well localized within the cavity First detector of single atoms on a chip ability to distinguish F=1 from F=2 states with 99.7% fidelity Demonstrated a Quantum Zeno effect w/o spontaneous emission.

44 Outlook Characterize the atomic motional state are we still in the ground state? Manipulate of pairs of atoms in the cavity Cavity-assisted entanglement generation Combine with other atom chip technology (state dependent mw potentials) Quantum memory with BEC and Fiber-cavity - Large collection efficiency - Long storage time laser cavity a b e

45 Single atom Vacuum Rabi splitting

46 Atomchip-based single atom detectors Fluorescence (Wilzbach et al. 0801.3255) Photoionization (Stibor et al PRA 76 (2007)) Cavity QED (Purdy et al. APB 90 (2008)) 1 2 3

47 Single atoms – light/matter interface Single photon source Atom-photon entanglement Photon-photon entanglement Long-distance atom-atom entanglement via entanglement swapping Quantum networks for quantum cryptography laser vacuum a b e - Probabilistic is OK (DLCZ 2002) atomic ensembles possible but coherence time ~ms. - Collection efficiency small with single atoms a cavity helps

48 Single atom temperature Release and recapture Mean energy < 100 K (trap depth 2.6 mK)

49 Single atom Rabi oscillations

50 Single atoms : some fascinating achievements Beugnon et al. Nature 440 (2006) Hong-Ou-Mandel effect Evidence of field quantisation & photon counting Brune et al. PRL 76 (1996) Massive multi-particle entanglement Mandel et al. Nature 425 (2003)

51 Single atoms toolbox Preparation & trapping 1-qubit gates 2-qubit gates State readout Requirements: - state dependent potentials - preparation in the trap ground state Scheme : controlled collisions Theory: Calarco et al., PRA 61 (2000) Experiment: Mandel et al. Nature 425 (2003) Böhi et al. preprint arXiv 0904.4837 Entangle atomic internal and external state

52 Single atoms toolbox Preparation & trapping 1-qubit gates 2-qubit gates State readout Requirements: - preparation of Rydberg states - small distance (<5 m) between atoms Scheme : Rydberg gate Theory: Jaksch et al. PRL 85 (2000) Experiment: Wilk et al. preprint arXiv:0908.0454 a r b d 1.d 2

53 Single atoms toolbox Preparation & trapping 1-qubit gates 2-qubit gates State readout Requirements: - optical cavity, strong coupling regime - good control over the coupling g Scheme : cavity-mediated interaction ea aa aa+1 photon ba You et al. PRA 67 (2003) g g aa ab ae

54 Single atoms toolbox Preparation & trapping 1-qubit gates 2-qubit gates State readout : need a cavity to enhance light/matter coupling and avoid spontaneous emission e b a For free space detection Signal = Spontaneous emission heating & depumping Non-destructive measurement? - Not necessary in principle - but very useful for preparation!

55 Detection of waveguided atoms Analysis Spontaneous emission: depumping to untrapped states. Some atoms lost before they reach maximum coupling Still: Demonstrates >50% efficiency single atom detection (absorption imaging, simulations) But: trapped atoms in the strong coupling region should lead to better results


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