Preparation, manipulation and detection of single atoms on a chip Guilhem Dubois Supervisor: Jakob Reichel Atomchips group, Laboratoire Kastler Brossel, ENS Paris.
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Preparation, manipulation and detection of single atoms on a chip Guilhem Dubois Supervisor: Jakob Reichel Atomchips group, Laboratoire Kastler Brossel, ENS Paris Single atoms : remarkable features b a • Well-controlled system! • Testbed for Quantum Mechanics Cooling & trapping • Qubit candidate? Tcoh > 10s 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 Single atoms toolbox 1. Preparation 2. Interaction 3. Detection Single atoms toolbox 1. Preparation 2. Interaction with … light fields (in free space, in a cavity) 3. Detection another single atom (atom-atom entanglement) atom-photon entanglement [Volz et al. PRL 96 (2006)] controlled collisions [Mandel et al. Nature 425 (2003)] Rydberg blockade [Gaëtan et al. Nat. Phys. 5 (2009)] non-classical states of light - Fock states [Deleglise Nature 455 (2008)] - polarisation-entangled photons [Wilk Science 317 (2007)] 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 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 ? 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 Atom-cavity system atom g e b coupling g Strong coupling regime : g >> k , g small mode volume good quality mirrors optical cavity k Cavity QED experiments single atom - single photon interaction Detection of single atoms Evidence of field quantisation & photon counter Quantum light sources Brune et al. PRL 76 (1996) Hijlkema PhD thesis (2007) Oettl et al. PRL 95 (2005) b e,0 b,0 g,1 b,1 +,1 coupling g energy e energy Resonant Jaynes-Cummings spectrum splitting 2g -,1 b,0 Interaction single atom - single photon visible! Principle of single atom detection in a cavity 1. Optimum measurement rate 1 measurement = 1 photon 2. With losses L : ¡signal = L £ ¡inc Detection with minimum backaction? Backaction characterized by Gsp For a free space detector: factor C ! 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 AutoCAD’s view Integrated atom chip-cavity system 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 Miniaturized Fabry-Perot cavity Miniaturized Fabry-Perot cavity - tunable - small mode volume w0=4 mm ; d=39 mm - integrated 150mm from chip surface Cavity QED Strong coupling regime! finesse coupling F = 38000 g /2p = 160 MHz cavity decay k /2p = 50 MHz atomic decay g /2 p = 3 MHz cooperativity C = g2/2kg = 85 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 Detection of waveguided atoms Principle APD BEC Atomic waveguide a Detection zone LASER … the easiest way to put SINGLE atoms in the cavity Detection of waveguided atoms Reference with no atoms Detection of waveguided atoms Single run with atoms Detection of waveguided atoms Experiment Threshold these are single atoms !!! 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 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) Positioning the BEC in the cavity BEC in magnetic trap N ~ a few 1000s input fibre Dipole trap @ 830nm output fibre • Initial cloud size ~1mm single-site loading possible. Probe light @ 780nm Y Laser detuning ΔL-A [GHz] Vacuum Rabi Splitting with collective enhancement How to get to the single atom regime? Y. Colombe, T. Steinmetz, G. Dubois, F. Linke, D. Hunger and J.Reichel Nature 450 (2007) 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 Weak MW pulse (@6.8 GHz) ~2% transfer probability/atom F=1 Reservoir (N~10) Usual strategy to obtain trapped single atoms “Wait and trap” scheme: dip ! • 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. “Preparation and detection” iterative sequence Detection mw Detection Reservoir preparation mw time Etc … F’=3 F=2 F=1 1000 ~10 0 or 1 atom in F=2? nAPD ~ 25 nAPD < 1 Analysis of detection pulses <n>=25 threshold <n>=0.35 successful transfers (~10%) • Transfer efficiency 10% • Relative transmission 1.4% unsuccessful transfers (~90%) after ~10 pulses Reliable preparation Lifetime of the atoms during detection single run or ?? Lifetime of the atoms during detection or ?? Fit • Average lifetime 1.2 ms • Limited by depumping to F=1 stat. limit depump limit Fidelity=99.7% + QND measurement 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 How can we measure spontaneous emission? Zeeman “random walk”: But not visible in lifetime ! Measurement and preparation of a specific Zeeman state (F=2;mF=0) Measurement of mF p p B p Diffusion in the Zeeman manifold Fit Detection figure of merit : backaction Better than a perfect free space detection ! Possible to prepare a single atom without changing the motional state ! 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 ??? 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 Quantum Zeno Effect Cavity & atomic excited state F=2;mF=0 b mw p F=1;mF=0 a Gm = Coherence decay rate between a and b Gm = Photon input rate ~ 20 £ Spontaneous emission rate 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. 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 e - Long storage time a b Single atom Vacuum Rabi splitting Atomchip-based single atom detectors 1. Fluorescence (Wilzbach et al. 0801.3255) 2. Photoionization (Stibor et al PRA 76 (2007)) 3. Cavity QED (Purdy et al. APB 90 (2008)) 1 2 3 Single atoms – light/matter interface e • Single photon source • Atom-photon entanglement • Photon-photon entanglement • Long-distance atom-atom entanglement via entanglement swapping Quantum networks for quantum cryptography - Probabilistic is OK (DLCZ 2002) atomic ensembles possible but coherence time ~ms. - Collection efficiency small with single atoms a cavity helps a b Single atom ‘temperature‘ Release and recapture Mean energy < 100 mK (trap depth 2.6 mK) Single atom Rabi oscillations Transfer probability 1 0.8 0.6 0.4 0.2 0 0 5 10 15 MW pulse duration [ms] 20 Single atoms : some fascinating achievements Hong-Ou-Mandel effect Evidence of field quantisation & photon counting Beugnon et al. Nature 440 (2006) Massive multi-particle entanglement Brune et al. PRL 76 (1996) Mandel et al. Nature 425 (2003) Single atoms toolbox • Preparation & trapping Scheme : controlled collisions • 1-qubit gates • 2-qubit gates • State readout Entangle atomic internal and external state Requirements: - state dependent potentials - preparation in the trap ground state Theory: Calarco et al. , PRA 61 (2000) Experiment: Mandel et al. Nature 425 (2003) Böhi et al. preprint arXiv 0904.4837 Single atoms toolbox • Preparation & trapping Scheme : Rydberg gate • 1-qubit gates r • 2-qubit gates d1.d2 • State readout b a Theory: Jaksch et al. PRL 85 (2000) Experiment: Requirements: Wilk et al. preprint arXiv:0908.0454 - preparation of Rydberg states - small distance (<5mm) between atoms Single atoms toolbox • Preparation & trapping • 1-qubit gates Scheme : cavity-mediated interaction • 2-qubit gates ea ae aa+1 photon • State readout g g ba aa You et al. PRA 67 (2003) Requirements: - optical cavity, strong coupling regime - good control over the coupling g ab aa Single atoms toolbox • Preparation & trapping • For free space detection Signal = Spontaneous emission heating & depumping • 1-qubit gates • 2-qubit gates • State readout : • Non-destructive measurement? - Not necessary in principle - but very useful for preparation! e need a cavity to enhance light/matter coupling and avoid spontaneous emission b a 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