Presentation on theme: "Preparation, manipulation and detection of single atoms on a chip"— Presentation transcript:
1Preparation, manipulation and detection of single atoms on a chip Guilhem DuboisSupervisor: Jakob ReichelAtomchips group, Laboratoire Kastler Brossel, ENS Paris
2Single atoms : remarkable features Tcoh > 10sWell-controlled system!Testbed for Quantum MechanicsQubit candidate?Cooling & trappingsingle atom characterized by a quantum structure of levels, which is perfecly knownif you add the 3 external dof, it is still a simple quant systemStable states+ mw transitionsTc limited by B field inhomogeneityLaser cooling, magnetic and optical trapsScalability: array of magnetic microtraps / optical latticesmanipulate single, identical atoms in parallel.2
3Outline Introduction: experiments with single atoms Cavity QED and single atom detectionExperimental setupDetection of waveguided atomsPreparation and detection of trapped single atomsDetection with minimum backactionQuantum Zeno effect
5Single atoms toolbox Preparation Interaction with … 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)]Terminology of QIP2-qubit -> entanglement5
6Single atoms toolboxPreparation : constraints deterministic specific internal state e.g. clock states specific motional state e.g. trap ground stateInteractionDetectionHere the preparation step is not step not easydifficult to have exactly one atom in a given trapfurthermore constraints depend on the exact experiment you want to achieve, but are generally of two forms : either you want to single out a specific internal state, like « clock states » which have long coherence time, and/or aim for a specific motional state, like the ground state of the trap.6
7Single atoms toolboxPreparation : feedback deterministic specific internal state e.g. clock states specific motional state e.g. trap ground stateInteractionDetection : here atom counting minimum backaction (spontaneous emission)Terminology of QIP2-qubit -> entanglementHow can we achieve that ?7
8Outline Introduction Cavity QED and single atom detection Experimental setupDetection of waveguided atomsPreparation and detection of trapped single atomsDetection with minimum backactionQuantum Zeno effect
9Atom-cavity system e b Strong coupling regime : g >> k , g opticalcavitykcouplinggSchematic atom+ cavityCoupling rates and decay ratesStrong coupling regime.Strong coupling regime : g >> k , g small mode volume good quality mirrors
10Cavity QED experiments single atom - single photon interactionDetection of single atomsOettl et al. PRL 95 (2005)Quantum light sourcesHijlkema PhD thesis (2007)Evidence of field quantisation & photon counterBrune et al. PRL 76 (1996)The experiments you can perform with a cavity QED system take advantage of the good control and the large value of the single atom – single photon interaction.1- In the experiments of Haroche group at ENS, the atom acts as a probe of the cavity field, and allows to count the number of photons in the cavity. It is the oscillations frequency of the atom which tells you how many photons are in the cavity. This expt is actually a very nice proof of field quantisation.2- The subject of a number of recent expts is to take advantage of the good control over the coupling to fashion the light-matter interaction and obtain sources of quantum light. Here I show of expt from the MPQ group which produces single photons on demand from a trapped atom in a cavity.3- Another class of experiment you can perform with an atom-cavity system is the detection of single atoms, for example here I show an expt performed in Zurich where they use the detector to probe the coherence properties of an atom laser beam.I should now get into more detail about the way a cavity is used to detect single atoms.
11Resonant Jaynes-Cummings spectrum b,0e,0energyb,1b,0+,1energycoupling g-,1splitting 2gebInteraction single atom - single photon visible!Now we consider the situation when the atom and the cavity mode are exactly resonant.The interaction between the atom and the cavity field is described by a simple Hamiltonian which models the coherent energy exchange between the atom and the cavity.Initially the level structure is like that, with a degeneracy between the atomic excited state and the one-photon state.2-The effect of the coupling is to lift this degeneracy to obtain new eigenstates of the system called the dressed states.3- this interaction with a single atom is visible in the transmission spectrum of the cavity as the initial transmission peak is splitted in two new peaks corresponding to the position of the dresses states.
12Principle of single atom detection in a cavity Optimum measurement rate 1 measurement = 1 photonWith losses L : ¡signal = L £ ¡inc1-The detection process with a strongly coupled cavity is optimum.I just need 1 photon to know if I have one atom or not.Or equivalently: if I have a single atom in the cavity, that can be in two state, one resonant one nonresonant, I would just have to send one photon to know in which state it is.If I had a single photon source, I would just need to trigger it once.
13Detection with minimum backaction? Backaction characterized by GspFor a free space detector:factor C !Now if you look again at this picture, there is something quite fascinating (and suspicious): you can detect the atom with light, but there is no light entering the cavity. We would therefore say that the detection is done with no spontaneous emission, no perturbation of atomic variables like motion, zeeman state, etc.In reality, there is always some light that enters the cavity, and you get some spontaneous emission in the external modes of the light fields (shown in green here). Nevertheless we can still compute it and find that the fraction of light that is scattered by the atom out of the cavity, compared to the light we sent in in very small : 1/CLet us know insist on the differences with a free space based detection scheme, such as fluorescence detection. Here, the signal that you get in olny due to spontaneous emission, and for the same signal, you have a factor C of difference between free space and cavity detection. The idea is then to make this factor as big as possible to obtain the smallest possible backaction on the atom.
14Outline Introduction Cavity QED and single atom detection Experimental setupDetection of waveguided atomsPreparation and detection of trapped single atomsDetection with minimum backactionQuantum Zeno effect
15AutoCAD’s view Integrated atom chip-cavity system The experimental setup consists in the combination of an atom chip, used to trap ultracold atoms, and a high-finesse Fabry-Perot cavity
16Atom chip basics 1cm Applications: - BEC - precise transport and positioning- atomic clocks and interferometers- single atom manipulation?Magnetic traps:- versatility- strong confinement close to the surfaceAtomchips are used to trap atoms, with magnetic fields. They are made of small wires, here it’s gold wires, on a planar surface, in which you drive current to create a magnetic trap, together with external bias fields.Many wires -> many trap configurations.Small distance atoms/chip -> good confinement. -> applications …
17Miniaturized Fabry-Perot cavity Mention this cavity is OUR designC02 laser ablation -> good surface quality, curved fiber tips -> HR coating
18Miniaturized Fabry-Perot cavity - tunable- small mode volume w0=4 mm ; d=39 mm- integrated 150mm from chip surfaceCavity QEDStrong coupling regime!finesse F = 38000coupling g /2p = 160 MHzcavity decay k / 2p = 50 MHzatomic decay g / 2 p = 3 MHzcooperativity C = g2/2kg = 85Mention this cavity is OUR design
19Outline Introduction Cavity QED and single atom detection Experimental setupDetection of waveguided atomsPreparation and detection of trapped single atomsDetection with minimum backactionQuantum Zeno effect
20Detection of waveguided atoms Principle APDBECAtomic waveguideaMention it is the easiest way to put SINGLE atoms in the cavityDetection zoneLASER… the easiest way to put SINGLE atoms in the cavity
21Detection of waveguided atoms Reference with no atoms
22Detection of waveguided atoms Single run with atoms
23Detection of waveguided atoms Experiment ThresholdExcellent signal to noise ratio for dilute BECs.From these results : not possible to conclude on a detection efficiency with this technique.Some atoms are missed – because they do not lead to a deep enough signal. these are single atoms !!!
24Outline Introduction Cavity QED and single atom detection Experimental setupDetection of waveguided atomsPreparation and detection of trapped single atomsDetection with minimum backactionQuantum Zeno effect
25Trapping & detecting the atoms in the cavity mode Transfer magnetic trap Optical dipole 830nmExperiments with BEC : see Colombe et al. Nature 450 (2007)
26Positioning the BEC in the cavity BEC in magnetic trapN ~ a few 1000sinput fibreoutput fibreInitial cloud size ~1mm single-site loading possible.YDipole trap@ 830nmProbe light@ 780nm
27Vacuum Rabi Splitting with collective enhancement How to get to the single atom regime?Laser detuning ΔL-A [GHz]The effect of a BEC in a cavity in to increase the coupling by srqt(N) compared to a single atom. We measured this increased coupling by performing a spectroscopy of the system for different values of N, and looking for the resonance peaks corresponding here to the collective dressed states. We find a very good agreement to the theory.2-We also see on this curve that is difficult to get to the small atom number regime. So how can we prepare trapped single atoms this way?Y. Colombe, T. Steinmetz, G. Dubois, F. Linke, D. Hunger and J.Reichel Nature 450 (2007)
28From 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'=0,1,2,3Cavity tuned to F=2 -> F’=3 transitionF=2F=1Reservoir (N~10)Weak MW pulse GHz)~2% transfer probability/atomTo do deterministic preparation of single atoms with cavity , you need to start with a very small could of 0 or a1 toms.Noise in external magnetic fields and current wires.Key idea: weak mw pulse to decrease the relative probability of multiple transfers.
29Usual 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 stateAnalogy with our scheme : position internal state.
30“Preparation and detection” iterative sequence timeReservoirpreparationmwDetectionmwDetectionEtc …F’=3F=2F=11000~10
32Analysis of detection pulses Transfer efficiency 10%Relative transmission 1.4%thresholdWe find that the results of detection pulses, in terms of APD counts, are distributed along a Bi-Poissonian distributionsuccessful transfers (~10%)unsuccessfultransfers (~90%)after ~10 pulses Reliable preparation
33Lifetime of the atoms during detection single runor??
34Lifetime of the atoms during detection Fitor??Average lifetime 1.2 msLimited by depumping to F=1Fidelity=99.7%+ QND measurementstat. limitdepump limit
35Outline Introduction Cavity QED and single atom detection Experimental setupDetection of waveguided atomsPreparation and detection of trapped single atomsDetection with minimum backactionQuantum Zeno effect
36How can we measure spontaneous emission? Zeeman “random walk”:But not visible in lifetime !
37Measurement and preparation of a specific Zeeman state (F=2;mF=0) Measurement of mFpppUntil now I did not talk about Zeeman stateZeeman state : not visible in cavity detection+ all Zeeman states are trapped-> however it is important to perturb the atomic state as little as possiblefor preparation in the ground state, for example, or a given Zeeman state.B
39Detection figure of merit : backaction Better than a perfect free space detection ! Possible to prepare a single atom without changing the motional state !Improvements require:- simultaneous R&T meas.- lower optical losses
40Detection without perturbation ? with L ~ 0.1 : C ~ 20expected value C ~ 85 ???What is the real measurement rate of the system?for a lossless observer ¡m = ¡inc = C ¡spcan we check that ???
41Outline Introduction Cavity QED and single atom detection Experimental setupDetection of waveguided atomsPreparation and detection of trapped single atomsDetection with minimum backactionQuantum Zeno effect
42Quantum Zeno Effect b mw p a m = Coherence decay rate between a and b Cavity & atomicexcited stateF=2;mF=0F=1;mF=0bmw pWhat does the QZE consist in? Consider the situation of two stable states, between which you can drive a Rabi oscillation with an external field.Now suppose that you continuously measure the population in one of the two states, for example by coupling to a fast-decaying 3rd state. This measurement will actually prevent the transitions between the two initial states. The reason is that in QM, the measurement perturbs the state. More precisely, the measurement destroys the coherence between the two states, which are necessary to establish the Rabi osc. Therefore, if you apply a pi-pulse on the initial transition, you find that the transfer probability will drop from 1 to 0 as you increase the meas rate.In the experiment, the transition we consider is the F=2 mF=0 -> F=1 mF=0 transition, and the measurement is performed by pumping the cavity which measure the value of F. The measurement rate is in fact the coherence decay rate of an initial state superposition.Here, we find that the coherence decay rate = photon input rate in the cavity mode;If we compare with the Zeeman diffusion expt. we find that this is approx 20 times larger than sp em rate.It is also equal to the optimum detection rate for the experimentator, if the system were lossless.So this expt actually demonstrates that the state of the system is measured (at least by the environment) with almost no sp. em.am = Coherence decay rate between a and bm = Photon input rate~ 20 £ Spontaneous emission rate
43Summary Preparation of trapped single atoms starting from a BEC: preparation in a specific Zeeman state qubit clock stateswell localized within the cavityFirst detector of single atoms on a chip ability to distinguish F=1 from F=2 states with 99.7% fidelityDemonstrated a Quantum Zeno effect w/o spontaneous emission.
44Outlook e laser cavity b a Characterize the atomic motional state are we still in the ground state?Manipulate of pairs of atoms in the cavity Cavity-assisted entanglement generationCombine with other atom chip technology (state dependent mw potentials)Quantum memory with BEC and Fiber-cavity- Large collection efficiency- Long storage timelasercavityabe
46Atomchip-based single atom detectors Fluorescence (Wilzbach et al )Photoionization (Stibor et al PRA 76 (2007))Cavity QED (Purdy et al. APB 90 (2008))123
47Single atoms – light/matter interface laservacuumabeSingle photon sourceAtom-photon entanglementPhoton-photon entanglementLong-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 helpsLambda scheme
48Single atom ‘temperature‘ Release and recaptureMean energy < 100 mK(trap depth 2.6 mK)
50Single atoms : some fascinating achievements Beugnon et al.Nature 440 (2006)Hong-Ou-Mandel effectEvidence of field quantisation & photon countingBrune et al. PRL 76 (1996)Massive multi-particle entanglementMandel et al. Nature 425 (2003)50
51Single atoms toolbox Preparation & trapping 1-qubit gates State readoutScheme : controlled collisionsEntangle atomic internal and external stateTheory:Calarco et al. , PRA 61 (2000)Experiment:Mandel et al. Nature 425 (2003)Böhi et al. preprint arXivfor this entanglement to succeed you need a perfect knowledge about the state initial state-> trap grd state!Requirements:- state dependent potentials- preparation in the trap ground state51
52Single atoms toolbox Preparation & trapping 1-qubit gates State readoutScheme : Rydberg gaterbd1.d2aTheory:Jaksch et al. PRL 85 (2000)Experiment:Wilk et al. preprint arXiv:Ref : Calarco proposalBloch realizationRequirements:- preparation of Rydberg states- small distance (<5mm) between atoms52
53Single atoms toolbox Preparation & trapping 1-qubit gates State readoutScheme : cavity-mediated interactionaeeaaa+1 photonggbaabaaaaYou et al. PRA 67 (2003)Ref : proposal You and LiRequirements:- optical cavity, strong coupling regime- good control over the coupling g53
54Single atoms toolbox Preparation & trapping 1-qubit gates State readout :For free space detection Signal = Spontaneous emission heating & depumpingNon-destructive measurement?- Not necessary in principle- but very useful for preparation!eQND measurement ->Not necessary for quantum computingBUT allows to do preparation in a well-defined state with active feedback. need a cavity to enhance light/matter coupling and avoid spontaneous emissionba54
55Detection of waveguided atoms Analysis Spontaneous emission: depumping to untrapped states.Some atoms lost before they reach maximum couplingStill: Demonstrates >50% efficiency single atom detection (absorption imaging, simulations)But: trapped atoms in the strong coupling region should lead to better results