Preparation, manipulation and detection of single atoms on a chip
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Transcript of Preparation, manipulation and detection of single atoms on a chip
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Guilhem Dubois
Supervisor: Jakob Reichel
Atomchips group, Laboratoire Kastler Brossel, ENS Paris
Preparation, manipulation and detection of single atoms on a chip
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Single atoms : remarkable features
• Well-controlled system!
• Testbed for Quantum Mechanics
• Qubit candidate? Cooling & trapping
a
b
Tcoh > 10s
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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
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Single atoms toolbox
1. Preparation 2. Interaction3. Detection
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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)]
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Single atoms toolbox
1. Preparation : constraints deterministic specific internal state e.g. clock states specific motional state e.g. trap ground state
2. Interaction3. Detection
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Single atoms toolbox
1. Preparation : feedback deterministic specific internal state e.g. clock states specific motional state e.g. trap ground state
2. Interaction3. Detection : here atom counting
minimum backaction (spontaneous emission)
How can we achieve that ?
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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
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Atom-cavity system
Strong coupling regime : g >>
small mode volume
good quality mirrors
e
b opticalcavity
atom
couplingg
a
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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)
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Resonant Jaynes-Cummings spectrum
g,1
b,0
e,0en
ergy
b,1
b,0
+,1
ener
gycoupling g-,1
splitting 2g
Interaction single atom - single photon visible!
e
b
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Principle of single atom detection in a cavity
1. Optimum measurement rate1 measurement = 1 photon
2. With losses L : ¡signal = L £ ¡inc
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Detection with minimum backaction?
Backaction characterized by sp
For a free space detector: factor C !
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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
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AutoCAD’s viewIntegrated atom chip-cavity system
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Atom chip basics1cm
Applications:
- BEC
- precise transport and positioning
- atomic clocks and interferometers
- single atom manipulation? Magnetic traps:
- versatility
- strong confinement close to the surface
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Miniaturized Fabry-Perot cavity
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Miniaturized Fabry-Perot cavity
finesse F = 38000
coupling g /2 = 160 MHz
cavity decay / 2 = 50 MHz
atomic decay / 2 = 3 MHz
cooperativity C = g2/2 = 85
Cavity QEDStrong coupling regime!
- tunable- small mode volumew0=4 m ; d=39 m
- integrated150m from chip surface
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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
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Detection of waveguided atoms Principle
LASER
APD
Atomic waveguide
Detection zone
a
BEC
… the easiest way to put SINGLE atoms in the cavity
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Detection of waveguided atoms
Reference with no atoms
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Detection of waveguided atoms
Single run with atoms
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Detection of waveguided atoms Experiment
Threshold
these are single atoms !!!
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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
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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)
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Positioning the BEC in the cavity
input fibre output fibre
YDipole trap @ 830nm
BEC in magnetic trapN ~ a few 1000s
Probe light @ 780nm
• Initial cloud size ~1m single-site loading possible.
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Vacuum Rabi Splitting with collective enhancement
Lase
r det
unin
g Δ
L-A [G
Hz]
Y. Colombe, T. Steinmetz, G. Dubois, F. Linke, D. Hunger and J.Reichel Nature 450 (2007)
How to get to the single atom regime?
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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=1Reservoir (N~10)
Weak MW pulse (@6.8 GHz)~2% transfer probability/atom
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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:
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“Preparation and detection” iterative sequence
time
F=2
F=1
1000 ~10
mw
Det
ectio
n
mw
Det
ectio
n
Etc …
Reservoirpreparation
F’=3
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0 or 1 atom in F=2?
nAPD ~ 25 nAPD < 1
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Analysis of detection pulses
successful transfers (~10%)
unsuccessfultransfers (~90%)
• Transfer efficiency 10%
• Relative transmission1.4%
<n>=0.35 <n>=25
thre
shol
d
after ~10 pulses Reliable preparation
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Lifetime of the atoms during detection
or ??single run
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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
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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
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How can we measure spontaneous emission?
Zeeman “random walk”:
But not visible in lifetime !
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Measurement and preparation of a specific Zeeman state (F=2;mF=0)
B
Measurement of mF
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Diffusion in the Zeeman manifold
Fit
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Detection figure of merit : backaction
Better than a perfect free space detection !
Possible to prepare a single atom without changing the motional state !
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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 ???
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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
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Quantum Zeno Effect
m = Coherence decay ratebetween a and b
mw
Cavity & atomic
excited state
F=2;mF=0
F=1;mF=0
m = Photon input rate
~ 20 £ Spontaneous emission rate
b
a
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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.
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Outlook
• Characterize the atomic motional stateare 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
lase
r
cavity
ab
e
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Single atom Vacuum Rabi splitting
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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
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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
lase
r
vacuum
ab
e
- Probabilistic is OK (DLCZ 2002)
atomic ensembles possible but coherence time ~ms.
- Collection efficiency small with single atoms
a cavity helps
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Single atom ‘temperature‘Release and recapture
Mean energy < 100 K
(trap depth 2.6 mK)
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Single atom Rabi oscillations
0 5 10 15 200
0.2
0.4
0.6
0.8
1
MW pulse duration [s]
Tran
sfer
pro
babi
lity
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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)
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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
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Single atoms toolbox• Preparation & trapping• 1-qubit gates• 2-qubit gates• State readout
Requirements:
- preparation of Rydberg states
- small distance (<5m) between atoms
Scheme : Rydberg gate
Theory:
Jaksch et al. PRL 85 (2000)
Experiment:
Wilk et al. preprint arXiv:0908.0454
a
r
b
d1.d2
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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
aaab
ae
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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
ba
• For free space detectionSignal = Spontaneous emission heating & depumping
• Non-destructive measurement? - Not necessary in principle - but very useful for preparation!
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Detection of waveguided atomsAnalysis
• 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