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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. Well-controlled system! Testbed for Quantum Mechanics Qubit candidate?. b. a. - PowerPoint PPT Presentation
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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
Single atoms : remarkable features
• Well-controlled system!
• Testbed for Quantum Mechanics
• Qubit candidate? Cooling & trapping
a
b
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. Interaction3. Detection
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)]
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
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 ?
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
Strong coupling regime : g >>
small mode volume
good quality mirrors
e
b opticalcavity
atom
couplingg
a
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)
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
Principle of single atom detection in a cavity
1. Optimum measurement rate1 measurement = 1 photon
2. With losses L : ¡signal = L £ ¡inc
Detection with minimum backaction?
Backaction characterized by sp
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 viewIntegrated atom chip-cavity system
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
Miniaturized Fabry-Perot cavity
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
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
LASER
APD
Atomic waveguide
Detection zone
a
BEC
… 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
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.
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?
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
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:
“Preparation and detection” iterative sequence
time
F=2
F=1
1000 ~10
mw
Det
ectio
n
mw
Det
ectio
n
Etc …
Reservoirpreparation
F’=3
0 or 1 atom in F=2?
nAPD ~ 25 nAPD < 1
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
Lifetime of the atoms during detection
or ??single run
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
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)
B
Measurement of mF
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
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
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 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
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
• 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
Single atom ‘temperature‘Release and recapture
Mean energy < 100 K
(trap depth 2.6 mK)
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
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)
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
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
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
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!
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