guilhem dubois supervisor: jakob reichel atomchips group, laboratoire kastler brossel, ens paris...

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. Interaction

3. 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. 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

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 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

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 trap

N ~ a few 1000s

Probe light @ 780nm

• Initial cloud size ~1m single-site loading possible.

Vacuum Rabi Splitting with collective enhancement

La

ser

det

uni

ng

ΔL

-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]

Tra

nsf

er p

rob

abili

ty

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