Single atom manipulations

Benoît Darquié, Silvia Bergamini, Junxiang Zhang, Antoine Browaeys and Philippe Grangier

Laboratoire Charles Fabry de l'Institut d'Optique Théorique et Appliquée

UMR 8501 du CNRS

91 403 Orsay

http://www.iota.u-psud.fr/~grangier/Quantum_optics.html

Introduction

• Experience :

• Context :

• Goals :

– two neutral atoms– trapped in two different dipole traps– confinement : m3

– a few microns away from one another

– entangle the atoms– make a quantum gate

study and manipulation of an optical dipole trap for single atoms

Principle of a dipole trap

Assumption : two-level atom, in a laser-field of frequency L, with a red detuning : = L - .

Atoms are trapped in the high intensityregionsThe transition frequency is shifted to theblue

01

4

)()(

21

r

rU

laser-induced non-dissipative force associated with a potential energy )(rU

with : - the Rabi frequency- the lightshift

)(1 r

For large detunings

h

|e

|g

atom in the laser field

hLh

two-level atom

We use a magneto-optical trap as a reservoir of cooled atoms : - to trap and cool atoms- to induce the fluorescence of the atoms (which will allow us to observe them)

Dipole trapDipole force= non-dissipative force=> we previously have to cool atoms

Focussing of a Titanium-Sapphire laser beamin the centre of this reservoir

dipole trap

Atoms are gathering at the focussing spot => dipole trap

Dimensions of the trap=

dimensions of the focussing spot

The microscope objective :MIGOU

Positionof the MOT

Trapping beam

Characteristics of MIGOU :– large numerical aperture : 0,7– diffraction limited spot– large working distance (~1cm)– ultra high vacuum compatible

waist of the beam < 1 m

Double use of MIGOU :– to secure the focussing of the trapping beam in the center of the MOT– to collect the fluorescence of trapped atoms with a large efficiency

~5 cm

Experimentalset-up

Vacuum chamber

MOT & dipole trap

x

y

z

Dipoletrap beam

Fluorescence

CCD camera

780 nm filters

Spatial filtering

Computers

Filtering pinhole

AvalanchePhotodiode

Pictures of the dipole trap on the CCD camera

scaling of imaging system :1 pixel = 1 m

• Continuous observation of the fluorescence of thedipole trap on the CCD caméra.• One picture every 200 ms. Y

X

Fluorescence(CCD)

X

Y

Fluorescence

10 000 counts(200 ms)

Images on theCCD camera

5 m

Single atom regime

120

80

40

0

2520151050Time (s)

Counting rate (counts/10ms)

1 atom

Background

Double trapMOT & dipole trap

secondtrapping beam.

4 m

What we observe on the CCD caméra

In single atom regime, there arefour likely configurations :

Temperature of the atoms and trap frequencies

• Goals :

• Requirements : – atom in the Lambe-Dicke regime : << 1

we have to measure the temperature of the atoms and the trap frequencies

– entangle the atoms– make a quantum gate

2

2

m

Tkxk B

Oscillation frequencies : principle of the measurement

• We trap one atom.• We switch off and on the dipole trap during t1.

If the atom is recaptured, it starts to oscillate in the trap.• We wait for t and then, we switch off and on the dipole trap during t2.

P(t) is the probability to recapture the atom after the whole sequence.

Dipole trapON

OFFt

t

P(t)

oscillate at 2fosc.

t1 t2

Oscillation frequencies : experimental results

0.8

0.6

0.4

0.2

0.0

Prob

abili

ty o

f re

capt

urin

g th

e at

om

14121086420Delay (s)

fosc=134 KHz

• w0 = 0.89 m • Ptrap = 2 mW

fr = 140 kHz , fz = 29 kHz}

Ptrap = 1,9 mW

1.0

0.8

0.6

0.4

0.2

0.0

Prob

abili

ty o

f re

capt

urin

g th

e at

om

14121086420Retard (s)

fosc = 108 Khz

Delay (s)

t1 = 2.5 st1 = 1 s

Ptrap = 1,5 mW

Temperature of the atom : time of flight experiments

• Time sequence:

Objective

MOT

1 : We trap one atom

Trappingbeam

Temperature of the atom : time of flight experiments

• Time sequence:

Objective

1 : We trap one atom

Trappingbeam

2 : We switch off the MOT

Temperature of the atom : time of flight experiments

• Time sequence:

1 : We trap one atom

2 : We switch off the MOT

3 : The trapping beam is switched off during t

ObjectiveTrapping

beam

MOT

4 : We check if the atom is still there

We measure the probability of recapturing the atom after t.

Temperature of the atom : results

1.0

0.8

0.6

0.4

0.2

0.0

Pro

babi

lity

of

reca

ptur

ing

the

atom

50x10-6

403020100t (seconds)

P = 2 mW Simulation with T = 140 K Simulation with T = 35 K

T = 35 K

P = 2 mWsimulation with T = 140 Ksimulation with T = 35 K

+

Conclusion and outlooks

• We are now able to evaluate the trap frequencies and the temperature of the atoms

• We need :– a better confinement– a smaller temperature

• Better confinement retro-reflexion of the trapping beam, standing wave

• Smaller temperatures Raman cooling

Lamb-Dicke parameters : r 0.5 z 2.5

Single atom manipulations

Benoît Darquié, Silvia Bergamini, Junxiang Zhang, Antoine Browaeys and Philippe Grangier

Laboratoire Charles Fabry de l'Institut d'Optique Théorique et Appliquée

UMR 8501 du CNRS

91 403 Orsay

http://www.iota.u-psud.fr/~grangier/Quantum_optics.html

Entanglement of two atoms

|>|>

probebeam

|>

|>|>

probebeam

|>

Atome 1 Atome 2

beam splitter

detector of -polarizedlight:

Entanglement of two atoms

Excitation by a photon of the probe beam: 20020000 21 ii bebe

detection of -polarizedligt:

|>|>

probebeam

|>

detection of -polarizedlight:

atoms behave as Young's slits

interferences

projection onto the state: 1001 21 ii ee

entanglement

Plan of my talk

• Principle of the optical dipole trap

• Implementing a dipole trap A microscope objective : MIGOU Experimental set-up Pictures of the dipole trap Double dipole trap

• Temperature of the atoms

• Oscillation frequencies of the dipole trap

• Conclusion and outlooks