Bose-Einstein Condensation
of a Metastable Helium Gas

Contact us : leduc@lkb.ens.fr - Leduc office : + 33 1 44 32 20 23


The Helium team

Julien Dugué, actuellement à Camberra en Australie
Christian Buggle Maximilien Portier Steven Moal

Nassim Zahzam

Claude Cohen-Tannoudji Michèle Leduc C.S. Unnikrishnan

Ph D Students :

Franck Pereira dos Santos (until February 2002)
Jérémie Léonard (until November 2003)
Steven Moal (since September 2003)
Maximilien Portier (since September 2004)
Julien Dugué (since September 2005)

Visitors since 1999 :

Carl J. Barrelet
Erwan Jahier
Allard Mosk

Tobias Mueller
Francesco Pavone
Francisco Perales
Hema Ramachandran
Ernst Rasel
Alice Sinatra
C.S. Unnikrishnan
Matthew Walhout
Junmin Wang
Umakant Rapol
Jae Wan Kim
Christian Buggle
Nassim Zahzam

Group leaders :

Claude Cohen-Tannoudji
Michèle Leduc

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Presentation

 We has a set-up producing Bose-Einstein Condensation (BEC) in a dilute gas of metastable helium in the 23S1 state. Our experimental set-up is similar to the one used for alkali BEC (Rb, Li, Na). Metastable helium is the first atom condensed in an excited state with a high energy of 19.8 eV. Its lifetime is long (8000s).

In order to obtain a magneto-optical trap with a large number of atoms, we have built a new source of metastable helium with better performance than the previous one. Then we have used standard laser cooling methods to collimate the atomic beam in order to increase its brightness. Atoms are slowed down in a long Zeeman slower before being trapped by the magneto-optical trap (MOT).

We have constructed a magnetic trap in which we transfered a cold gas of metastable helium atoms. Then by evaporative cooling the Bose-Einstein Condensation was achieved.

We have studied the elastic and melastic collisions between ploarised atoms at very low temperature. We studied the hydrodynamic modes in the ultracold gas by generating collective modes of excitation. We were then involved in the creation of ultracold molecules by photoassocation with one or several laser beams tuned close to the 23S-23P transition.We have produced giants dimers in a purely long range molecular potential. We keep using the photo association method with one and two photons for an accurate value of the scattering length between two metastable atoms
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Experimental set-up
 

The experimental set-up consists of the following steps :  a source of metastable helium, collimation-deflection of the atomic beam, two Zeeman slowers and a quartz cell where atoms are trapped.

 
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Metastable Helium source
 

Metastable Helium atoms are produced by an electric discharge in a small volume of helium gas in its ground state. The source  is cooled down by liquid nitrogen.  The discharge takes place between a metallic tip (cathode) and an aluminum plate (anode) with a potentiel difference of 700 V. This discharge can produce a metastable atomic flux of  2 ×1014 atoms/s/sr with a mean velocity of about 1000 m/s. The discharge yields about 1 metastable atom for 104 ground state helium atoms.
 

Atomic source


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Collimation, deflection and Zeeman slowing of the atomic beam

 The laser is a semiconductor DBR diode followed by a doped fibre amplifier, operating at 1083 nm (2S-2P transition)
The atomic beam is collimated in the transverse directions using the radiation pressure of a laser beam which is forced to bounce many times across it. Due to the collimation, the atomic flux is of the order of 2 ×1011 atoms/s. Then metastable atoms are separated by deflection from the large number of ground state atoms and other residual products of the source (ions, UV photons...), using a converging laser. Finally the atoms are slowed down in a 2 meter long Zeeman slower in two parts. Their velocity is reduced  from 1000 m/s down to 40 m/s.

Zeeman slower

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The Magneto-Optical Trap (MOT)

 

After the Zeeman slower, the atoms are trapped in a MOT. This trap takes place at the intersection of 6 laser beams perpendicular to the cell windows. The trap parameters are detuning of  - 45 MHz , laser intensity s I = 300 Isat. The density of the atomic cloud  in the MOT is limited by Penning collisions between metastable helium atoms which are increased in the presence of the MOT light (see our bibliography). The MOT traps 109 atoms in a  volume of 0.1 cm3, at a temperature of 1 mK.

  

The magnetic trap
 
 

 A magnetic trap was built in a Ioffe-Pritchard configuration, in order to trap the metastable helium cloud. After switching off the MOT field, the sample is further cooled down to about 300 microK by an optical molasses. Then the atoms are optically pumped in order to increase the tranfer efficiency into the magnetic trap. Up to 3 ×108 atoms can be tranfered in the trap. After compression of the trap and compensation of the field bias, the temperature of the cloud reaches 1.2 mK and its lifetime is between 30s and 80s depending on experimental parameters. We then start evaporative cooling performed by RF-induced spin flips. The evaporation frequency is ramped down from 160 MHz to about 12 MHz in 15 seconds. Finally we suddently release the cloud by switching off the current in the coils. We probe the cloud by absorption imaging on a CCD camera at 1.083 micrometers. At a final evaporative frequency below 13 MHz, une double structure appears in the absorption images. We attribute it to a Bose-Einstein Condensate in the presence of a thermal cloud.

Top view of the quartz cell

The 2 white coils are part of the 3 coils in the Ioffe-Pritchard magnetic trap. The 2 large coils are used for compensation of the field bias. 
 

Trap parameters : 

Gradients = 260 G/cm 
Curvature = 200 G/cm2


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The Bose-Eintein Condensate
 

We have observed our first Bose-Einstein condensate of metastable helium atoms on the 2Oth of February 2001. The critical temperature is 4.7± 0.5 microK for a cloud of about 8 millions atoms. The figures below show a 3D image and a 1D profile of the same cloud after 4ms of expansion time. One can clearly see the difference between the condensed and the thermal fraction which is fitted by a bosonic distribution in the 1D profile.
 

3D image and 1D profile of the atomic density

 
The figure below shows images of a pure condensate containing 400 000 atoms for different expansion time after switching off the trap. Initially the elliptical shape of the condensate corresponds to the geometry of the trap. As the expansion time increases, the ellipticity of the condensate undergoes an inversion due to mean-field interactions between atoms. This inversion is the strongest evidence for the existence of a Bose-Einstein condensate.
 
 

Time of flight experiment : the ellipticity of the cloud undergoes an inversion.

From these images it is possible to extract the scattering length for collisions between two polarized helium atoms in the metastable triplet state :  a =16± 8 nm.

The measurement of the number of atoms in the condensate as a function of trapping time yields a lifetime of order 2 seconds. It also places an upper bound on the 2 and 3-body inelastic collision rate constants between metastable helium atoms (see our article).
 

NB: The condensation of metastable helium was first observed on the 12th of February 2001 in IOTA, at the université d'Orsay-Paris11.
 

 
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Hydrodynamic modes in the ultracold gas

We study the behavior of an ultracold cloud of helium gas at temperatures just above the Bose Einstein transition, a domain where the elastic collisions play a major role for understanding the dynamics of the evaporative cooling and of the condensation. In the case of metastable helium, the atomic cloud is rather dense near the transition and the value of the scattering length a was found large (16 nm): one thus expects the regime to be more hydrodynamic than collisionless (mean free path larger than the cloud dimensions and not the reverse). To study this we generate collective excitation modes in the gas et we study both their frequency and their damping as a function of temperature. We show that, when the rate of elastic collisions increases, one moves from the collisionless regime to the hydrodynamic regime, in which one enters at temperatures of the order of 3 Tc. However we find a discrepancy between the collisional parameters deduced from the present study and those derived from measurement of the condensate expansion (see our preliminary article on cond-mat/0208263)




Oscillation of the ellipticity of theatomic cloud for the
monopole quadrupole m = 0 excitation mode



Frequency and damping of this excitation mode. The curve is calculated, our experimental results are shown as crosses for T=3Tc


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Photoassociation and creation of ultracold molecules of metastable helium

We started a photoassociation (PA) study in a gas of metastable helium atoms near the Bose Einstein transition. For PA we use a laser beam tuned near the helium transition 23S1-23P0,1,2 transition of the helium atom at 1083 nm. The goal is to create ultracold molecules in bound states of the S-P and S-S molecular potentials. The PA beam is sent through the atomic cloud with the magnetic trap on. The frequency is slightly detuned to the red, the detuning is varied between a few MHz up to a few 10 GHz. The intensity of the PA pulse is weak enough so as not to destroy too fast the atom pair through light assisted Penning ionization collisions. The molecular lines are observed through the atom losses optically detected on the absorption of a resonant probe beam.

A large number of molecular lines are observed that are under identification from what is known of the S-P molecular potentials. Interesting results concern lines in the vicinity of the 2S-2P0 transition : we observe 5 narrow lines, the position of which can be exactly calculated from the weakly attractive long range potential C3/R3. The corresponding molecules are very loosely bound with the 2 atoms being at very large distance of each other.

In parenthesis, calculated values

We have studied frequency shifts of photoassociation lines under light irradiation. The photoassociation laser excites a pair of spin polarized metastable helium atoms to a molecular bound state in the purely long range potential 0u+. The frequency displacements are proportional to the laser intensity.


1-Two resonance curves obtained for two different intensities of the photoassociation laser, (a) : 9 mW/cm2, (b) : 5W/cm2 .The signal shows the temperature raise of the atomic cloud as a function of the frequency detuning of the photoassociation laser from the 23S1-23P0 atomic transition. The resonance peak (b) at high intensity is shifted; the frequency shift is proportional to the laser intensity

The frequency shifts result of the optical coupling of the excited molecular state with the continuum of scattering states of the two colliding atoms and with the bound states of the interaction potential between the two metastable atoms. The comparison between the predicted and measured shifts lead to a value of the s-wave scattering length for the spin polarized helium atoms: a = 7.2 ± 0.6 nm, lower than values previously measured by other methods.



2-Ratio of the frequency shifts for photoassociation lines obtained with 23S1 helium atoms excited to v=0,1 and 2 molecular levels in the 0u+ potential, plotted as a function of the value of the s-wave scattering length a . Curves are theoretical; grey areas represent the experimental uncertainties; (a): with σ- polarization, (b): with linear polarization. Comparison between theory and experiment provides: a = 7.2 ± 0.6 nm.

Using this value of a provided by the light shifts, we were able to make a much more precise measurement by a two-photon photoassociation experiment. We determined the position of the least bound state (v=14) in the interaction potential between the two metastable atoms, using a 3 level scheme involving the excited molecular bound state v=0 in the potential 0u+ .



3-Principle of a two-photon photoassociation experiment. A first laser excites two free atoms towards a bound molecular level in the 0u+ potential. This state is connected to the bound molecular state v = 14 of the ground potential by a second laser L2.

A very narrow atom-molecule dark resonance is observed, the position of which leads to the energy of the v=14 state, from which the following value of the scattering length can be deduced: a = 7.512 ± 0.005 nm.



4-Two-photon photoassociation experiment, according to the scheme of the previous figure. Laser L2 is at fixed frequency, laser L1 is scanned. (a): L2 at resonance, with a large intensity, an Autler-Townes doublet appears; (b): L2 slightly off resonance, a Raman peak with a Fano profile appears; (c): L2 at resonance with a weak intensity; the narrow central, peak is attributed to an atom-molecule dark resonance.

This study continues with the atom-molecule dark resonance in order to extract informations on the ionizing Penning collisions between two metastable helium atoms when inhibited by the spin polarization.


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Recent publications:
 

2005

Accurate determination of the scattering length of metastable Helium atoms using dark resonances between atoms and exotic molecules,
S.Moal, M.Portier, J.Kim, J.Dugué, U.D.Rapol, M.Leduc, and C.Cohen-Tannoudji
submitted to Phys. Rev. Lett., cond-mat/0509286 download from cond-mat

Frequency shifts of photoassociative spectra of ultracold metastable Helium atoms : a new measurement of the s-wave scattering length,
J.Kim, S.Moal, M.Portier, J.Dugué, M.Leduc, and C.Cohen-Tannoudji
accepted for publication by Europhys. Lett., physics/0507180 download from physics

Observation of the Photoassociation Resonance of ultracold mestable He Dimers by Mechanical Effects,
J.Kim, S.Moal, M.Portier and M.Leduc,
Las. Phys. 15 7 p.1075 (2005)

Rotationally induced Penning ionization of ultracold photoassociated helium dimers
J.Léonard, A.P.Mosk, M.Walhout, M.Leduc, M.van Rijnbach, D.Nehari, and P.van der Straten
Europhys. Lett., 70 (2), 190-196 (2005) download from edpsciences

2004

Analysis of Photoassociation Spectra for Giant Helium Dimers
J.Leonard, A.P.Mosk, M.Walhout, P.Van der Straten, M.Leduc, and C.Cohen-Tannoudji,
Phys. Rev. A 69, 032702 (2004) download from APS

Prospects for measurement and control of the scattering length of metastable helium using photoassociation techniques,
J.C.J.Koelemeij, and M.Leduc,
Eur. Phys. J. D 31, 263-271 (2004) download from edpsciences

Photoassociation experiments with ultracold metastable Helium,
J.Kim, U.D.Rapol, S.Moal, J.Leonard, M.Walhout, and M.Leduc
Eur. Phys. J. D 31, 227-237 (2004) download from edpsciences

2003

Giant Helium Dimers Produced by Photoassociation of Ultracold Metastable Atoms,
J.Leonard, M.Walhout, A.P.Mosk, T.Müller, M.Leduc, and C.Cohen-Tannoudji,
Phys. Rev.Lett. 91, 073203 (2003) download from APS

2002

Hydrodynamic modes in a trapped gaz of metastable helium above the Bose-Einstein transition,
M.Leduc, J.Léonard, F.Pereira Dos Santos, E.Jahier, S.Schwartz and C.Cohen-Tannoudji,
Physica Acta Polonica B 33, p 2213 (2002) download from cond-mat

Production of a Bose-Einstein Condensate of metastable helium atoms
F.Pereira Dos Santos, J.Léonard, Junmin Wang, C.J.Barrelet, F.Perales, E.Rasel, C.S.Unnikrishnan, M.Leduc and C.Cohen-Tannoudji,
Eur. Phys. J. D 19, 103-109 (2002) download from edpsciences

2001

Bose-Einstein Condensation of Metastable Helium,
F.Pereira Dos Santos, J.Léonard, Junmin Wang, C.J.Barrelet, F.Perales, E.Rasel, C.S.Unnikrishnan, M.Leduc, and C.Cohen-Tannoudji,
Phys. Rev. Lett. 86, 3459 (2001) download from APS

Penning collisions of laser-cooled metastable helium atoms,
F.Pereira Dos Santos, F.Perales, J.Léonard, A.Sinatra, Junmin Wang, F.Saverio Pavone, E.Rasel, C.S.Unnikrishnan, and M.Leduc,
Eur. Phys. J. D 14, 15-22 (2001) download from edpsciences

Efficient magneto-optical trapping of a metastable helium gas,
F.Pereira Dos Santos, F.Perales, J.Léonard, A.Sinatra, Junmin Wang, F.Saverio Pavone, E.Rasel, C.S.Unnikrishnan, and M.Leduc,
Eur. Phys. J. AP 14, 69-76 (2001) download from edpsciences

1999

White light transverse cooling of a helium beam,
E.Rasel, F.Pereira Dos Santos, F.Saverio Pavone, F.Perales, C.S.Unnikrishnan, and M.Leduc,
Eur. Phys. J. D 7, 311-316 (1999) download from edpsciences