Un condensat de chrome pour l’étude des interactions dipolaires.

1. Laboratoire de Physique des Lasers Université Paris Nord Villetaneuse - France Un condensat de chrome pour l’étude des interactions dipolaires. Bruno Laburthe Tolra

2. Statistical physics at very low T • Bose-Einstein condensates • Degenerate Fermi gases • What about interactions ? • In most experiments (alkali) – Van-der-Waals interactions • ‘short-range’ (1/r6) • isotropic • Study of dipole-dipole interactions in Quantum Degenerate gases • Dipole-dipole interactions : • ‘long range’ (1/r3) • anisotropic répulsive attractive • chromium: • magnetic moment : 6µB=> dipole dipole interactions x 36 • 1 boson et 1 fermion S=3

3. First BEC : team of T. Pfau (Stuttgart 2005) Phys. Rev. Lett. 94, 160401 (2005) • Ballistic expansion of the BEC modified by dipole dipole interactions • Tune contact interactions using Feshbach resonances: dipolar interaction larger than Van-der-Waals interaction • When edd~1, condensate not stable. Stability depends on trapping geometry. • Collapse of condensate reveals dipolar pattern. Phys. Rev. Lett. 95, 150406 (2005) Nature. 448, 672 (2007) répulsive attractive And… collective excitations, Tc, spinor physics, strong rf fields…

4. All optical production of a Chromium BEC • A Cr BEC in strongrffield • An rf-assisted d-waveFeshbachresonance • Des outils pour les interactions dipolaires

5. 7P4 7P3 650 nm 600 425 nm 550 5S,D (2) (1) 500 427 nm Z 450 500 550 600 650 700 750 7S3 1 10 0 10 -1 10 Phase Sapce Density -2 10 -3 10 -4 10 3 2 4 6 8 10x10 Time (ms) How to make a Chromium BEC in 14s and one slide ? • An atom: 52Cr • An experiment • A small MOT N = 4.106 T=120 μK • A dipole trap • A BEC • An evaporation ramp • A crossed dipole trap

6. All optical production of a Chromium BEC • A Cr BEC in strongrffield • An rf-assisted d-waveFeshbachresonance • Otherthingswecan do

7. Eigenenergies Rf power Control of the Landé factor On canmodify the Landé factor of the atomsgJwithverystrong off resonantrffields. If the RF frequencyωislargerthan the Larmor frequencyω0, gJismodified : • Serge Haroche thesis • S.Haroche, et al., PRL 24 16 (1970) 3 2 1 0 -1 Can we use thisdegeneracy for spinorphysics ? See L. Santos et al., PRA 75, 053606 (2007) -2 -3

8. Modified motion of dressed atoms in a magnetic potential Timescales for adiabaticity of dressing

9. Collision properties of off-resonantly rf dressed states : Elastic s-wave collisions: Rf does not couple different molecular potentials -> s-wave elastic collisions should be unchanged. Dipolar interactions: « geometrical averaging ? » (non calculated) répulsive attractive q

10. Inelastic collision properties of off-resonantly rf dressed states : Beware of the lowest energy state argument !! Two timescales : collision time << dressing time. • No emission of rf photons during a collision • An inelastic collision in a fixed (rf) field -> Dipolar relaxation • Roughly ok for thermalization ? Other atoms ?

11. All optical production of a Chromium BEC • A Cr BEC in strongrffield • An rf-assisted d-waveFeshbachresonance • Otherthingswecan do

12. Superelastic collision A d-wave Feshbach resonance in chromium 0.4 At ultra-low temperature scattering is inhibited in l>0, because atoms need to tunnel through a centrifugal barrier to collide: collisions are « s-wave ». In a « d-wave » Feshbach resonance, tunneling is resonantly increased by the presence of a bound molecular state. 0.3 0.2 Energy (arb.) 0.1 0.0 -0.1 -0.2 1 2 3 4 Internuclear distance (arb.) To probe a feshbach resonance: 3 body losses Tunneling to short internuclear distance is increased by a Feshbach resonance. A third atom triggers superelastic collisions, leading to three-body losses, as the kinetic gained greatly exceeds the trap depth

13. Original temperature dependence Three-body losses measured Feshbach coupling psd • Conclusions: • - « 2-body » three-body losses. • Loss parameter proportionnal to T • -Feshbach coupling measured; very narrow • Useful to tailor anisotropic interactions ?

14. Rf spectrocopy of the Feshbach resonance 0.4 Resonant (three-body) losses when w=Eb-Ei 0.3 0.2 0.1 0.0 Rf photon -0.1 -0.2 1 2 3 4 We modulate the magnetic field close to the Feshbach resonance. The colliding pair of atoms emits a photon while it is colliding, and the pair of atoms is transfered into a bound molecule Rf spectroscopy: not so high precision…

15. Amplitude of lossesanalysis: A radio-frequency assisted d-wave Feshbach resonance in the strong field regime We describe a four body process (three atoms and one photon) by a simple analytical Bessel function !

16. All optical production of a Chromium BEC A Cr BEC in strongrffield An rf-assisted d-waveFeshbachresonance Otherthingswecan do

17. Rfspectro, quadraticligth shifts (QLS) and state preparation Magnetic field - Rf spectroscopy – magnetic field characterization - A BEC near B=0 - Prepare a condensate in arbitrary m states

18. Stern-Gerlach experiments / Rabi oscillations -3 -2 -1 0 1 2 3 2 mm Single shot image from BEC 10 000 atoms Spin population measurement Useful for spinor physics and dipolar physics (spin exchange, relaxation at zero field)

19. Collective excitations « Historically » collective excitations are an excellent tool to probe interactions in condensates . Dipolar effects can be revealed, by measuring collective excitation frequencies to better than 5 percent. Requirements and open questions: • Velocity smaller than sound velocity: dRTF<<RTF • Beware of anharmonicity !! • How far in the Thomas Fermi regime is needed ? Today: shot to shot noise in the TF radius too important. Work on laser pointing stabilization (PZT on mirror). Pressure-driven dynamics Too wide oscillations to reveal dipole interaction

20. 1D Optical lattices • Soon in the lab... • 2D dipolar gases. • repulsive interactions: reduction of three-body recombination events ? (discussions P. Pedri) • Other lattice geometry will need other experimental apparatus

21. Fermion: (a long way) towards a dipolar Fermi sea • 53Cr: MOT • R. Chicireanu et al. • Phys. Rev. A 73, 053406 (2006) N = 5.105 fermions T=120 μK density = 2.5 1010 atoms /cm3 Loading rate = 107 atoms/s • A MOT for a mixture (52Cr- 53Cr): N52,53 ~ 105 atoms Route to degeneracy unknown: Sympathetic cooling ? Scattering cross-section ? Trapping geometry ? Feshbach resonances ? New science chamber design needed • Fermions : non vanishing interactions when T→0 • Thermalization in a polarized Fermi gas ?

22. Thanks! Former PhDs: A. Pouderous R. Chicireanu PhD: Q. Beaufils (2nd year) ATER: T. Zanon (leaving) Permanent people: B. Laburthe-Tolra, E. Maréchal, L. Vernac, (R. Barbé), J.C. Keller O. Gorceix Newt year Paolo Pedri (post-doc, theory) P. Bismut, B. Pasquiou (thèse) Collabration Anne Crubellier (Laboratoire Aimé Cotton) • Fermions: • Phys. Rev. A 73, 053406 (2006) • Cr Metastable: • Phys. Rev. A 76, 023406 (2007) • Optical trapping metastable: • Eur. Phys. J. D 45 189 (2007) • Rf sweeps: • Phys. Rev. A , 77 , 053413 (2008) • BEC: • Phys.Rev. A 77, 061601(R) (2008) Money: Conseil Régional d’Ile de France (Contrat Sésame) Ministère de l’Education, de l’Enseignement Supérieur et de la Recherche European Union (FEDER – Objectif 2) IFRAF (Institut Francilien de Recherche sur les Atomes Froids)