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

Dipole-dipole interactions : long range (1/r 3 ) anisotropic répulsive attractive chromium: Study of dipole-dipole interactions in Quantum Degenerate gases 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/r 6 ) isotropic magnetic moment : 6µ B => dipole dipole interactions x 36 1 boson et 1 fermion S=3

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 dd ~1, condensate not stable. Stability depends on trapping geometry. Collapse of condensate reveals dipolar pattern. First BEC : team of T. Pfau (Stuttgart 2005) Phys. Rev. Lett. 94, (2005) Phys. Rev. Lett. 95, (2005) Nature. 448, 672 (2007) And… collective excitations, Tc, spinor physics, strong rf fields… répulsive attractive

All optical production of a Chromium BEC All optical production of a Chromium BEC A Cr BEC in strong rf field A Cr BEC in strong rf field An rf-assisted d-wave Feshbach resonance An rf-assisted d-wave Feshbach resonance Des outils pour les interactions dipolaires Des outils pour les interactions dipolaires

x Time (ms) Phase Sapce Density How to make a Chromium BEC in 14s and one slide ? 425 nm 427 nm 650 nm 7S37S3 5 S,D 7P37P3 7P47P4 An atom: 52 Cr N = T=120 μK (1) (2) Z An experiment A small MOT A dipole trap A crossed dipole trap An evaporation ramp A BEC

All optical production of a Chromium BEC All optical production of a Chromium BEC A Cr BEC in strong rf field A Cr BEC in strong rf field An rf-assisted d-wave Feshbach resonance An rf-assisted d-wave Feshbach resonance Other things we can do Other things we can do

Control of the Landé factor On can modify the Landé factor of the atoms g J with very strong off resonant rf fields. If the RF frequency ω is larger than the Larmor frequency ω 0, g J is modified : Serge Haroche thesis S.Haroche, et al., PRL (1970) Rf power Eigenenergies Can we use this degeneracy for spinor physics ? See L. Santos et al., PRA 75, (2007)

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

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

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

All optical production of a Chromium BEC All optical production of a Chromium BEC A Cr BEC in strong rf field A Cr BEC in strong rf field An rf-assisted d-wave Feshbach resonance An rf-assisted d-wave Feshbach resonance Other things we can do Other things we can do

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. Superelastic collision 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 A d-wave Feshbach resonance in chromium Internuclear distance (arb.) Energy (arb.)

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

Rf spectrocopy of the Feshbach resonance Resonant (three-body) losses when =E b -E i Rf photon 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…

A radio-frequency assisted d-wave Feshbach resonance in the strong field regime Amplitude of losses analysis: 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 !

All optical production of a Chromium BEC All optical production of a Chromium BEC A Cr BEC in strong rf field A Cr BEC in strong rf field An rf-assisted d-wave Feshbach resonance An rf-assisted d-wave Feshbach resonance Other things we can do Other things we can do

Rf spectro, quadratic ligth shifts (QLS) and state preparation - Rf spectroscopy – magnetic field characterization - A BEC near B=0 - Prepare a condensate in arbitrary m states

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

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: Pressure-driven dynamics Too wide oscillations to reveal dipole interaction Velocity smaller than sound velocity: R TF <<R TF 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).

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

Fermion: (a long way) towards a dipolar Fermi sea 53 Cr: MOT N = fermions T=120 μK density = atoms /cm 3 Loading rate = 10 7 atoms/s R. Chicireanu et al. Phys. Rev. A 73, (2006) A MOT for a mixture ( 52 Cr- 53 Cr): N 52,53 ~ 10 5 atoms Route to degeneracy unknown: Sympathetic cooling ? Scattering cross-section ? Trapping geometry ? Feshbach resonances ? New science chamber design needed Fermions : non vanishing interactions when T0 Thermalization in a polarized Fermi gas ?

Money: Conseil Régional dIle de France (Contrat Sésame) Ministère de lEducation, de lEnseignement Supérieur et de la Recherche European Union (FEDER – Objectif 2) IFRAF (Institut Francilien de Recherche sur les Atomes Froids) Fermions: Phys. Rev. A 73, (2006) Cr Metastable: Phys. Rev. A 76, (2007) Optical trapping metastable: Eur. Phys. J. D (2007) Rf sweeps: Phys. Rev. A, 77, (2008) BEC: Phys.Rev. A 77, (R) (2008) Thanks! Former PhDs: A. Pouderous R. Chicireanu PhD: Q. Beaufils (2 nd 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)