1. Dipolar relaxation in a Chromium Bose Einstein Condensate Benjamin Pasquiou Laboratoire de Physique des Lasers Université Paris Nord Villetaneuse - France Quentin Beaufils, Gabriel Bismut, Paolo Pedri, Bruno Laburthe-Tolra, Etienne Maréchal, Laurent Vernac, Olivier Gorceix.

2.  Chromium : S=3 in the ground state  Large magnetic dipole-dipole interactions Long range (1/r 3 ) Anisotropic ( contrary to contact interactions) Large dipole-dipole interactions + No hyperfine interactions a useful system to study dipolar relaxation Chromium BEC : strong dipolar interactions ++ -- + + - -

3.  Tune scattering length using Feshbach resonances : dipolar interactions larger than contact interactions T.Lahaye et al, Nature. 448, 672 (2007)  Effect of dipole-dipole interactions : T.Lahaye et al, PRL 101, 080401 (2008)  Collapse of a purely dipolar condensate Chromium BEC : strong dipolar interactions collisions with change of total magnetization gain of angular momentum

4. Outline  I) All optical condensation of 52 Cr.  II) Dipolar relaxation in a Chromium BEC

5.  All optical evaporation Oven at 1500 °C 425 nm 427 nm 650 nm 7S37S3 5 S,D 7P37P3 7P47P4  An atom: 52 Cr N = 4.10 6  An oven  A small MOT  A dipole trap  A crossed dipole trap  A BEC every 15 s  A Zeeman slower I) 1 - Overview of the production of a Cr BEC

6.  All optical evaporation 425 nm 427 nm 650 nm 7S37S3 5 S,D 7P37P3 7P47P4  An atom: 52 Cr N = 4.10 6  An oven  A small MOT  A dipole trap  A crossed dipole trap  A BEC every 15 s  A Zeeman slower N = only 4.10 6 bosons! Loading rate = 3.5 10 8 atoms/s Inelastic light assisted collisions (dominant process) R. Chicireanu et al. Phys. Rev. A 73, 053406 (2006) 2 to 3 orders of magnitude larger than in alkalis I) 2 - Cr Magneto-optical traps

7.  All optical evaporation  An atom: 52 Cr N = 4.10 6  An oven  A small MOT  A dipole trap  A crossed dipole trap  A BEC every 15 s  A Zeeman slower IPG fiberized laser - 50W @ 1075 nm Horizontal beam - waist ≈ 40 µm Accumulation of metastable atoms in the Optical Dipole Trap (ODT). These atoms are shielded from light assisted collisions. R Chicireanu et al., Euro Phys J D 45, 189 (2007) 425nm 7P47P4 7S37S3 5D45D4 I) 3 - Accumulation of metastable atoms in an ODT

8. What for : Load all magnetic sublevels How : During loading of the OT, magnetic forces are averaged out by rapidly spin flipping the atoms RF Sweep m>0 m<0 Plus two major improvements : (i) Cancel magnetic forces with an rf field (ii) Depump towards metastable state : 5 S 2 5S25S2 7S37S3 7P37P3 7P47P4 425nm 427nm 633nm 663nm 654nm Load 5 D 4 et 5 D 3 : 1.2 million atoms RF Sweeps : 2 million atoms Q. Beaufils et al., Phys. Rev. A 77, 053413 (2008) What we expect : A lower inelastic loss parameter ? A larger loading rate ? (i)*(ii) Load 5 D 4 et 5 S 2 and rf sweeps : 5 to 6 million atoms in the single beam ODT (1075 nm, 35 W) More than in the MOT! Loading time : 100 ms Temperature : 100 µK.

9.  All optical evaporation 425 nm 427 nm 650 nm 7S37S3 5 S,D 7P37P3 7P47P4  An atom: 52 Cr  An oven  A small MOT  A dipole trap  A crossed dipole trap  A BEC every 15 s  A Zeeman slower I) 4 - Evaporative cooling and Chromium BEC In situ TF radii : 4 and 5 µm Density : 6.10 13 atoms/cm 3 - 2.10 14 atoms/cm 3 Condensates lifetime : a few seconds. Chemical potentential : about 1 kHz - 4 kHz  Atoms back in the ground state, in the lowest energy Zeeman state m = -3  15 seconds evaporation ramp Pure BEC: 10 000 to 20 000 atoms Q.Beaufils et al., Phys. Rev. A 77, 061601(R) (2008)

10. Outline  I) All optical condensation of 52 Cr  II) Dipolar relaxation in a Chromium BEC

11. What is dipolar relaxation ? Only two channels for dipolar relaxation in m = 3 (no relaxation in m = -3) : II) 1 – Dipolar relaxation Δm S = -1 Δm S = -2 Our BEC is in m = -3 Zeeman substate Change to m = +3 to see dipolar relaxation use of rf sweep We observe dipolar relaxation Not seen in Rb BEC (negligible) The kinetic energy gain makes the atoms leave the trap

12.  Experimental procedure Fit gives β  Typical results II) 2 – Experimental procedure Rf sweep 2 Produce BEC m = -3 detect BEC m = -3 Rf sweep 1 BEC m = +3, varying time Time (ms) Atom number Static magnetic field two-body collision rate In a BEC : BEC lost

13. It has been shown ( S.Hensler, Appl. Phys. B, 77, 765 (2003) ) that the Born approximation is valid for B 10 G… not in between ! II) 2 – Comparison theory - experiment  BEC m = +3 measurements Born approximation predictions (BEC) Magnetic field (G) Two body loss parameter 10 13 cm 3 /s -1

14. It has been shown ( S.Hensler, Appl. Phys. B, 77, 765 (2003) ) that the Born approximation is valid for B 10 G… not in between ! II) 2 – Comparison theory - experiment  BEC m = +3 measurements Born approximation predictions (BEC) Thermal gas 5 µK measurements Born approximation predictions (thermal gas) Magnetic field (G) Two body loss parameter 10 13 cm 3 /s -1

15. It has been shown ( S.Hensler, Appl. Phys. B, 77, 765 (2003) ) that the Born approximation is valid for B 10 G… not in between ! II) 2 – Comparison theory - experiment  BEC m = +3 measurements Born approximation predictions (BEC) First theoretical calculations (A. Crubellier) Thermal gas 5 µK measurements Born approximation predictions (thermal gas) Magnetic field (G) Two body loss parameter 10 13 cm 3 /s -1

16. l = 0 l = 2 Avoided crossing gap ≈ V dd E = g J µ B B II) 3 – Interpretation Interparticle distance Interatomic potentials aSaS Interparticle distance = a s Zero coupling Determination of scattering lengths S=6 and S=4 (in progress, Anne Crubellier)

17. Summary  All optical production of a chromium BEC.  Observation of the evolution of dipolar relaxation in a thermal gas and a BEC, with a static magnetic field.  Good agreement with Born approximation, but observation of a reduction of dipolar relaxation for a range of field. Discrepancy due to a zero coupling between input and output channel.

18. Other work on dipolar relaxation  Dipolar relaxation in reduced dimensions 1D Lattice (retro-reflected Verdi laser) Cr BEC diffracted by lattice Rf sweep 2 Produce BEC m = -3 detect BEC m = -3 Rf sweep 1 BEC m = +3, varying time Static magnetic field Load optical lattice  Control of dipolar relaxation with strong rf field We observe experimentally and caracterize rf assisted dipolar relaxation, in presence of a strong off-resonance rf magnetic field

19. Future  Optical lattices – dipolar gases in reduced dimensions.  Feshbach resonances – pure dipolar gases.  Fermions – degenerate Fermi sea of polarized atoms with dipole- dipole interactions.

20. Have left: T. Zanon, R. Barbé, A. Pouderous, R. Chicireanu Collaboration: Anne Crubellier (Laboratoire Aimé Cotton) B. Pasquiou O. Gorceix Q. Beaufils P. Pedri B. Laburthe L. Vernac J. C. Keller E. Maréchal G. Bismut