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Dipolar relaxation in a Chromium Bose Einstein Condensate

Dipolar relaxation in a Chromium Bose Einstein Condensate. 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. Benjamin Pasquiou.

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Dipolar relaxation in a Chromium Bose Einstein Condensate

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  1. Dipolar relaxation in a Chromium Bose Einstein Condensate 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. Benjamin Pasquiou

  2. Chromium BEC : strong dipolar interactions • Chromium : S=3 in the ground state • Large magnetic dipole-dipole interactions • Long range (1/r3) • Anisotropic ( contrary to contact interactions) + - + + - - + - Large dipole-dipole interactions + No hyperfine interactions a useful system to study dipolar relaxation

  3. Chromium BEC : strong dipolar interactions • Collapse of a purely dipolar condensate T.Lahaye et al, PRL 101, 080401 (2008) • 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 : collisions with change of total magnetization gain of angular momentum

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

  5. 7P4 7P3 650 nm 425 nm 5S,D 427 nm 7S3 I) 1 - Overview of the production of a Cr BEC • An atom: 52Cr • An oven • A Zeeman slower • A small MOT Oven at 1500 °C N = 4.106 • A dipole trap • A BEC every 15 s • All optical evaporation • A crossed dipole trap

  6. 7P4 7P3 650 nm 425 nm 5S,D 427 nm 7S3 I) 2 - Cr Magneto-optical traps • An atom: 52Cr • An oven • A Zeeman slower • A small MOT N = only 4.106 bosons! Loading rate = 3.5 108 atoms/s N = 4.106 Inelastic light assisted collisions (dominant process) 2 to 3 orders of magnitude larger than in alkalis R. Chicireanu et al. Phys. Rev. A 73, 053406 (2006) • A dipole trap • A BEC every 15 s • All optical evaporation • A crossed dipole trap

  7. I) 3 - Accumulation of metastable atoms in an ODT • An atom: 52Cr • An oven • A Zeeman slower • A small MOT • IPG fiberized laser - 50W @ 1075 nm • Horizontal beam - waist ≈ 40 µm 7P4 5D4 425nm N = 4.106 Accumulation of metastable atoms in the Optical Dipole Trap (ODT). These atoms are shielded from light assisted collisions. 7S3 R Chicireanu et al., Euro Phys J D 45, 189 (2007) • A dipole trap • A BEC every 15 s • All optical evaporation • A crossed dipole trap

  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 7P4 m<0 m>0 7P3 654nm 425nm 633nm 663nm 427nm 5S2 7S3 Plus two major improvements : (i) Cancel magnetic forces with an rf field • (i)*(ii) Load 5D4 et 5S2 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. RF Sweeps : 2 million atoms Q. Beaufils et al., Phys. Rev. A 77, 053413 (2008) (ii) Depump towards metastable state : 5S2 • Whatweexpect : • A lowerinelasticlossparameter ? • A largerloading rate ? Load 5D4 et 5D3 : 1.2 million atoms

  9. 7P4 7P3 650 nm 425 nm 5S,D 427 nm 7S3 I) 4 - Evaporative cooling and Chromium BEC • An atom: 52Cr • An oven • A Zeeman slower • A small MOT • 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 In situ TF radii : 4 and 5 µm Density : 6.1013 atoms/cm3- 2.1014 atoms/cm3 Condensates lifetime : a few seconds. Chemical potentential : about 1 kHz - 4 kHz Q.Beaufils et al., Phys. Rev. A 77, 061601(R) (2008) • A dipole trap • A BEC every 15 s • All optical evaporation • A crossed dipole trap

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

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

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

  13. II) 2 – Comparison theory - experiment It has been shown (S.Hensler, Appl. Phys. B, 77, 765 (2003)) that the Born approximation is valid for B < 1G and B > 10 G… not in between ! Born approximation predictions (BEC) • BEC m = +3 measurements Two body loss parameter 1013 cm3/s-1 Magnetic field (G)

  14. II) 2 – Comparison theory - experiment It has been shown (S.Hensler, Appl. Phys. B, 77, 765 (2003)) that the Born approximation is valid for B < 1G and B > 10 G… not in between ! Born approximation predictions (BEC) • BEC m = +3 measurements Born approximation predictions (thermal gas) Thermal gas 5 µK measurements Two body loss parameter 1013 cm3/s-1 Magnetic field (G)

  15. II) 2 – Comparison theory - experiment It has been shown (S.Hensler, Appl. Phys. B, 77, 765 (2003)) that the Born approximation is valid for B < 1G and B > 10 G… not in between ! Born approximation predictions (BEC) • BEC m = +3 measurements First theoretical calculations (A. Crubellier) Born approximation predictions (thermal gas) Thermal gas 5 µK measurements Two body loss parameter 1013 cm3/s-1 Magnetic field (G)

  16. II) 3 – Interpretation Avoided crossing gap ≈ Vdd Interparticle distance l = 0 E = gJ µB B Interatomic potentials l = 2 aS Interparticle distance = as Zero coupling Determination of scattering lengths S=6 and S=4 (in progress, Anne Crubellier)

  17. Summary All opticalproduction of a chromium BEC. Observation of the evolution of dipolar relaxationin a thermal gas and a BEC, with a static magnetic field. Good agreement with Born approximation, but observation of a reduction of dipolar relaxationfor a range of field. Discrepancy due to a zero couplingbetween input and output channel.

  18. Otherwork on dipolar relaxation Rf sweep 2 Rf sweep 1 • Dipolar relaxation in reduced dimensions 1D Lattice (retro-reflected Verdi laser) Produce BEC m = -3 BEC m = +3, varying time detect BEC m = -3 Static magnetic field Load optical lattice Cr BEC diffracted by 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. P. Pedri G. Bismut L. Vernac B. Pasquiou Q. Beaufils O. Gorceix E. Maréchal J. C. Keller B. Laburthe Have left: T. Zanon, R. Barbé, A. Pouderous, R. Chicireanu Collaboration:Anne Crubellier (Laboratoire Aimé Cotton)

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