Laboratoire de Physique des Lasers Universit Paris Nord

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

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

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

I) 1 - Overview of the production of a Cr BEC § An atom: 52 Cr 7 P § An oven § A Zeeman slower 4 Oven at 1500 °C 7 P 650 nm 3 425 nm 5 S, D 427 nm 7 S § A small MOT N = 4. 106 3 § A dipole trap § A BEC every 15 s § All optical evaporation § A crossed dipole trap

I) 2 - Cr Magneto-optical traps § An atom: 52 Cr 7 P § An oven § A Zeeman slower 4 7 P 650 nm 3 425 nm 5 S, D N = only 4. 106 bosons! Loading rate = 3. 5 108 atoms/s 427 nm 7 S Inelastic light assisted collisions (dominant process) 3 § A small MOT N = 4. 106 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

I) 3 - Accumulation of metastable atoms in an ODT § An atom: 52 Cr 7 S § A Zeeman slower § A small MOT 4 5 D 425 nm 7 P § An oven 4 • IPG fiberized laser - 50 W @ 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. 3 N = 4. 106 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

Plus two major improvements : (i) Cancel magnetic forces with an rf field • What for : Load all magnetic sublevels RF Sweep • How : m<0 m>0 During loading of the OT, magnetic 5 D et 5 S and rf sweeps : Loadout forces are(i)*(ii) averaged 4 by rapidly 2 RF Sweeps : 2 million atoms spin flipping the atoms 5 to 6 million atoms in the single beam ODT (1075 nm, 35 W) Q. Beaufils et al. , Phys. Rev. A 77, 053413 (2008) More than in the MOT! (ii) Depump towards metastable state : 5 S 2 • Loading time : 100 ms • : Temperature : 100 µK. • What we expect • A lower inelastic loss parameter ? A larger loading rate ? 7 P 7 P 4 3 425 nm 654 nm 633 nm 427 nm Load 5 D 4 et 5 D 3 : 1. 2 million atoms 663 nm 5 S 7 S 3 2

I) 4 - Evaporative cooling and Chromium BEC § An atom: 52 Cr 7 P § A Zeeman slower 4 7 P 650 nm 3 425 nm 5 S, D 427 nm 7 S § An oven § 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 3 In situ TF radii : 4 and 5 µm Density : 6. 1013 atoms/cm 3 - 2. 1014 atoms/cm 3 Condensates lifetime : a few seconds. Chemical potentential : about 1 k. Hz - 4 k. Hz Q. Beaufils et al. , Phys. Rev. A 77, 061601(R) (2008) § A BEC every 15 s § All optical evaporation § A crossed dipole trap § A dipole trap

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

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) : Δm. S = -1 Δm. S = -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

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

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

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 < 1 G and B > 10 G… not in between ! Born approximation predictions (thermal gas) Two body loss parameter 1013 cm 3/s-1 Thermal gas 5 µK measurements § Born approximation predictions (BEC) BEC m = +3 measurements Magnetic field (G)

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 < 1 G and B > 10 G… not in between ! Born approximation predictions (thermal gas) Two body loss parameter 1013 cm 3/s-1 Thermal gas 5 µK measurements § Born approximation predictions (BEC) BEC m = +3 measurements First theoretical calculations (A. Crubellier) Magnetic field (G)

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

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.

Other work on dipolar relaxation § Dipolar relaxation in reduced dimensions 1 D Lattice (retro-reflected Verdi laser) Static magnetic field ep we ce d oa L ic pt al ti t a l o Produce BEC m = -3 R eep w fs 1 BEC m = +3, varying time 2 s Rf detect BEC m = -3 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

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

ix ce or . G O Q . B ea uf rth bu La ils e r lle B. J. C. sq Pa B. Pe dr i ac rn Ve L. Ke ui ou l ha E. M ar éc ut G . B ism Have left: T. Zanon, R. Barbé, A. Pouderous, R. Chicireanu Collaboration: Anne Crubellier (Laboratoire Aimé Cotton)