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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

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

slide2

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

slide3

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

outline
Outline
  • I) All optical condensation of 52Cr.
  • II) Dipolar relaxation in a Chromium BEC
slide5

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
slide6

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
slide7

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
slide8
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

slide9

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
outline10
Outline
  • I) All optical condensation of 52Cr
  • II) Dipolar relaxation in a Chromium BEC
slide11

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

slide12

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

slide13

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)

slide14

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)

slide15

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)

slide16

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)

summary
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.

slide18

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

future
Future

Optical lattices– dipolar gases in reduced dimensions.

Feshbach resonances– pure dipolar gases.

Fermions – degenerate Fermi sea of polarized atoms with dipole-dipole interactions.

slide20

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)