Transcript Evidence for collisions in a magnetically guided beam

Strong dipolar effects in a Chromium BEC
A quantum ferrofluid
T. Koch, T. Lahaye, B. Fröhlich, J. Metz, M. Fattori,
A. Griesmaier, S. Giovanazzi and T. Pfau
5. Physikalisches Institut, Universität Stuttgart
Assisi – June 6th 2007
Interacting quantum systems in AMO physics
Contact interaction
Dipole-dipole interaction
Coulomb interaction
Short range
Isotropic
Long range - Anisotropic
Long range
Isotropic
MIT
Innsbruck
New physics in dipolar quantum gases
Dipole-dipole interactions are:
- anisotropic
- instability
- modified dispersion relation
(roton)
- new equilibrium shapes
(biconcave BEC)
- long range
- new quantum phases in optical lattices
- supersolid phase
pancake
Outline – BEC with MDDI
• Chromium
How to get a Chromium BEC?
Dipolar expansion
• Demagnetization cooling
• Strong dipolar effects in a Cr BEC
• Outlook
I. Chromium
Yb
Cr  [ Ar ]3d 5 4s1
Ground state 7S3
Magnetic dipole moment m = 6mB.
Way to BEC
• Continously loaded Ioffe Pritchard trap
(CLIP-trap)
J. Stuhler et al. PRA 64, 031405 (2001); P. O. Schmidt et
al. J. Opt. B 5, S170 (2003)
• Doppler cooling in compressed IP-trap
P. O. Schmidt, et al., J. Opt. Soc. Am. B 20, 5 (2003)
>108 atoms in the ground state
phase space density ~10-7
• Rf-evaporation
• Stop by dipolar relaxation!
No cold & dense cloud (no BEC) in MT!
S. Hensler et al., Appl. Phys. B 77, 765 (2003)
m=3
m=2
m=1
+E
+2E
Transfer to optical dipole trap
optical pumping in mj=-3
s-
7P
3
7S
3
Advantages:
• all magnetic substates are trapped
(no dip. relaxation)
• operation at arbitrary magnetic offset
field (Feshbach resonance)
mj= -3
mj= +3
Forced evaporation in ODT
horizontal
beam
vertical
beam
BEC with up to 100.000 atoms
Dipolar expansion of a BEC
Density
PRL 95, 150406 (2005).
PRA 74, 013621 (2006).
Mean-field potential
due to MDDI
First Observation of mechanical effect of a
homogenous magnetic field on a gas
Elongation along magnetization direction!
II. Demagnetization cooling
Why another cooling scheme ?????
► doppler cooling techniques
limited by reabsorption
► evaporative cooling
throw away 99 % of your atoms
► demagnetization cooling
Kastler, Journal de physique et le radium 11, 255 (1950).
Cirac, Lewenstein, Phys Rey A 52, 6 (1995).
basic idea
1. Initialization
2. Lowering B-field
Needed:
1. Suitable level scheme
2. Strong enough coupling
m = -1
m = -2
m = -3
3. Optical pumping
7P
3
s-
7S
3
-E
mj= +3
mj= -3
s1  S 3
T0? Solid vs.gas
decrease of B-field
solid
gas
kB
kB
spins
phonons
phonons
kB
kB
kB
Tf
spins
c
 i  1
Ti c f
Tf
Ti
kB
kB

ci 3

cf 4
But we can pump back !
kB
Results: Single step
1G
50mG
M. Fattori et.al. Nature Physics 2 , 765 (2006)
Experimental challenges
bad polarization due to
(a) badly polarized light
(b) transverse magnetic fields

(a) polarization quality 1/1000
(b) transverse fields
below 5mG
Results: Optimized ramps
d ln 
  11
d ln N
III. Strong dipolar effects in a BEC
Strength of the dipole-dipole interaction:
Heteronuclear molecules
(electric dipole moment d )
Atoms with large magnetic
dipole moment m.
Chromium: 6mB.
Large d (~1 Debye):
No BEC yet
Small edd… but a tunable
BEC !!!
Griesmaier et.al. PRL 97, 250402 (2006)
Griesmaier et.al. PRL 94, 160401 (2005)
Tuning a with a Feshbach resonance
collision with molecular potential V(R):
V’(R) with Ms’ ≠ Ms + B-field + coupling:
V’(R) V(R)
Vc
Ec
 a ! describes scattering @ low T
 a is modified !
scattering length a can be tuned with B-field !
Tuning a with a Feshbach resonance
Broadest resonance at 589.1 G ( = 1.7 G)
Field stability better than 10-4 required!
[J. Werner et al., PRL 94, 183201, (2005)]
Tuning the scattering length
Without MDDI: measure a through the released energy a ~ R5 / N
Correct for the MDDI effects (hydrodynamic theory, TF regime).
Aspect ratio vs. edd
Theory without any adjustable parameter !!!
Dipolar expansion with tunable εdd
„εdd=0“
εdd=0.5
εdd=0.16
εdd=0.75
Stuhler et.al. PRL 95 , 150406 (2005)
Lahaye et.al. Nature in press
UseLimits:
of a Feshbach
inelasticresonance
losses
1 / e lifetime of the condensate:
Lifetime [ms]
1000
100
10
-15
-10
-5
0
5
Magnetic field B-B0 [G]
10
15
Summary and Outlook
I. Dipole-dipole interaction
& ultracold Cr atoms
II. Demagnetization cooling
III. New regime of strong
dipolar interactions
 New physics
1D lattice:
A stack of pancakes
Thanks for your attention!
The Cr team:
T. Lahaye
B. Fröhlich
M. Fattori
T. Koch
T. Pfau A. Griesmaier J. Metz
Theory:
http://www.pi5.uni-stuttgart.de/
S. Giovanazzi
SFB/TR 21
SPP1116
Summary and Outlook
• By tuning a we enter a new regime
Ø stabilize the BEC with respect to dipolar collapse?
Ø study spectrum of excitations?
Ø (more) stable molecules?
• One-dimensional optical lattice: a stack of pancake traps.
Ø stabilize the BEC with respect to dipolar collapse?
Ø study spectrum of excitations?
Ø (more) stable molecules?
title