T. Koch, T. Lahaye, B. Fröhlich, J. Metz, M. Fattori, A. Griesmaier, S. Giovanazzi and T. Pfau
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Transcript of T. Koch, T. Lahaye, B. Fröhlich, J. Metz, M. Fattori, A. Griesmaier, S. Giovanazzi and T. Pfau
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
Strong dipolar effects in a Chromium BEC A quantum ferrofluid
Interacting quantum systems in AMO physics
Long range Isotropic
Short rangeIsotropic
Coulomb interactionDipole-dipole interactionContact interaction
MIT Innsbruck
Long range - Anisotropic
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
• ChromiumHow to get a Chromium BEC?Dipolar expansion
• Demagnetization cooling
• Strong dipolar effects in a Cr BEC • Outlook
Outline – BEC with MDDI
I. Chromium
Yb
5 1[ ]3 4Cr Ar d s
Ground state 7S3
Magnetic dipole moment = 6B.
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-trapP. 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)
+E +2Em = 3m = 2m = 1
Transfer to optical dipole trap
Advantages:• all magnetic substates are trapped (no dip. relaxation)• operation at arbitrary magnetic offset field (Feshbach resonance)
optical pumping in mj=-3
mj= -3 mj= +37S3
7P3
Forced evaporation in ODT
BEC with up to 100.000 atoms
horizontal beam
verticalbeam
Dipolar expansion of a BEC
Elongation along magnetization direction!
Density
Mean-field potentialdue to MDDI
PRL 95, 150406 (2005).PRA 74, 013621 (2006).
First Observation of mechanical effect of a homogenous magnetic field on a gas
II. Demagnetization cooling
Why another cooling scheme ?????
► doppler cooling techniqueslimited by reabsorption
► evaporative coolingthrow 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 3. Optical pumping2. Lowering B-field
Needed:1. Suitable level scheme2. Strong enough coupling
31 S
mj= -3 mj= +37S3
7P3
-Em = -1m = -2m = -3
T0? Solid vs.gas
1f i
i f
T cT c
decr
ease
of B
-fie
ld
solid
kB
spins phonons
gas
kB
kBkB
kB
kBkB
kB
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f i
i f
T cT c
spins phonons
But we can pump back !
Results: Single step
M. Fattori et.al. Nature Physics 2 , 765 (2006)
1G
50mG
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
ln 11ln
dd N
Atoms with large magnetic dipole moment .
Chromium: 6B.
Small dd… but a tunableBEC !!!
III. Strong dipolar effects in a BEC
Strength of the dipole-dipole interaction:
Heteronuclear molecules(electric dipole moment d )
Large d (~1 Debye):
No BEC yet Griesmaier et.al. PRL 97, 250402 (2006) Griesmaier et.al. PRL 94, 160401 (2005)
Tuning a with a Feshbach resonance
scattering length a can be tuned with B-field !
V(R)
collision with molecular potential V(R):
Ec
a ! describes scattering @ low T
V’(R)
V’(R) with Ms’ ≠ Ms + B-field
Vc
a is modified !
+ coupling:
[J. Werner et al., PRL 94, 183201, (2005)]
Broadest resonance at 589.1 G ( = 1.7 G)Field stability better than 10-4 required!
Tuning a with a Feshbach resonance
Tuning the scattering lengthWithout MDDI: measure a through the released energy a ~ R5 / N
Correct for the MDDI effects (hydrodynamic theory, TF regime).
Aspect ratio vs. dd
Theory without any adjustable parameter !!!
Dipolar expansion with tunable εdd
εdd=0.16
„εdd=0“
εdd=0.75
εdd=0.5
εdd=0.16
„εdd=0“„εdd=0“
Stuhler et.al. PRL 95 , 150406 (2005)
Lahaye et.al. Nature in press
1 / e lifetime of the condensate:
Limits: inelastic losses
-15 -10 -5 0 5 10 1510
100
1000
Li
fetim
e [m
s]
Magnetic field B-B0 [G]
Use of a Feshbach resonance
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!
T. Lahaye B. Fröhlich M. Fattori T. Koch T. Pfau A. Griesmaier J. Metz
Theory:
S. Giovanazzi
http://www.pi5.uni-stuttgart.de/
SFB/TR 21 SPP1116
The Cr team:
Summary and Outlook
• 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?
• 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?
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