Towards a finite ensemble of ultracold fermions
Timo OttensteinMax-Planck-Institute for Nuclear Physics Heidelberg
19th International IUPAP Conference on few-body problems in physics
Fermi seafew fermions
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 2 / 13
Motivation
• few fermion systems in nature– quarks in hadrons– nucleons in nuclei– electrons in atoms
• challenging problems– pairing– energy spectrum of the system
Ultracold atoms offer a simple and easy accessible model system in a tabletop experiment!
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 3 / 13
Our System
Ultracold 6Li atoms in the two lowest magnetic substates in an optical dipole trapEvaporative cooling ~100 nK
Interaction described by one single parameter the s-wave scattering length a! a can be tuned by means of broad Feshbach resonance! for a range of interaction potential, properties of the system are universal!
À
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 4 / 13
Experimental Challenges
1. Preparation of a deeply degenerate Fermi gas with defined atom number Precise control over quantum states in the trap
2. Detection and quantitative measurementsa) Counting single atomsb) Spatially resolved single atom detection
1 2 N
On demandwith high fidelity!
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 5 / 13
Our Approach
• switch off shallow trap
micrometer size trap
• thermal equilibrium: T/TF decreases by a factor of ~6!
TkBoptical beam trap 2
0w
P
we aim for:
obtain high occupation probability close to 1
state of the art:
degenerate Fermi gas (N~105)in a shallow optical dipole trap, T ≈ 0.05 TF
B. Demarco, et al., Science 285, 1703 (1999)
M. Bartenstein, et al., PRL 92, 12 (2004)
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 6 / 13
)(xB
“tilt the trap“
Control of the atom number
We aim for a control of the atom number on the single particle level.
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 7 / 13
The microtrap
20
04
mw
Ur
focus (~3m) of a red detuned beam high field seeking atoms
trap frequencies: r ~ 2 × 3.8 kHz
z ~ 2 × 160 Hz
use high NA aspheric lens for microtrap
(P = 1 mW)
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 8 / 13
Atoms in the microtrap!
N = 150.000T = 200 nKT/TF = 0.27
N = 5.000T = 200 nKdeeply degenerate
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 9 / 13
Current status
Apply magnetic field gradient after transfer of the atoms into the microtrap.
Observed atomnumber statistics for highest value of magneticfield gradient.
N = 120 +/- 11
Observed atom number fluctuations also caused by imaging technique! Go for single atom detection using fluorescence imaging.
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 10 / 13
Fluorescence imaging
Proof of principle experiment:Measure fluorescence signal of single atoms in a weakMagneto-optical trap.
CCD
We are able to detect single atoms as discrete steps in the fluorescence signalon the CCD camera.
0 50 100 150 200 250 3000.0
0.5
1.0
1.5
2.0
2.5
3.0
2 atoms
3 atoms
Flu
ore
scen
ce s
ign
al [
a.u
.]
Time [a.u.]
no atoms
1 atom
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 11 / 13
Conclusion
- Ultracold atoms provide a clean and easy accessible model system for finite fermionic systems in nature
Current status:
- Implementation of a microtrap in the experimental setup- Control of the atom number in the regime of ~100 atoms- Fluorescence detection of single atoms in a weak MOT
Next steps:
- Design of a new lens system for a tighter focus and higher imaging resolution
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 12 / 13
Let’s do physics!
• spill atoms from the trap with interaction switched on and compare with ideal gas case How do interactions change the energy of the system? For a , what is in a finite system?
ideal Fermi gas
a 0 ?• Probe single particle excitations in a finite Fermi system by radio frequency (RF) spectroscopy
|1>
|2>
|3>RF
Timo Ottenstein – 19th International IUPAP Conference on few-body problems 13 / 13
Thank you!
Thomas Lompe
Gerhard Zuern
Friedhelm Serwane
Selim Jochim
Timo Ottenstein
Andre Wenz(currently
@ UC Berkeley)
The ultracold quantum gases group @ MPIK Heidelberg