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Towards a finite ensemble of
ultracold fermions
Fermi sea
few fermions
Timo Ottenstein
Max-Planck-Institute for Nuclear Physics Heidelberg
19th International IUPAP Conference
on few-body problems in physics
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
2 / 13
Our System
scattering length (1000 a0)
Ultracold 6Li atoms in the two lowest magnetic substates in an optical dipole trap
Evaporative cooling  ~100 nK
4
2
0
-2
-4
0
500
1000
1500
magnetic field (G)
Interaction described by one single parameter the s-wave scattering length a!
 a can be tuned by means of broad Feshbach resonance!
for a À range of interaction potential, properties of the system
are universal!
Timo Ottenstein – 19th International IUPAP Conference on few-body problems
3 / 13
Experimental Challenges
1. Preparation of a deeply degenerate Fermi gas with defined atom number
 Precise control over quantum states in the trap
On demand
with high
fidelity!
1
2
N
2. Detection and quantitative measurements
a) Counting single atoms
b) Spatially resolved single atom detection
Timo Ottenstein – 19th International IUPAP Conference on few-body problems
4 / 13
Our Approach
state
of the
we aim
for:art:
obtain highFermi
occupation
degenerate
gas (N~105)
1
inprobability
a shallow close
opticaltodipole
trap,
T ≈ 0.05 TF
kBT  
P
optical beam trap  
M. Bartenstein, et al., PRL 92, 12 (2004)
w02
B. Demarco, et al., Science 285, 1703 (1999)
micrometer size trap
• thermal equilibrium: T/TF decreases by a factor of ~6!
• switch off shallow trap
Timo Ottenstein – 19th International IUPAP Conference on few-body problems
5 / 13
Control of the atom number
“tilt the trap“
B (x)
We aim for a control of the atom number on the single particle level.
Timo Ottenstein – 19th International IUPAP Conference on few-body problems
6 / 13
The microtrap
use high NA aspheric lens for microtrap
focus (~3m) of a red detuned beam
high field seeking atoms
trap frequencies:
r ~ 2 × 3.8 kHz
z ~ 2 × 160 Hz
r 
4U 0
mw02
Timo Ottenstein – 19th International IUPAP Conference on few-body problems
(P = 1 mW)
7 / 13
Atoms in the microtrap!
N = 150.000
T = 200 nK
T/TF = 0.27
N = 5.000
T = 200 nK
deeply
degenerate
Timo Ottenstein – 19th International IUPAP Conference on few-body problems
8 / 13
Current status
Apply magnetic field gradient after
transfer of the atoms into the microtrap.
Observed atomnumber statistics
for highest value of magnetic
field 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
9 / 13
Fluorescence imaging
Proof of principle experiment:
Measure fluorescence signal of single atoms in a weak
Magneto-optical trap.
CCD
Fluorescence signal [a.u.]
3.0
3 atoms
2.5
2.0
2 atoms
1.5
1 atom
1.0
We are able to detect single atoms as
discrete steps in the fluorescence signal
on the CCD camera.
0.5
no atoms
0.0
0
50
100
150
200
250
300
Time [a.u.]
Timo Ottenstein – 19th International IUPAP Conference on few-body problems
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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
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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?
a0
• Probe single particle excitations in a
finite Fermi system by radio
frequency (RF) spectroscopy
?
ideal Fermi gas
|3>
RF
|2>
|1>
Timo Ottenstein – 19th International IUPAP Conference on few-body problems
12 / 13
Thank you!
The ultracold quantum gases group @ MPIK Heidelberg
Andre Wenz
(currently
@ UC Berkeley)
Timo Ottenstein
Friedhelm Serwane
Gerhard Zuern
Selim Jochim
Thomas Lompe
Timo Ottenstein – 19th International IUPAP Conference on few-body problems
13 / 13