1. Towards a finite ensemble of ultracold fermions Timo Ottenstein Max-Planck-Institute for Nuclear Physics Heidelberg 19th International IUPAP Conference on few-body problems in physics Fermi sea few fermions

2. 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!

3. Timo Ottenstein – 19th International IUPAP Conference on few-body problems 3 / 13 Our System Ultracold 6 Li atoms in the two lowest magnetic substates in an optical dipole trap Evaporative 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

4. 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 measurements a)Counting single atoms b)Spatially resolved single atom detection 1 2N On demand with high fidelity!

5. Timo Ottenstein – 19th International IUPAP Conference on few-body problems 5 / 13 Our Approach switch off shallow trap micrometer size trap thermal equilibrium: T/T F decreases by a factor of ~6! optical beam trap we aim for: obtain high occupation probability close to 1 state of the art: degenerate Fermi gas (N~10 5 ) in a shallow optical dipole trap, T ≈ 0.05 T F B. Demarco, et al., Science 285, 1703 (1999) M. Bartenstein, et al., PRL 92, 12 (2004)

6. Timo Ottenstein – 19th International IUPAP Conference on few-body problems 6 / 13 “tilt the trap“ Control of the atom number We aim for a control of the atom number on the single particle level.

7. Timo Ottenstein – 19th International IUPAP Conference on few-body problems 7 / 13 The microtrap focus (~3  m) 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)

8. Timo Ottenstein – 19th International IUPAP Conference on few-body problems 8 / 13 Atoms in the microtrap! N = 150.000 T = 200 nK T/TF = 0.27 N = 5.000 T = 200 nK deeply degenerate

9. 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 magnetic field gradient. N = 120 +/- 11 Observed atom number fluctuations also caused by imaging technique!  Go for single atom detection using fluorescence imaging.

10. 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 weak Magneto-optical trap. CCD We are able to detect single atoms as discrete steps in the fluorescence signal on the CCD camera.

11. 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

12. 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

13. 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