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!
À Our System Ultracold 6Li 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!
Experimental Challenges • Preparation of a deeply degenerate Fermi gas with defined atom number • Precise control over quantum states in the trap • 2. Detection and quantitative measurements • Counting single atoms • Spatially resolved single atom detection On demand with high fidelity! 2 N 1
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) optical beam trap Our Approach micrometer size trap • thermal equilibrium: T/TF decreases by a factor of ~6! • switch off shallow trap
Control of the atom number “tilt the trap“ We aim for a control of the atom number on the single particle level.
The microtrap 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)
Atoms in the microtrap! N = 150.000 T = 200 nK T/TF = 0.27 N = 5.000 T = 200 nK deeply degenerate
Current status Observed atomnumber statistics for highest value of magnetic field gradient. Apply magnetic field gradient after transfer of the atoms into the microtrap. N = 120 +/- 11 Observed atom number fluctuations also caused by imaging technique! Go for single atom detection using fluorescence imaging.
CCD Fluorescence imaging Proof of principle experiment: Measure fluorescence signal of single atoms in a weak Magneto-optical trap. We are able to detect single atoms as discrete steps in the fluorescence signal on the CCD camera.
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
|3> RF |2> |1> 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? ? • Probe single particle excitations in a finite Fermi system by radio • frequency (RF) spectroscopy a 0 ideal Fermi gas
Thank you! The ultracold quantum gases group @ MPIK Heidelberg Andre Wenz (currently @ UC Berkeley) Timo Ottenstein Friedhelm Serwane Gerhard Zuern Selim Jochim Thomas Lompe