motivation

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