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Ultracold Quantum Gases: An Experimental Review Herwig Ott University of Kaiserslautern OPTIMAS Research Center.

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Presentation on theme: "Ultracold Quantum Gases: An Experimental Review Herwig Ott University of Kaiserslautern OPTIMAS Research Center."— Presentation transcript:

1 Ultracold Quantum Gases: An Experimental Review Herwig Ott University of Kaiserslautern OPTIMAS Research Center

2 Outline Laser cooling, magnetic trapping and BEC Optical dipole traps, fermions Optical lattices: Superfluid to Mott insulator transition Magnetic microtraps: Atom chips and 1D physics

3 Outline Feshbach resonances: taming the interaction The BEC-BCS transition Single atom detection

4 Lab impressions from all over the world Tübingen Munich Austin Osaka

5 Magneto-optical trap (MOT) MOT: 3s, 1 x 10 9 atoms

6 MOT: Limits and extensions Temperature: 50 – 150 µK for alkalis Atom number: 1 … 10 9 Narrow transitions: below 1µK (e.g. Strontium) Single atom MOT (strong quadrupole field) Huge loading rate (Zeeman slower, 2D-MOT)

7 The beauty of magneto-optical traps sodium lithium strontium ytterbiumerbiumdysprosium

8 Magnetic trapping Working principle: Magnetic field minimum provides trapping potential Evaporative cooling with radio frequency induced spin flips Technical issues: heat production in the coils, control of field minimum Pros: robust, large atom number Cons: long cooling cycle (20 s – 60 s), limited optical access

9 Magnetic traps for neutral atoms Ioffe- Pritchard trap 4 cm Clover leaf trap

10 Imaging an ultracold quantum gas „Time of flight“ technique Credits: Immanuel Bloch

11 „Standard“ Bose-Einstein condensation classical gas coherent matter wave T c ~ 1µK Bose-Einstein condensation

12 The first BEC 1995: Cornell and Wieman, Boulder

13 The early phase: expansion: MIT Boulder Duke condensate fraction speed of sound

14 The early phase: Interference between two condensates (MIT) MIT

15 The early phase: Vortices Boulder

16 Optical dipole traps Working principle: exploit AC Stark shift single beam dipole trapcrossed dipole trap 1 mm

17 Optical dipole traps Requirements for a good dipole trap: a lot of laser power: nm available Pro: independent of magnetic sub-level, magnetic field becomes free parameter Con: high power laser, stabilization, limited trap depth -> smaller atom number Arbitrary trapping potentials possible

18 Ultracold Fermi gases The challenge: 1.Identical fermions do not collide at ultralow temperatures 2.Fermions are more subtle than bosons -> everything is more difficult The solution: Take tow different spin-states or admix bosons Duke university

19 Ultracold Fermi gases Bose-Fermi mixtures Bosons (rubidium) Fermions (potassium) After release from the trap Florence

20 Optical lattices Band structure Laser configuration 2D lattice (makes 1D tubes) 3D lattice

21 Optical lattices Expansion of a superfluid: interference pattern visible Expansion without coherence Munich

22 Optical lattices Superfluidity: tunneling dominates Mott insulator: Interaction energy Dominates (no interference)

23 Atoms meet solids: atom chips Working principle: make miniaturized magnetic traps with minaturized electric wires: Magnetic field of a wire Homogeneous Offest-field Trapping potential for the atoms along the wire => one-dimensional geometry

24 Atom chips Todays‘s setup: Basel

25 Atom chips: 1D physics Radial confinement leads to stronger interaction Lieb-Liniger interaction parameter: Induced antibunching: Tonks-Girardeau gas Penn state

26 Newton‘s cradle with atoms Penn State

27 Feshbach resonances Microscopic innteraction mechanisms between the ultacold atoms: s-wave scattering, and (more and more often) dipole-dipole interaction Change the s-wave scattering length via magnetic field: Working principle:

28 Generic properties of a Feshbach resonance The situation for fermionic 6 Li: Attractive interaction Repulsive interaction Unitary regime

29 Making ultracold molecules Evaporative cooling in a dipole trap a = a 0 a = a 0 Maximum possible number of trapped non-interacting fermions Innsbruck

30 Molecules form Bose-Einstein condensates Result: bimodal distribution of molecular density distribution Condensate fraction Boulder Two fermionic atoms form a bosonic molecule

31 Controlling the interaction between fermions a>0: weak repulsive interaction, BEC of molecules a<0: weak attractive interaction, BCS type of pairing What happens in between?

32 Test superfluidity with creation of vortices Set atoms in rotation and test superfluidity by the formation of vortices MIT

33 Unitary regime Result: fermion are superfluid across the crossover MIT

34 Dynamic of inelastic processes Lifetime of the vortices MIT

35 Single atom detection Fluorescence imaging: -shine resonant light on atoms and keep them trapped at the same time -collect enough photons to detect the atoms Single atoms in a 1D optical lattice Bonn

36 Single atom detection in a 2D system The Mott insulator state Munich

37 Single atom detection with electron microscopy Come and see tomorrow!


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