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

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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

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

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Lab impressions from all over the world Tbingen Munich Austin Osaka

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

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

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The beauty of magneto-optical traps sodium lithium strontium ytterbiumerbiumdysprosium

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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

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

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

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Standard Bose-Einstein condensation classical gas coherent matter wave T c ~ 1K Bose-Einstein condensation

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The first BEC 1995: Cornell and Wieman, Boulder

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The early phase: 1995 - 1999 expansion: MIT Boulder Duke condensate fraction speed of sound

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The early phase: 1995 - 1999 Interference between two condensates (MIT) MIT

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The early phase: 1995 - 1999 Vortices Boulder

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

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Optical dipole traps Requirements for a good dipole trap: a lot of laser power: 100 W @ 1064 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

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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

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

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

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

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

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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

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Atom chips Todayss setup: Basel

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

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Newtons cradle with atoms Penn State

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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:

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

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Making ultracold molecules Evaporative cooling in a dipole trap a = + 3500 a 0 a = - 3500 a 0 Maximum possible number of trapped non-interacting fermions Innsbruck

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

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Controlling the interaction between fermions a>0: weak repulsive interaction, BEC of molecules a