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Ultracold Quantum Gases:An Experimental ReviewHerwig OttUniversity of KaiserslauternOPTIMAS Research Center1OutlineLaser cooling, magnetic trappingand BEC

Optical dipole traps, fermions

Optical lattices:Superfluid to Mott insulator transition

Magnetic microtraps: Atom chips and 1D physics

2OutlineFeshbach resonances: taming the interaction

The BEC-BCS transition

Single atom detection

3Lab impressions from all over the world

TbingenMunichAustinOsakaMagneto-optical trap (MOT)

MOT: 3s, 1 x 109 atomsMOT: Limits and extensionsTemperature: 50 150 K for alkalis

Atom number: 1 109Narrow transitions: below 1K (e.g. Strontium)Single atom MOT(strong quadrupole field)

Huge loading rate (Zeeman slower, 2D-MOT)

The beauty of magneto-optical traps

sodium

lithium

strontium

ytterbium

erbiumdysprosiumMagnetic trappingWorking principle: Magnetic field minimum provides trapping potentialEvaporative 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

Magnetic traps for neutral atoms

Ioffe- Pritchard trap4 cm

Clover leaf trapImaging an ultracold quantum gasTime of flight techniqueCredits: Immanuel Bloch

10Standard Bose-Einstein condensation

classical gas

coherent matter waveTc ~ 1KBose-Einstein condensation

The first BEC1995: Cornell and Wieman, Boulder

The early phase: 1995 - 1999expansion:

MITBoulderDukecondensate fractionspeed of soundThe early phase: 1995 - 1999Interference between two condensates (MIT)

MITThe early phase: 1995 - 1999Vortices

BoulderOptical dipole trapsWorking principle: exploit AC Stark shift

single beam dipole trapcrossed dipole trap1 mmOptical dipole trapsRequirements 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

Ultracold Fermi gasesThe challenge:

Identical fermions do not collide at ultralow temperaturesFermions are more subtle than bosons -> everything is more difficult

The solution: Take tow different spin-states or admix bosons

Duke universityUltracold Fermi gasesBose-Fermi mixtures

Bosons (rubidium)Fermions (potassium)After release from the trapFlorenceOptical lattices

Band structureLaser configuration

2D lattice (makes 1D tubes)3D latticeOptical latticesExpansion of a superfluid: interference pattern visible

Expansion without coherenceMunichOptical latticesSuperfluidity: tunneling dominates

Mott insulator:

Interaction energyDominates(no interference)

Atoms meet solids: atom chipsWorking principle: make miniaturized magnetic traps with minaturized electric wires:

Magnetic field of a wireHomogeneous Offest-fieldTrapping potential for the atoms along the wire

=> one-dimensional geometryAtom chipsTodayss setup:

BaselAtom chips: 1D physics

Radial confinement leads to stronger interactionLieb-Liniger interaction parameter: Induced antibunching: Tonks-Girardeau gas

Penn stateNewtons cradle with atoms

Penn StateFeshbach resonancesMicroscopic innteraction mechanisms between the ultacold atoms:

s-wave scattering, and (more and more often) dipole-dipole interactionChange the s-wave scattering length via magnetic field:Working principle:

Generic properties of a Feshbach resonanceThe situation for fermionic 6Li:

Attractive interactionRepulsive interactionUnitary regimeMaking ultracold molecules

Evaporative cooling in a dipole trapa = + 3500 a0a = - 3500 a0Maximum possible number of trapped non-interacting fermionsInnsbruckMolecules form Bose-Einstein condensates

Result: bimodal distribution of molecular density distribution

Condensate fractionBoulderTwo fermionic atoms form a bosonic moleculeControlling the interaction between fermionsa>0: weak repulsive interaction, BEC of molecules

a