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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