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