430 likes | 702 Views
Ultracold Quantum Gases: An Experimental Review. Herwig Ott University of Kaiserslautern OPTIMAS Research Center. Outline. Laser cooling , magnetic trapping and BEC Optical dipole traps, fermions Optical lattices : Superfluid to Mott insulator transition
E N D
Ultracold Quantum Gases:An Experimental Review Herwig Ott University of Kaiserslautern OPTIMAS Research Center
Outline • Laser cooling, magnetictrappingand BEC • Optical dipole traps, fermions • Optical lattices:Superfluid to Mott insulatortransition • Magneticmicrotraps: Atom chipsand 1D physics
Outline • Feshbachresonances: tamingtheinteraction • The BEC-BCS transition • Single atomdetection
Lab impressionsfrom all overtheworld Munich Tübingen Osaka Austin
Magneto-optical trap (MOT) MOT: 3s, 1 x 109 atoms
MOT: Limits andextensions Temperature: 50 – 150 µK foralkalis Atom number: 1 … 109 Hugeloading rate (Zeeman slower, 2D-MOT) Single atom MOT (strong quadrupolefield) Narrow transitions: below 1µK (e.g. Strontium)
The beauty of magneto-optical traps sodium strontium lithium ytterbium dysprosium erbium
Magnetictrapping Working principle:Magneticfieldminimumprovidestrapping potential Evaporativecoolingwithradiofrequencyinducedspinflips Technical issues: heatproduction in thecoils, control of fieldminimum Pros: robust, large atomnumber Cons: longcoolingcycle (20 s – 60 s), limited opticalaccess
Magnetic traps for neutral atoms Ioffe- Pritchardtrap Cloverleaftrap 4 cm
Imaging an ultracoldquantum gas „Time offlight“ technique Credits: Immanuel Bloch
coherent matter wave „Standard“ Bose-Einstein condensation classical gas Tc ~ 1µK Bose-Einstein condensation
The first BEC 1995: Cornell andWieman, Boulder
The earlyphase: 1995 - 1999 expansion: condensatefraction Duke speed of sound Boulder MIT
The earlyphase: 1995 - 1999 Interferencebetweentwocondensates (MIT) MIT
The earlyphase: 1995 - 1999 Vortices Boulder
Optical dipole traps Working principle: exploit AC Stark shift single beam dipoletrap crosseddipoletrap 1 mm
Optical dipole traps Arbitrarytrappingpotentialspossible Requirementsfor a gooddipoletrap: a lot of laserpower: 100 W @ 1064 nmavailable Pro: independent of magnetic sub-level, magneticfieldbecomesfreeparameter Con: high power laser, stabilization, limited trapdepth -> smalleratomnumber
Ultracold Fermi gases The challenge: • Identicalfermions do not collideatultralowtemperatures • Fermions aremoresubtlethanbosons -> everythingismoredifficult The solution: Take tow different spin-statesoradmixbosons Duke university
After release from the trap Bosons (rubidium) Fermions (potassium) Ultracold Fermi gases Bose-Fermi mixtures Florence
Optical lattices Laser configuration 2D lattice (makes 1D tubes) 3D lattice Band structure
Optical lattices Expansion of a superfluid: interferencepatternvisible Expansion withoutcoherence Munich
Optical lattices Superfluidity: tunnelingdominates Mott insulator: Interaction energy Dominates (nointerference)
Atoms meetsolids: atomchips Working principle: makeminiaturizedmagnetic traps withminaturizedelectricwires: HomogeneousOffest-field Magneticfield of a wire Trapping potential fortheatomsalongthewire => one-dimensional geometry
Atom chips Todays‘ssetup: Basel
Atom chips: 1D physics Radial confinementleadstostrongerinteraction Lieb-Linigerinteractionparameter: Inducedantibunching: Tonks-Girardeau gas Penn state
Newton‘scradlewithatoms Penn State
Feshbachresonances Microscopicinnteractionmechanismsbetweentheultacoldatoms: s-wavescattering, and (moreandmoreoften) dipole-dipole interaction Change the s-wavescatteringlength via magneticfield: Working principle:
Genericproperties of a Feshbachresonance The situationforfermionic6Li: Unitaryregime Repulsive interaction Attractiveinteraction
Making ultracoldmolecules Evaporativecooling in a dipoletrap Maximum possiblenumber of trapped non-interactingfermions a = + 3500 a0 a = - 3500 a0 Innsbruck
Molecules form Bose-Einstein condensates Twofermionicatoms form a bosonicmolecule Result:bimodaldistribution of moleculardensitydistribution Condensatefraction Boulder
Controlling theinteractionbetweenfermions a>0: weak repulsive interaction, BEC of molecules a<0: weakattractiveinteraction, BCS type of pairing Whathappens in between?
Test superfluiditywithcreation of vortices Set atoms in rotationandtestsuperfluiditybytheformationof vortices MIT
Unitaryregime Result: fermionare superfluid acrossthecrossover MIT
Dynamic of inelasticprocesses Lifetime of thevortices MIT
Single atomdetection Fluorescenceimaging: • shine resonant light on atomsandkeepthemtrappedatthe same time • collectenoughphotonstodetecttheatoms Single atoms in a 1D opticallattice Bonn
Single atomdetection in a 2D system The Mott insulatorstate Munich
Single atomdetectionwithelectronmicroscopy Comeandseetomorrow!