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 • Magnetic microtraps: Atom chips and 1D physics Outline • Feshbach resonances: taming the interaction • The BEC-BCS transition • Single atom detection Lab impressions from all over the world Munich Tübingen Osaka Austin Magneto-optical trap (MOT) MOT: 3s, 1 x 109 atoms MOT: Limits and extensions Temperature: 50 – 150 µK for alkalis Atom number: 1 … 109 Single atom MOT (strong quadrupole field) Narrow transitions: below 1µK (e.g. Strontium) Huge loading rate (Zeeman slower, 2D-MOT) The beauty of magneto-optical traps sodium ytterbium lithium dysprosium strontium erbium 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 Magnetic traps for neutral atoms Ioffe- Pritchard trap 4 cm Clover leaf trap Imaging an ultracold quantum gas „Time of flight“ technique Credits: Immanuel Bloch „Standard“ Bose-Einstein condensation classical gas T Tc dB h mv T 1 2 T Tc dB d Tc ~ 1µK T 0 coherent matter wave Bose-Einstein condensation The first BEC 1995: Cornell and Wieman, Boulder The early phase: 1995 - 1999 expansion: condensate fraction Duke speed of sound T N0 1 N Tc MIT 3 Boulder The early phase: 1995 - 1999 Interference between two condensates (MIT) MIT The early phase: 1995 - 1999 Vortices Boulder Optical dipole traps Working principle: exploit AC Stark shift single beam dipole trap crossed dipole trap 1 mm Optical dipole traps Arbitrary trapping potentials possible 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 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 Ultracold Fermi gases Bose-Fermi mixtures After release from the trap Bosons (rubidium) Fermions (potassium) Florence Optical lattices Laser configuration 2D lattice (makes 1D tubes) 3D lattice Band structure Optical lattices Expansion of a superfluid: interference pattern visible Expansion without coherence Munich Optical lattices Superfluidity: tunneling dominates Mott insulator: Interaction energy Dominates (no interference) 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 Atom chips Todays‘s setup: Basel Atom chips: 1D physics Radial confinement leads to stronger interaction Lieb-Liniger interaction parameter: Induced antibunching: Tonks-Girardeau gas Penn state Newton‘s cradle with atoms Penn State 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: Generic properties of a Feshbach resonance The situation for fermionic 6Li: Unitary regime Repulsive interaction Attractive interaction Making ultracold molecules Evaporative cooling in a dipole trap a = + 3500 a0 Maximum possible number of trapped non-interacting fermions a = - 3500 a0 Innsbruck Molecules form Bose-Einstein condensates Two fermionic atoms form a bosonic molecule Result: bimodal distribution of molecular density distribution Condensate fraction Boulder Controlling the interaction between fermions a>0: weak repulsive interaction, BEC of molecules a<0: weak attractive interaction, BCS type of pairing What happens in between? Test superfluidity with creation of vortices Set atoms in rotation and test superfluidity by the formation of vortices MIT Unitary regime Result: fermion are superfluid across the crossover MIT Dynamic of inelastic processes Lifetime of the vortices MIT Single atom detection Fluorescence imaging: - shine resonant light on atoms and keep them trapped at the same time collect enough photons to detect the atoms Single atoms in a 1D optical lattice Bonn Single atom detection in a 2D system The Mott insulator state Munich Single atom detection with electron microscopy Come and see tomorrow!