Ultracold Quantum Gases An Experimental Review Herwig Ott

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 chipsand 1 D 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: 3 s, 1 x 10 9 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, 2 D-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 Tc ~ 1µK 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 Boulder MIT

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 2 D lattice (makes 1 D tubes) 3 D lattice Band structure

Optical lattices Expansion of a superfluid: interference patternvisible 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: 1 D 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 6 Li: Unitary regime Repulsive interaction Attractive interaction

Making ultracold molecules Evaporative cooling in a dipole trap a = + 3500 a 0 Maximum possible number of trapped non-interacting fermions a = - 3500 a 0 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 andtest 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 1 D optical lattice Bonn

Single atom detection in a 2 D system The Mott insulatorstate Munich

Single atom detection with electron microscopy Come and see tomorrow!