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!