Presentation on theme: "Creating new states of matter:"— Presentation transcript:
1 Creating new states of matter: Experiments with ultra-cold Fermi gasesSelim JochimMPI für Kernphysik andUniversität HeidelbergHenning MoritzETH Zürich
2 IntroductionMajor breakthroughs in this field have made this field an exciting one in the past decadeFermi Superfluidity, Crossover to a gas of Bosons (weakly bound molecules)With tunable interactions: Model system for High-TC superconductors, Neutron stars, Quark-Gluon Plasma and more ….
3 What is an ultracold quantum gas? Gas shows “quantum” effects when the wave packets start to overlap
4 Fermions and Bosons: Bosons Fermions At zero temperature …. Fermi energyEF=kBTFBose-Einstein condensationDegenerate Fermi gas
5 What makes ultracold gases special? Compare with superfluids, like He, or superconductors:Density is way lower -> dilute gas makes description very simpleLab-in-a-trap type of systems with many easy-to-use knobs, such astemperatureconfinement (single well, periodic …),Interactions (even do controlled “chemistry”!)
7 Fermi degenerate gases Two isotopes of Lithium in the same trap in thermal equilibrium
8 Superfluid Fermi Gases: Molecular condensatesLook like a normal BECAre normal BECsA little bit of cheating?
9 Observe superfluidity A rotating superfluid cloud needs to exhibit vortices
10 What will the course be about? Today:How do we make/manipulate/detect ultracold gasesLaser coolingTrappingEvaporative cooling in conservative potentialsDetection and manipulation of ultracold atoms
11 2nd dayHow to cool a Fermi gas - special challenges, - like forbidden collisions - Pauli blocking, etc.Scattering lengthConcept of Feshbach resonance to tune interactions make things interesting!Making ultracold molecules, BEC of molecules
12 3rd day BEC of molecules BEC/BCS crossover Gap, collective excitations/ Cooper pairs superconductivityVorticesImbalanced spin mixtures
13 4th day Condensed Matter Physics with atoms? Periodic potentials, bosonic Case: Mott isolatorFermions: The Fermi SurfaceInteractions of Fermions in optical latticesLow dimensional systemsFuture directions with optical latticesFinal discussion
14 Spontaneus light force: photon momentum (recoil)scattering rateLithium:acceleration:Frisch 1933: Deflection of a sodium beam using a Na-lamp:
15 Model: 2 level atom: Spontaneous scattering rate: s0: saturation G Line width
19 How cold can we get? T = /2kB Spontaneous emission causes heating, due to randomly distributed emission.stationary state when heating rate=cooling rateminimal, whenT = /2kB ≈ a few MHz Tmin typically 0.1…0.25 mKPrediction by Hänsch, Schawlow, Wineland, Dehmelt (1975)
20 Much lower temperatures observed!!! Time-of flight measurement:
21 Sub Doppler and sub recoil cooling So far we only considered a 2-level atom,typically, there are several Zeeman-sublevels.different Zeeman-sublevel experience different“light shifts”, “dressed atom” picture:Rabi frequency
22 Sisyphus cooling Light shift on Zeeman level (Clebsch Gordan coefficients)Counter propagating Laser beams with orthogonalpolarization create a polarization grating:
23 Sideband cooling Quantization of trap potential |e> |g> Condition for sideband cooling:“Lamb-Dicke regime”:Localize atoms better than Dx<< l|g>Used in this way in ion traps!
24 Raman-sideband cooling Optical pumpingRaman-couplingA little more complicated, but universal!e.g. in optical lattice!
25 Magneto-optical trapOptical molasses + magnetic field + polarisation:
26 MOT in 3D Quadrupole field through anti-Helmholtz coils, Counterpropagating laser beams in x,y,z, with proper polarization
27 How to load a MOT?Most simple technique: Load atoms from vapor! but: trapping velocity is limited to v ≈ a few 10 m/s, e.g. Rb., Cs. only a small fraction of the Boltzmann distribution can be trapped!also: atomic vapor limits the vacuum and causes trap loss (Especially critical for subsequent experiments!)
28 Loading from and atomic beam Atoms with a low vapor pressure: need to be evaporated from an oven.(need to compensate Doppler shift!)Slow an atomic beam? make use of spontaneous light scattering!
29 Zeeman slower Make use of Zeeman tuning: E.g.: Li, Na “Extend” MOT to obtain slow atomic beamApply magnetic field, such thatE.g.: Li, Na
31 (Density) limitation of the MOT What limits the (phase space) density in a MOT?Collisions with background gas ( vapor cell!)Light assisted collisions:e.g.: photo association!max. phase space density: ≈10-5
32 How to obtain a quantum gas? So far: No success with exclusively optical cooling, but it provides excellent starting conditionsAlso: No success without optical cooling!!!
33 Conservative potentials for atoms Spatially varying magnetic field (magnetic trap): trap polarized atomsFar detuned laser fields (induce dipole)
34 Magnetic trap Simplest configuration: quadrupole field (MOT) There is a problem, when the atoms get colder:µBMajorana spin flips at B=0!Orientation of the magnetic field should not change faster than Larmor frequencyB
35 Ways around the zero: Time Orbiting Potential (TOP) Trap: Rotate zero of magnetic field fast enough such that the atoms don’t take notice ……but slower than the Larmor frequencyTime averaged potential!
36 Trap with offset field “Ioffe”-Bars with minimum (0G) in the center “Pinch”-coils produce an offset fieldand confine the atoms axially Ioffe Pritchard-trap
37 Optical traps (dipole force) Electric field induces dipole:Ep
38 oscillating E-FeldE-field oscillates slower than resonance (red detuned light) dipole oscillates in phaseIntensity maximum is trap (e.g. focus)E-field oscillates faster than resonance (blue detuned)Dipole phase is shifted by pIntensity minimum is trap (e.g. hollow beam)
39 optical dipole interaction dipole potentialscattering rate„red“ detuning (w<w0)„blue“ detuning (w>w0)optical dipole forceFdip = - Udipoptical dipole potentialattractionrepulsionFor most applications: Need to go for very large detunings!
40 Why an optical trap? Challenge: Typically, very large intensities are required to create the desired potentialAlso, photon scattering has to be taken care of!Potential is independent of spin state, magnetic fieldVery flexible opportunities to shape potentials, e.g. optical lattice
41 Evaporative coolingIdea: Remove hottest atoms, while thermal equilibrium is maintainedImportant figure of merit: Gain in phase space density per loss of particles
42 EV cooling techniquesIn magnetic traps, use RF fields to convert atoms to a high-field seeking state at distinct magnetic field (i.e. position)potentialposition
43 EV cooling techniquesIn optical traps, reduce trap depth by reducing laser power.
44 Evaporative cooling Important quantities: Truncation parameter: Ratio of good to bad collisions:Bad collisions: E.g. dipolar relaxation, three-body recombination ….
45 Optimize EV cooling Efficiency limited by Collision rate Losses Background gas (increase collision rate) Binary collisions (scales just as EV cooling) Three body collisions (go for low density)Heating Photon scattering Parametric heating Anti-evaporation (e.g. Majorana spin flips)Trap geometry
46 Efficiency Graph: Typical efficiencies …. EV cooling efficiency truncation parameter h
47 Optimize EV coolingGeometry matters when the gas becomes (close to) hydrodynamic, e.g. trap frequency < collision rate:Example for inefficient geometry:Magnetic trap with gravitational sag
48 Which trap to use? Magnetic trap: Easy evaporation, Well defined potentialConstant trap frequencyOptical trapMore freedom with trap potentialsCan trap atoms in absolute (magnetic) ground stateHave to take care of photon scattering (use far off-resonant traps!)
49 Absorption imaging resonant cross section of the atoms ~l2 (depends on Clebsch-Gordan coefficients)Considerable absorption already at very low density:Image shadow on CCD!Important advantage: “See” ALL scattered photons
50 Absorption imaging This is the quantity we measure In the same way, measure momentum distribution:Time of flight (TOF): measure spatial distribution after a certain time of flight
51 Challenges when cooling Fermions Identical ultracold particles do not collide (s-waves).“Pauli blocking” makes cooling of a degenerate Fermi gas very inefficient.Also: Very low temperatures required to observe superfluidity:
52 Idea: Use Bosons to cool Fermions Bosons can be cooled with “established” technologyNot the first degenerate Fermi gas, but a very instructive one:6Li cooled by bosonic 7Li (Rice U., ENS Paris):Difference of just one neutron makes all the difference!
53 6Li+7Li cooled togetherTwo MOTs for the two isotopes (10GHz isotope shift)Magnetic trap traps both isotopes …
54 Challenges to achieve very low T Bosons condense to BEC -> heat capacity drops to zero, no more cooling effectInteractions between Fermions are necessary to observe interesting physics -> spin mixture is neededTo study pairing effects, wish to tune pairing energy!All of this: Tomorrow by Henning Moritz
55 Literature Metcalf and van der Straaten: “Laser cooling and trapping” Ketterle, Durfee and Stamper-Kurn “Making, probing and understanding Bose-Einstein condensates”