Report

“Characterizing many-body systems by observing density fluctuations” Wolfgang Ketterle Massachusetts Institute of Technology MIT-Harvard Center for Ultracold Atoms 8/7/2010 QFS 2010 Satellite Workshop Grenoble Next challenge Magnetic ordering - quantum magnetism (ferromagnetism, antiferromagnetism, spin liquid, …) Dominant entropy: spin entropy Bosonic or fermionic Hubbard Hamiltonian is equivalent to spin Hamiltonian (for localized particles) Duan, Demler, Lukin (2003) Magnetic Ground States Z-Ferromagnet: XY-Ferromagnet: Antiferromagnet: Towards quantum magnetism • Characterization of new quantum phases density fluctuations to determine compressibility, spin susceptibility and temperature • New cooling scheme spin gradient demagnetization cooling Single site resolution in a 2D lattice across the superfluid to Mott insulator transition Greiner labs (Harvard) Science , 6/17/2010 Bloch group, Garching preprint, June 2010 Not only the mean of the density distribution of ultracold gases is relevant. The fluctuations around the average can contain very useful Information. New methods to detect interesting new phases of matter Density fluctuations fluctuation-dissipation theorem n N T atomic density atom number in probe volume V isothermal compressibility Crossover or phase transitions, signature in T: Mott insulator, band insulator are incompressible Sub-shot noise counting of (small number of) bosons: Raizen, Oberthaler, Chin, Greiner, Spreeuw, Bloch, Steinhauer New methods to detect interesting new phases of matter Density fluctuations fluctuation-dissipation theorem n N T atomic density atom number in probe volume V isothermal compressibility ideal classical gas Poissonian fluctuations non-interacting Fermi gas 3 T 2nEF sub-Poissonian Pauli suppression of fluctuations Spin fluctuations: relative density fluctuations fluctuation-dissipation theorem M magnetization (N –N) V probe volume (∆)2 = ( ) spin susceptiblity Crossover or phase transitions, signature in : For a paired or antiferromagnetic system, = 0, For a ferromagnetic system, diverges. C. Sanner, E.J. Su, A. Keshet, R. Gommers, Y. Shin, W. Huang, and W. Ketterle: Phys. Rev. Lett. 105, 040402 (2010). related work: Esslinger group, PRL 105, 040401 (2010). Advantages: Expansion: more magnifies spatial spatial resolution scale elements than for in-trap imaging adjustment locally preserves of optimum Fermi-Dirac opticaldistribution density through with same ballistic T/Texpansion F no same high fluctuations magnification as innecessary situ You want to scatter many photons to lower the photon shot noise, but …. imprinted structure in the atomic cloud flat background (very good fringe cancellation) IMPRINT MECHANISMS -Intensities close to the atomic saturation intensity -Recoil induced detuning (Li-6: Doppler shift of 0.15 MHz for one photon momentum) -Optical pumping into dark states for the very light Li atoms, the recoil induced detuning is the dominant nonlinear effect 6 photons/atom transmission optical density noise OD variance variance for Poissonian statistics variance due to photon shot noise atom number variance Noise thermometry T/TF = 0.23 (1) T/TF = 0.33 (2) T/TF = 0.60 (2) Shot noise hot cold Counting N atoms m times: Poissonian variance: N Two standard deviations of the variance: 2N 2 m “Pauli suppression” in Fermi gases • two particle effects, at any temperature (but cold helps) Hanbury-Brown Twiss effect, antibunching electrons: Basel, Stanford 1999 neutral atoms: Mainz (2006), Orsay (2007) • two particle effects, at low temperature (but not degenerate) freezing out of collisions (when db<range of interactions): elastic collisions JILA (1997) clock shifts MIT (2003) • many-body effects, requires T << TF freezing out of collisions (between two kinds of fermions) JILA (2001) suppression of density fluctuations MIT (2010) suppression of light scattering (requires EF>Erecoil) not yet observed Suppression of light scattering in Fermi gases so far not observed For 20 years: Suggestions to observe suppression of light scattering (Helmerson, Pritchard, Anglin, Cirac, Zoller, Javanainen, Jin, Hulet, You, Lewenstein, Ketterle, Masalas, Gardiner, Minguzzi, Tosi) But: Light scattering d/dq S(q) is proportional to density fluctuations which have now been directly observed. Note: For our parameters, only scattering of light by small angles is suppressed. Total suppression is only 0.3 % - does not affect absorption imaging. Noninteracting mixture 2 (∆(N−N)) = (∆(N+N)) 2 Paired mixture 2 (∆(N−N)) << (∆(N+N)) 2 Using dispersion to measure relative density |e> =-3/2, =-1,0,1 |2> =-1/2, =0 |1> =-1/2, =1 ∝ 1 − 2 ∝ 1 + 1 3 2 Absorption imaging of dispersive speckle Propagation after a phase grating: a phase oscillation becomes an amplitude oscillation 20 40 Phase fluctuations lead to amplitude fluctuations after spatial propagation 60 80 100 120 140 20 40 60 80 100 120 140 0 527G 790G a=0 a>0 preliminary data 915G a<0 BEC II Ultracold fermions: Lattice density fluct. Christian Sanner Aviv Keshet Ed Su Wujie Huang Jonathon Gillen $$ NSF ONR MURI-AFOSR DARPA BEC III Na-Li Ferromagnetism Caleb Christensen Ye-ryoung Lee Jae Choi Tout Wang Gregory Lau D.E. Pritchard BEC IV Rb BEC in optical lattices Patrick Medley David Weld Hiro Miyake D.E. Pritchard