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World of zero temperature --- introduction to systems of ultracold atoms National Tsing-Hua University Daw-Wei Wang

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Temperature ? What we mean by “ultracold” ?

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Why low temperature ? Ans: To see the quantum effects ! Uncertainty principle:

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(after Nature, 416, 225 (’02))

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How to reach ultracold temperature ? Normal pressure: He 4 4.2 K We need very low density to avoid strong binding, i.e. we need low “kinetic energy”, not low “potential energy” !

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How to reach ultracold temperature ? 1. Laser cooling ! (1997 Nobel Price) Use red detune laser + Doppler effect

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How to reach ultracold temperature ? 2. Evaporative cooling ! Reduce potential barrial +thermal equilibrium

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Typical experimental environment MIT

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How to do measurement ? Trapping and cooling Perturbing Releasing and measuring BEC (2001 Nobel Price)

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What is Bose-Einstein condensation ? Therefore, when T-> 0, noninteracting bosons like to stay in the lowest energy state, i.e. BEC

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How about fermions in T=0 ? Therefore, when T-> 0, noninteracting fermions form a compact distribution in energy level. E D(E) Fermi sea

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BEC and Superfluidity of bosons Superfluid Normal fluid v repulsion Landau’s two-fluid model BEC = superfluidity uncondensate condensate (after Science, 293, 843 (’01))

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Phonons and interference in BEC Phonon=density fluctuation Interference Matter waves ? (after Science 275, 637 (’97))

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Vortices in condensate Vortex = topological disorder E L 1320 Vortices melting, quantum Hall regime ? (after Science 292, 476 (’01)) (after PRL 87, 190401 (’01))

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Spinor condensation in optical trap Na B E F=2 F=1 (see for example, cond-mat/0005001)

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Boson-fermion mixtures Sympathetic cooling collapse E D(E) Interacting fermi sea Fermions are noninteracting ! rf-pulse phonon-mediated interaction fermion phonon (after Science 291, 2570 (’01)) (after Nature 412, 295 (’01))

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Feshbach Resonance (i) Typical scattering: (ii) Resonant scattering: B a

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Molecule and pair condensate (JILA, after Nature 424, 47 (’03)) (MIT group, PRL 92, 120403 (’04)) (Innsbruck, after Science 305, 1128 (’04))

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Optical lattice 3D lattice 1D lattice Entanglement control other lattice

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Mott-Insulator transition (after Nature 415, 39 (’02)) n=1 n=2 n=3 superfluid

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Atom laser Bragg scattering Continuous source for coherent atoms

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Transport in 1D waveguide Interference ? Finite temperature + semiconductor technique wave guide wire

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Interdisciplinary field Ultracold atoms Traditional AMO Quantum Information Nonlinear Physics Precise measurement Condensed matter

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