Data: Sexta-Feira 13/03/2015, Sala A5-01, 11 am.

Palestrante: Laurent Vernac (Université Paris Nord)

Título: Quantum dipolar gases for quantum magnetism

Resumo: Bose Einstein Condensates (BEC) have been obtained with ultra cold atoms in 1995. In these quantum gases all atoms are in the same quantum state, which gives them unique properties in terms of coherence, and allows for observation of matter interference, superfluidity, or transition from superfluid to Mott insulator states in an optical lattice.
I will present experimental results obtained in our group with chromium atoms. Due to the high spin (S=3) in the ground state of chromium, there are large magnetic dipole-dipole interactions (DDIs) between the atoms, which are long range, and anisotropic. They confer specific properties to chromium BECs. For example, the excitation spectrum depends on the direction of the magnetic field: a Cr BEC can be seen as an anisotropic superfluid [1]. DDIs allow as well the change of the magnetization of the sample, which leads to spontaneous depolarization of the BEC at low magnetic fields, and multi-components BECs [2,3].
The main part of my talk will be devoted to the results we have obtained in a 3D optical lattice. We have studied spin exchange dynamics due to DDIs in the Mott insulator state, with one or two atoms per lattice site [4]. Our system is well described by an effective XXZ spin Hamiltonian, with Ising and exchange terms. We thus obtain an interesting system for quantum simulation, which realizes a spin model used in condensed matter physics, with true spin-spin interactions. As correlations build up between the few thousands of atoms loaded in the lattice, our system should reveal as well the existence of entanglement between many particles, through the finding of a relevant experimental entanglement witness.
I will conclude by perspectives offered by our recent production of a chromium quantum gas with the fermionic isotope (Fermi Sea) [5].

[1] Anisotropic Excitation Spectrum of a Dipolar Quantum Bose Gas G. Bismut et al., Phys. Rev. Lett. 109, 155302 (2012)

[2] Spontaneous demagnetization of a dipolar spinor Bose gas at ultra-low magnetic field B. Pasquiou, Phys. Rev. Lett. 106, 255303 (2011)

[3] Thermodynamics of a Bose-Einstein condensate with free magnetization B. Pasquiou et al., Phys. Rev. Lett., 108, 045307 (2012)

[4] Nonequilibrium Quantum Magnetism in a Dipolar Lattice Gas A. de Paz et al., Phys. Rev. Lett., 111, 185305 (2013)

[5] Chromium Dipolar Fermi Sea B. Naylor et al., Phys. Rev. A 91, 011603(R) (2015)

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