Bragg spectroscopy of 1D and 3D Bose Einstein condensates in optical lattices

The work I will present is focused on the experimental investigation of the response function of a three-dimensional (3D) and one-dimensional (1D) Bose Einstein condensate in the presence of a 1D optical lattice. By means of Bragg spectroscopy [1] we induce an elementary excitation of the BEC in the optical lattice and we observe different resonances in the energy transfer, corresponding to the transitions between momentum states in different Bloch bands. The experimental results demonstrate that the resonance frequency and the transition strength in the presence of the longitudinal optical lattice are well described by a mean-field theory both in 1D and in 3D case [2]. Nevertheless, we also observe a relevant broadening of the resonance in the 1D case with respect to the 3D case, which seems to be an effect of the reduced dimensionality. As it is known, in 1D systems correlations can play a crucial role [3]: for this reason, we also investigated the 3D-1D crossover by varying the optical con¯nement of each 1D tube, without superimposing a longitudinal lattice. - References [1] J. Stenger et al., Phys. Rev.Lett. 82,4569 (1999). [2] C. Menotti et al., Phys. Rev. A 67, 053609 (2003). [3] B. Laburthe Tolra et al., Physical Review Letters 92, 190401 (2004).