Leonardo Fallani

Optically induced lensing effect on a Bose-Einstein condensate expanding in a moving lattice

We report the experimental observation of a lensing effect on a Bose-Einstein condensate expanding in a moving 1D optical lattice. The effect of the periodic potential can be described by an effective mass dependent on the condensate quasimomentum. By changing the velocity of the atoms in the frame of the optical lattice, we induce a focusing of the condensate along the lattice direction. The experimental results are compared with the numerical predictions of an effective 1D theoretical model. In addition, a precise band spectroscopy of the system is carried out by looking at the real-space propagation of the atomic wave packet in the optical lattice.

Excitations of Bose-Einstein condensates in a one-dimensional periodic potential

We report on the experimental investigation of the response of a three-dimensional Bose-Einstein condensate (BEC) in the presence of a one-dimensional (1D) optical lattice. By means of Bragg spectroscopy we probe the band structure of the excitation spectrum in the presence of the periodic potential. We selectively induce elementary excitations of the BEC choosing the transferred momentum and we observe different resonances in the energy transfer, corresponding to the transitions to different bands. The frequency, the width, and the strength of these resonances are investigated as functions of the amplitude of the 1D optical lattice.

Decoherence effects in superradiant light scattering from a moving Bose-Einstein condensate

Abstract We present the results of an experiment on superradiant Rayleigh scattering from a moving Bose-Einstein condensate, where a superposition of two atomic wavepackets is created by the interaction of a far-detuned laser beam with the condensate. The system is described by the CARL-BEC model which is a generalization of the Gross-Pitaevskii model to include the self-consistent evolution of the scattered field. The experiment gives evidence of a damping of the matter-wave grating which depends on the initial velocity of the condensate. We describe this damping in terms of a phase-diffusion decoherence process, in good agreement with the experimental results. In a second experiment we add a seeding beam counterpropagating with respect to the pump beam. Varying intensity and frequency of the seed, we investigate the transition from the superradiant regime to the Rabi oscillations regime, evidencing fundamental differences between these two processes.

Quantum physics: United through repulsion

Mutually repulsive atoms placed at periodic intervals in a ‘crystal of light’ can, counterintuitively, be forced into stable couplings. That theoretical prediction has just seen experimental confirmation.An unlikely unionIn physics it is common for objects that attract each other to form stable bound states by lowering their energy. But under certain conditions, stable composite objects exist even for repulsive interactions. The creation of one such exotic bound state is reported this week. It consists of a pair of ultracold rubidium atoms in an optical lattice. The pairs are stable because two atoms sitting on a given site of an optical lattice with strong repulsive interactions cannot decay as they cannot convert their potential energy into kinetic energy, a phenomenon explained by the constraints of the Bose–Hubbard model for the structure of ultracold quantum gases.