Nature of condensation in the momentum space for an interacting Bose system

R. I. M. A. Rashid

Earlier it has been shown that the properties of superfluid helium four have an intimate connection with a non-vanishing coherent pairing as well as the generalized Bose-Einstein condensation. Here, the nature of condensation for an interacting Bose system is re-examined. It is shown that, under conditions of constant temperature and pressure, minimization of the Gibbs free energy leads to a generalized condensation of the type first proposed by Girardeau. Extrapolation of these ideas to quasiparticle models of interacting Bose systems such as helium-II leads to the conclusion that the condensation in momentum space should also be regarded as generalized in the same sense. A bonus of this approach is the disappearance of macroscopic fluctuations in the density encountered in the usual theory of the condensed ideal Bose system.

Joseph V. Pulé

Salvatore Butera, David Clément, Iacopo Carusotto

We investigate the position- and momentum-space two-body correlations in a weakly interacting, harmonically trapped atomic Bose-Einstein condensed gas at low temperatures. The two-body correlations are computed within the Bogoliubov approximation and the consequences of the finite system size are highlighted in contrast to the spatially homogeneous case. In the position space, we recover the antibunching induced by the repulsive interatomic interaction in the condensed fraction localized around the trap center and the bunching in the outer thermal cloud. In the momentum space, bunching signatures appear for either equal or opposite values of the momentum and display peculiar features as a function of the momentum and the temperature. In analogy to the optical Hanbury-Brown and Twiss effect, the amplitude of the bunching signal at closeby momenta is fixed by the chaotic nature of the matter field state and its linewidth is shown to be set by the (inverse of the) finite spatial size of the associated in-trap momentum components. In contrast, the linewidth of the bunching signal at opposite momenta is only determined by the condensate size.

Yu. Kagan, Boris V. Svistunov, Gora V. Shlyapnikov +1 more

Morawetz

Johannes Otterbach, Mikhail Lemeshko

We propose a technique for engineering momentum-dependent dissipation in Bose-Einstein condensates with nonlocal interactions. The scheme relies on the use of momentum-dependent dark states in close analogy to velocity-selective coherent population trapping. During the short-time dissipative dynamics, the system is driven into a particular finite-momentum phonon mode, which in real space corresponds to an ordered structure with nonlocal density-density correlations. Dissipation-induced ordering can be observed and studied in present-day experiments using cold atoms with dipole-dipole or off-resonant Rydberg interactions. Because of its dissipative nature, the ordering does not require artificial breaking of translational symmetry by an optical lattice or harmonic trap. This opens up a perspective of direct cooling of quantum gases into strongly interacting phases.