A stochastic approach to time-dependent BEC

Abstract

We propose a stochastic description of the dynamics of a Bose-Einstein condensate within the context of Nelson stochastic mechanics. We start from the $N$ interacting conservative diffusions, associated with the $N$ Bose particles, and take an infinite particle limit. We address several aspects of this formulation. First, we consider the problem of extending to a system with self-interaction the variational formulation of Nelson stochastic mechanics due to Guerra and Morato. In this regard we discuss two possible extensions, one based on a doubling procedure and another based on a constraint Eulerian type variational principle. Then we consider the infinite particle limit from the point of view of the $N$-particles Madelung equations. Since conservative diffusions can be identified with proper infinitesimal characteristics pairs $(ρ_N(t), v_N(t))$, a time marginal probability density and a current velocity field, respectively, we consider a finite Madelung hierarchy for the marginals pairs $(ρ_{N,n}(t), v_{N,n}(t))$, obtained by properly conditioning the processes. The infinite Madelung hierarchy arises from the finite one by performing, for each fixed $n$, a mean-field scaling limit in $N$. Finally, we introduce a $n$-particle conditioned diffusions which naturally parallels the quantum mechanical approach and is a new approach within the context of Nelson stochastic mechanics. We then prove the convergence, in the infinite particle limit, of the law of such a conditioned process to the law of a self-interacting diffusion which describes the condensate.