# Solvable Model of a Generic Trapped Mixture of Interacting Bosons: Reduced Density Matrices and Proof of Bose-Einstein Condensation

## TL;DR

This paper presents an exactly solvable model of a two-species bosonic mixture in a harmonic trap, analytically proving Bose-Einstein condensation and properties of reduced density matrices in the infinite-particle limit.

## Contribution

It provides an analytical proof of Bose-Einstein condensation and reduced density matrix properties for a solvable two-species bosonic mixture model.

## Key findings

- Both species are 100% condensed in the infinite-particle limit.
- Inter-species reduced density matrix is separable and product form.
- Energy and densities match Gross-Pitaevskii predictions in the limit.

## Abstract

A mixture of two kinds of identical bosons, species $1$ and species $2$, held in a harmonic potential and interacting by harmonic intra-species and inter-species particle-particle interactions is discussed. To prove Bose-Einstein condensation of the mixture three steps are needed. First, we integrate the all-particle density matrix, employing a four-parameter matrix recurrence relations, down to the lowest-order intra-species and inter-species reduced density matrices of the mixture. Second, the coupled Gross-Pitaevskii (mean-field) equations of the mixture are solved analytically. Third, we analyze the mixture's reduced density matrices in the limit of an infinite number of particles of both species $1$ and $2$ (when the interaction parameters, i.e., the products of the number of particles times the intra-species and inter-species interaction strengths, are held fixed) and prove that: (i) Both species $1$ and $2$ are 100\% condensed; (ii) The inter-species reduced density matrix per particle is separable and given by the product of the intra-species reduced density matrices per particle; and (iii) The mixture's energy per particle, and reduced density matrices and densities per particle all coincide with the Gross-Pitaevskii quantities. Finally, when the infinite-particle limit is taken with respect to, say, species $1$ only (with interaction parameters held fixed) we prove that: (iv) Only species $1$ is 100\% condensed and its reduced density matrix and density per particle, as well as the mixture's energy per particle, coincide with the Gross-Pitaevskii quantities of species $1$ alone; and (v) The inter-species reduced density matrix per particle is nonetheless separable and given by the product of the intra-species reduced density matrices per particle.