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

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.

31 pages

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• Solvable model of a generic driven mixture of trapped Bose-Einstein condensates and properties of a many-boson Floquet state at the limit of an infinite number of particles
• Effects Beyond Center-of-Mass Separability in a Trapped Bosonic Mixture: Exact Results
• Properties of a trapped multiple-species bosonic mixture at the infinite-particle-number limit: A solvable model
• Entanglement and correlations in an exactly-solvable model of a Bose-Einstein condensate in a cavity
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• Correlation Effects in a Trapped Bose-Fermi Mixture: Exact Results
• Fragmentation of a trapped multiple-species bosonic mixture
• Fragmentation of identical and distinguishable bosons' pairs and natural geminals of a trapped bosonic mixture