Fragmentation of identical and distinguishable bosons' pairs and natural geminals of a trapped bosonic mixture

TL;DR

This paper investigates pair fragmentation and natural geminals in a trapped bosonic mixture using a solvable model, revealing how entanglement persists even with infinite particles and elucidating the role of center-of-mass coordinates.

Contribution

It introduces explicit calculations of natural geminals and the Schmidt decomposition for bosonic mixtures, providing new insights into pair fragmentation and entanglement.

Findings

Pair fragmentation properties are analyzed in a solvable model.

Entanglement between species persists at large particle numbers.

Center-of-mass coordinates govern inter-species entanglement.

Abstract

In a mixture of two kinds of identical bosons there are two types of pairs, identical bosons' pairs, of either species, and pairs of distinguishable bosons. In the present work fragmentation of pairs in a trapped mixture of Bose-Einstein condensate is investigated using a solvable model, the symmetric harmonic-interaction model for mixtures. The natural geminals for pairs made of identical or distinguishable bosons are explicitly contracted by diagonalizing the intra-species and inter-species reduced two-particle density matrices, respectively. Properties of pairs' fragmentation in the mixture are discussed, the role of the mixture's center-of-mass and relative center-of-mass coordinates is elucidated, and a generalization to higher-order reduced density matrices is made. As a complementary result, the exact Schmidt decomposition of the wavefunction of the bosonic mixture is constructed. The entanglement between the two species is governed by the coupling of their individual center-of-mass coordinates, and it does not vanish at the limit of an infinite number of particles where any finite-order intra-species and inter-species reduced density matrix per particle is 100\% condensed. Implications are briefly discussed.