The Loop-Gas Approach to Bose-Einstein Condensation for Trapped Particles

TL;DR

This paper explores Bose-Einstein condensation in trapped particles using a loop-gas approach, revealing how long permutation cycles form the condensate and analyzing the accuracy of standard approximations in the thermodynamic limit.

Contribution

It applies the loop-gas framework to harmonic traps, providing exact solutions and insights into the role of permutation cycles in BEC, especially in the thermodynamic limit.

Findings

Condensate consists of very long permutation cycles.

WKB approximation becomes accurate for non-condensate in the TDL.

Exact density matrix describes condensate behavior in the thermodynamic limit.

Abstract

We examine Bose-Einstein condensation (BEC) for particles trapped in a harmonic potential by considering it as a transition in the length of permutation cycles that arise from wave-function symmetry. This ``loop-gas'' approach was originally developed by Feynman in his path-integral study of BEC for an homogeneous gas in a box. For the harmonic oscillator potential it is possible to treat the ideal gas exactly so that one can easily see how standard approximations become more accurate in the thermodynamic limit (TDL). One clearly sees that the condensate is made up of very long permutation loops whose length fluctuates ever more wildly as the number of particles increases. In the TDL, the WKB approximation, equivalent to the standard approach to BEC, becomes precise for the non-condensate; however, this approximation neglects completely the long cycles that make up the condensate. We examine the exact form for the density matrix for the system and show how it describes the condensate and behaves in the TDL.