Quantum Noise, Scaling and Domain Formation in a Spinor BEC

TL;DR

This paper analyzes quantum fluctuations and domain formation in a spinor Bose-Einstein condensate of 87Rb atoms, revealing how initial quantum dynamics seed ferromagnetic domains despite the system's overall purity.

Contribution

It introduces a novel analogy between spinor BEC quantum fluctuations and two-mode squeezing, and shows how these fluctuations lead to domain formation.

Findings

Quantum fluctuations act as seeds for domain formation.

Reduced density matrix for one spin component appears thermal.

System remains pure despite local thermal states.

Abstract

In this paper we discuss Bose-Einstein spinor condensates for F=1 atoms in the context of 87Rb, as studied experimentally by the Stamper-Kurn group [Sadler et al. Nature (2006)]. The dynamical quantum fluctuations of a sample that starts as a condensate of N atoms in a pure F=1, m_F = 0 state are described in analogy to the `two-mode squeezing' of quantum optics in terms of an su(1,1) algebra. In this system the initial m_F=0 condensate acts as a source (`pump') for the creation pairs of m_F =+1,-1 atoms. We show that even though the system as a whole is described by a pure state with zero entropy, the reduced density matrix for the m_F = +1 degree of freedom, obtained by tracing out the m_F = -1,0 degrees of freedom, corresponds to a thermal state. Furthermore, these quantum fluctuations of the initial dynamics of the system provide the seeds for the formation of domains of ferromagnetically aligned spins.