Coherent seeding of the dynamics of a spinor Bose-Einstein condensate: from quantum to classical behavior

TL;DR

This paper investigates how quantum fluctuations and coherent seeding influence the dynamics of a spinor Bose-Einstein condensate, revealing conditions where classical or quantum descriptions are valid and demonstrating the transition from quantum to classical behavior.

Contribution

It experimentally and theoretically analyzes the interplay of quantum fluctuations and coherent seeding in spinor BEC dynamics, highlighting the regimes where mean-field and semi-classical models are applicable.

Findings

Quantum fluctuations trigger reversible dynamics near equilibrium.

A small coherent seed stabilizes classical mean-field behavior.

Quantum fluctuation-driven collapse persists across many initial states.

Abstract

We present experiments revealing the competing effect of quantum fluctuations and of a coherent seed in the dynamics of a spin-1 Bose-Einstein condensate, and discuss the relevance of a mean-field description of our system. We first explore a near-equilibrium situation, where the mean-field equations can be linearized around a fixed point corresponding to all atoms in the same Zeeman state $m=0$. Preparing the system at this classical fixed point, we observe a reversible dynamics triggered by quantum fluctuations, which cannot be understood within a classical framework. We demonstrate that the classical description becomes accurate provided a coherent seed of a few atoms only is present in the other Zeeman states $m= \pm 1$. In a second regime characterized by a strong non-linearity of the mean-field equations, we observe a collapse dynamics driven by quantum fluctuations. This behavior cannot be accounted for by a classical description and persists for a large range of initial states. We show that all our experimental results can be explained with a semi-classical description (truncated Wigner approximation), using stochastic classical variables to model the quantum noise.