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 nonlinearity 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 semiclassical description (truncated Wigner approximation), using stochastic classical variables to model the quantum noise.

中文翻译：

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自旋玻色-爱因斯坦凝聚物动力学的相干注入：从量子行为到经典行为

我们目前的实验揭示了自旋1 Bose-Einstein凝聚体动力学中量子涨落和相干种子的竞争效应，并讨论了系统平均场描述的相关性。我们首先探索一种接近平衡的情况，其中均值场方程可以在与相同塞曼状态的所有原子相对应的固定点周围线性化$米=0$。在这个经典的固定点上准备系统，我们观察到由量子涨落触发的可逆动力学，这在经典框架内是无法理解的。我们证明，如果在其他塞曼状态中仅存在几个原子的相干种子，则经典描述将变得准确$米=\pm \mathrm{1个}$。在以均场方程的强非线性为特征的第二种情况下，我们观察到由量子涨落驱动的坍塌动力学。这种行为不能用经典的描述来解释，并且在大范围的初始状态下仍然存在。我们表明，使用随机经典变量对量子噪声进行建模，可以用半经典描述（截断的维格纳近似）来解释我们所有的实验结果。