Direct evaluation of coherence in a Magnon Bose-Einstein Condensate

The spontaneous emergence of coherence is a defining feature of multi-body quantum systems, underlying phenomena from superconductivity to quantum information processing. While Bose-Einstein condensates (BECs) provide a unique setting for exploring this process, yet direct observation of how a condensate acquires a coherent global phase has remained out of reach. Here, we present the first time-resolved measurement of spontaneous phase formation in a magnon BEC. Using a phase-referenced detection technique, we track the phase of the condensate coherent state relative to an external reference, revealing how an initially incoherent magnon gas thermalizes and undergoes a spontaneous transition into a coherent quantum state with a well-defined macroscopic phase.This observation provides direct evidence of spontaneous symmetry breaking in a quasiparticle condensate, confirming a central prediction of BEC theory that extends across diverse quantum systems.相干性的自发涌现是多体量子系统的标志性特征，为超导、量子信息处理等诸多物理现象提供了核心支撑。尽管玻色-爱因斯坦凝聚体（Bose-Einstein Condensates，BECs）为探究这一过程提供了独特的实验平台，但直接观测凝聚体如何获得相干全局相位的工作始终未能实现。本研究首次完成了磁振子玻色-爱因斯坦凝聚体（magnon BEC）中相位自发形成过程的时间分辨测量。我们采用相位参考探测技术，以外部参考为基准追踪凝聚体相干态的相位演化，揭示了初始处于非相干状态的磁振子气如何实现热化，并经自发相变转变为具有明确宏观相位的相干量子态。本次观测为准粒子（quasiparticle）凝聚体中的自发对称性破缺提供了直接实验证据，验证了适用于各类量子系统的BEC理论核心预言。