标题： 准粒子玻色-爱因斯坦凝聚体中相位相干性的自发出现 显示英文标题

标题： Spontaneous Emergence of Phase Coherence in a quasiparticle Bose-Einstein Condensate

摘要： 自爱因斯坦在量子力学初期提出以来，玻色-爱因斯坦凝聚体（BECs）由于其能够在宏观尺度上展示量子现象的特性，正在物理学的各个领域引起越来越多的关注。它们是各种物理系统中许多引人入胜的观察对象，从液氦到稀释的原子气体。除了像原子和复合玻色子如库珀对或激子这样的真实粒子外，这种现象也出现在准粒子气体中，如极化子和磁子——磁性介质中自旋波激发的量子。BEC状态的基本特性是其相干性，这由相应波函数的精确定义的相位表示，该相位自发产生并涵盖聚集在其谱底部的所有粒子。到目前为止，BEC相仅在依赖空间相位差的现象中被揭示，如干涉、二阶相干性和宏观BEC运动——超电流、超流性和约瑟夫森振荡。在这里，我们提出了一种直接测量磁子BEC相干态相位相对于外部参考信号的方法。我们报告了自由演化的磁子气体中自发相干性的出现，这表现为磁子凝聚成具有最小能量和明确相位的均匀进动状态。这些发现证实了所有关于准粒子凝聚体的假设基本特性，为这类系统提供了新的自由度，并开启了使用微波频率磁子BEC进行信息处理的可能性。 显示英文摘要

摘要： Since their prediction by Einstein at the dawn of quantum mechanics, Bose-Einstein condensates (BECs), owing to their property to show quantum phenomena on macroscopic scales, are drawing increasing attention across various fields in physics. They are the subject of many fascinating observations in various physical systems, from liquid helium to diluted atomic gases. In addition to real particles like atoms and composite bosons such as Cooper pairs or excitons, this phenomenon is also observed in gases of quasiparticles such as polaritons and magnons - quanta of spin-wave excitations in magnetic media. The fundamental property of the BEC state is its coherence, which is represented by a precisely defined phase of the corresponding wave function, which arises spontaneously and encompasses all particles gathered at the bottom of their spectrum. Until now, the BEC phase was only revealed in phenomena depending on the spatial phase difference, such as interference, second order coherence, and macroscopic BEC motions - supercurrents, superfluidity and Josephson oscillations. Here, we present a method for the direct time-domain measurement of the magnon BEC coherent state phase relative to an outside reference signal. We report the emergence of spontaneous coherence from a freely evolving magnon gas, which manifests as the condensation of magnons into a uniform precession state with minimal energy and a well-defined phase. These findings confirm all postulated fundamental properties of quasiparticle condensates, provide access to a new degree of freedom in such systems, and open up the possibility of information processing using microwave-frequency magnon BECs.