Phase fluctuations determine the low-energy properties of quantum condensates. However, at the condensation threshold, both density and phase fluctuations are relevant. While strong emphasis has been given to the investigation of phase fluctuations, which dominate the physics of the quantum system away from the critical point, number fluctuations have been much less explored even in thermal equilibrium. In this work, we report experimental observation and theoretical description of fluctuations in a circularly confined nonequilibrium Bose-Einstein condensate of polaritons near the condensation threshold. We observe critical fluctuations, which combine the number fluctuations of a single-mode condensate state and competition between different states. The latter is analogous to mode hopping in photon lasers. Our theoretical analysis indicates that this phenomenon is of a quantum character, while classical noise of the pump is not sufficient to explain the experiments. The manifestation of a critical quantum state competition unlocks possibilities for the study of condensate formation while linking to practical realizations in photonic lasers.

中文翻译：

---

受限驱动耗散量子凝聚体中的临界涨落

相位涨落决定了量子凝聚体的低能特性。然而，在凝结阈值处，密度和相位波动都是相关的。虽然人们非常重视对相位涨落的研究，相位涨落在远离临界点的量子系统的物理过程中占主导地位，但即使在热平衡状态下，对数涨落的研究也少得多。在这项工作中，我们报告了在凝结阈值附近的圆约束非平衡玻色-爱因斯坦凝聚极化激元中涨落的实验观察和理论描述。我们观察到临界涨落，它结合了单模凝聚态的数量波动和不同状态之间的竞争。后者类似于光子激光器中的模式跳跃。我们的理论分析表明这种现象具有量子特征，而泵浦的经典噪声不足以解释实验。关键量子态竞争的表现释放了研究凝聚态形成的可能性，同时与光子激光器的实际实现联系起来。