标题： 李-杨零点的观测 显示英文标题

标题： Observation of Lee-Yang zeros

摘要： 李-杨零点是磁化强度复平面上使自旋系统配分函数为零的点，因此自由能发散。 李-杨零点及其推广在很多体系统中普遍存在，并且它们完全表征了自由能的解析性质，从而决定了系统的热力学性质。 确定李-杨零点不仅对热力学和统计物理的概念完整性具有根本的重要性，而且在研究多体系统方面也具有技术上的实用性。 然而，由于李-杨零点仅在磁化强度的复数值下出现，这是不物理的，因此在实验中从未观察到李-杨零点。 在此，我们报告了首次观察到的李-杨零点，通过测量与伊辛型自旋浴耦合的探测自旋的量子相干性。 根据最近的提议，探测自旋的量子演化引入了一个复相位因子，因此在浴中有效实现了虚磁场所。 从测量的李-杨零点，我们重建了自旋浴的自由能并确定了其相变温度。 这个实验证明了量子相干探测作为一种研究复平面上热力学的有效方法，这可能会揭示出一系列新的现象，这些现象如果物理参数被限制为实数将无法被观察到。 显示英文摘要

摘要： Lee-Yang zeros are points on the complex plane of magnetic field where the partition function of a spin system is zero and therefore the free energy diverges. Lee-Yang zeros and their generalizations are ubiquitous in many-body systems and they fully characterize the analytic properties of the free energy and hence thermodynamics of the systems. Determining the Lee-Yang zeros is not only fundamentally important for conceptual completeness of thermodynamics and statistical physics but also technically useful for studying many-body systems. However, Lee-Yang zeros have never been observed in experiments, due to the intrinsic difficulty that Lee-Yang zeros would occur only at complex values of magnetic field, which are unphysical. Here we report the first observation of Lee-Yang zeros, by measuring quantum coherence of a probe spin coupled to an Ising-type spin bath. As recently proposed, the quantum evolution of the probe spin introduces a complex phase factor, and therefore effectively realizes an imaginary magnetic field on the bath. From the measured Lee-Yang zeros, we reconstructed the free energy of the spin bath and determined its phase transition temperature. This experiment demonstrates quantum coherence probe as a useful approach to studying thermodynamics in the complex plane, which may reveal a broad range of new phenomena that would otherwise be inaccessible if physical parameters are restricted to be real numbers.