理学院数理讲坛（2017年第02讲）
题 目：光和原子复合系统超辐射量子相变
报告人：金光日 教授
单 位：北京交通大学
时 间：2017年03月29日（星期三）下午15:00-16:00
地 点：龙赛理科楼北楼311
报告摘要：The single-mode Dicke model is well known to undergo a quantum phase transition from the so-called normal phase to the superradiant phase (hereinafter called the ‘superradiant quantum phase transition’). Normally, quantum phase transitions can be identified by the critical behavior of quantities such as entanglement, quantum fluctuations, and fidelity. In this talk, we present out recent work about the quantum Fisher information (QFI) and its behavior of both the field mode and the atoms in the ground state of the Dicke Hamiltonian. For a finite but large number of atoms, we show that near the critical atom-field coupling, the QFI of the atomic and the field subsystems can surpass their classical limits, due to the appearance of nonclassical quadrature squeezing. As the coupling increases far beyond the critical point, each subsystem becomes a highly mixed state, which degrades the QFI and hence the ultimate phase sensitivity. In the thermodynamic limit, we present the analytical results of the QFI and their relationship with the reduced variances of the field mode and the atoms. For each subsystem, we find that there is a singularity in the derivative of the QFI at the critical point, a clear signature of the quantum criticality in the Dicke model.