A central problem in many-body quantum physics is the determination of the ground state of a thermodynamically large physical system. We construct a cluster expansion for ground states of local Hamiltonians, which naturally incorporates physical requirements inherited by locality as conditions on its cluster amplitudes. Applying a diagrammatic technique we derive the relation of these amplitudes to thermodynamic quantities and local observables. Moreover we derive a set of functional equations that determine the cluster amplitudes for a general Hamiltonian, verify the consistency with perturbation theory and discuss non-perturbative approaches. Lastly we verify the persistence of locality features of the cluster expansion under unitary evolution with a local Hamiltonian and provide applications to out-of-equilibrium problems: a simplified proof of equilibration to the GGE and a cumulant expansion for the statistics of work, for an interacting-to-free quench.
Department of Physics, Sharif University of Technology（谢里夫理工大学物理系）；Research Center for High Energy Physics, Department of Physics, Sharif University of Technology（谢里夫理工大学物理系高能物理研究中心）
Emergent eigenstate solution and emergent Gibbs ensemble for expansion dynamics in optical lattices
在量子动力学的突现本征态解[Phys. Rev. X 7，021012（2017）]中，人们可以构造一个局部算符（突现哈密顿量），其时间演化态是本征态。在这里，我们表明，这样的解决方案存在的膨胀动力学的Tonks-Girardeau气体在光晶格后关闭幂律（例如，谐波或四次）限制势，其是不涉及升压算子的几何猝灭。对于系统，最初是在基态，并在膨胀过程中进行动态费米子化，我们表明，他们仍然在基态的紧急本地哈密顿量在任何时候。另一方面，对于非零初始温度下的系统，膨胀动力学可以描述为构造涌现局部哈密顿量的吉布斯系综（涌现吉布斯系综）。
