Guangcan Yang
A explicit formula on semiclassical Green functions in mixed position and momentum spaces is given, which is based on Maslov's multi-dimensional semiclassical theory. The general formula includes both coordinate and momentum representations of Green functions as two special cases of the form.
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A mixed semiclassical initial value representation expression for spectroscopic calculations is derived. The formulation takes advantage of the time-averaging filtering and the hierarchical properties of different trajectory based propagation methods. A separable approximation is then introduced that greatly reduces (about an order of magnitude) the computational cost compared with a full Herman-Kluk time-averaging semiclassical calculation for the same systems. The expression is exact for the harmonic case and it is tested numerically for a Morse potential coupled to one or two additional har…
We derive a semiclassical approximation to the Husimi functions of stationary states of spin systems. We rederive the Bohr-Sommerfeld quantization for spin by locating the poles of the corresponding local Green function. The residues correspond to the Husimi functions, which are seen to agree very well with exact calculations.
The recently introduced mixed time-averaging semiclassical initial value representation molecular dynamics method for spectroscopic calculations [M. Buchholz, F. Grossmann, and M. Ceotto, J. Chem. Phys. 144, 094102 (2016)] is applied to systems with up to 61 dimensions, ruled by a condensed phase Caldeira-Leggett model potential. By calculating the ground state as well as the first few excited states of the system Morse oscillator, changes of both the harmonic frequency and the anharmonicity are determined. The method faithfully reproduces blueshift and redshift effects and the importance of t…
Mixed eigenstates in the Dicke model: Statistics and power-law decay of the relative proportion in the semiclassical limit
本研究在Dicke模型中探讨混合本征态随半经典极限的变化，利用Husimi函数分类本征态并定义相空间重叠指数，发现系统尺寸增大时混合本征态相对比例呈幂律衰减，拓展了均匀半经典凝聚原理的适用范围。
Mixed Quantum Classical (MQC)-IVR is a recently introduced semiclassical framework that allows for selective quantization of the modes of a complex system. In the quantum limit, MQC reproduces the semiclassical Double Herman-Kluk IVR results, accurately capturing nuclear quantum coherences and conserving zero-point energy. However, in the classical limit, while MQC mimics the Husimi-IVR for real-time correlation functions with linear operators, it is significantly less accurate for non-linear correlation functions with errors even at time zero. Here, we identify the origin of this discrepancy …
Mixed Quantum-Semiclassical Simulation
本研究探讨混合量子-半经典系统的量子模拟，发现场模拟可获与作用量比成比例的改进，并在引力模型中验证了经典介体不产生可蒸馏纠缠的特性。
We present the analytical solution in closed form for the semiclassical limit of the quantum mechanical Coulomb Green function in position space in n dimensions. We utilize a projection method which has its roots in Lambert's theorem and which allows us to treat the system as an essentially one dimensional problem. The semiclassical result assumes a simple analytical form and is well suited for a numerical evaluation. The method can also be extended to classically forbidden space regions. Already for moderately large principal quantum numbers nu >= 5, the semiclassical Green function is found …
On the classical geometry of chaotic Green functions and Wigner functions
本文通过勒让德变换在加倍相空间中构造预解曲面，为混沌系统的半经典表示提供经典基础，并揭示其复数性质类似多维海绵，同时用次级周期轨道改进迹重求和。
The resonant interaction between x-ray photons and nuclei is one of the most exciting subjects of the burgeoning field of x-ray quantum optics. A resourceful platform used so far are thin-film x-ray cavities with embedded layers or Mössbauer nuclei such as $^{57}\mathrm{Fe}$. A new quantum optical model based on the classical electromagnetic Green's function is developed to investigate theoretically the nuclear response inside the x-ray cavity. The model is versatile and provides an intuitive picture about the influence of the cavity structure on the resulting spectra. We test its predictive p…
General semiclassical expression for quantum fidelity (Loschmidt echo) of arbitrary pure and mixed states is derived. It expresses fidelity as an interference sum of dephasing trajectories weighed by the Wigner function of the initial state, and does not require that the initial state be localized in position or momentum. This general dephasing representation is special in that, counterintuitively, all of fidelity decay is due to dephasing and none due to the decay of classical overlaps. Surprising accuracy of the approximation is justified by invoking the shadowing theorem: twice--both for ph…
Quantum chaos and semiclassical behavior in mushroom billiards I: Spectral statistics
本文研究蘑菇台球中量子混沌的谱统计，发现大波数下能级间距分布符合Berry-Robnik分布，低波数下可用Berry-Robnik-Brody分布描述弱动力学局域化，并验证了Yan（2025）的能级间距比解析理论。
We study the phase-space representation of dynamics of bosons in the semiclassical regime where the occupation number of the modes is large. To this end, we employ the van Vleck-Gutzwiller propagator to obtain an approximation for the Green's function of the Wigner distribution. The semiclassical analysis incorporates interference of classical paths and reduces to the truncated Wigner approximation (TWA) when the interference is ignored. Furthermore, we identify the Ehrenfest time after which the TWA fails. As a case study, we consider a single-mode quantum nonlinear oscillator, which displays…
The Heisenberg evolution of a given unitary operator corresponds classically to a fixed canonical transformation that is viewed through a moving coordinate system. The operators that form the bases of the Weyl representation and its Fourier transform, the chord representation, are, respectively, unitary reflection and translation operators. Thus, the general semiclassical study of unitary operators allows us to propagate arbitrary operators, including density operators, i.e. the Wigner function. The various propagation kernels are different representations of the superoperators which act on th…
Nonlinear optical signals from an assembly of N noninteracting particles consist of an incoherent and a coherent component, whose magnitudes scale \sim N and \sim N(N-1), respectively. A unified microscopic description of both types of signals is developed using a quantum electrodynamical (QED) treatment of the optical fields. Closed nonequilibrium Green's function expressions are derived that incorporate both stimulated and spontaneous processes. General (n+1)-wave mixing experiments are discussed as an example of spontaneously generated signals. When performed on a single particle, such sign…
Power-law decay of the fraction of the mixed eigenstates in kicked top model with mixed-type classical phase space
该研究分析 kicked top 模型中混合本征态随系统尺寸增大呈幂律衰减，验证了半经典极限下仅存规则与混沌本征态，支持 Berry-Robnik 图像。
Quantum chaos and semiclassical behavior in mushroom billiards II: Structure of quantum eigenstates and their phase space localization properties
研究Bunimovich蘑菇台球中本征态的相空间局域化，通过Poincaré-Husimi表示和熵度量发现混沌本征态局域化规律，并验证混合本征态分数随半经典参数幂律衰减符合PUSC原理。
Further results on the power-law decay of the fraction of the mixed eigenstates in kicked-top model with mixed-type classical phase space
本研究利用Krylov子空间技术拓展受激陀螺模型的研究规模，验证完全混沌情形下的Wehrl熵规律，发现混合型相空间中混合本征态占比随系统规模呈幂律衰减，为相关半经典理论提供支撑。
Semiclassical Husimi distributions for non-Hermitian quantum systems
本文在非厄米量子系统中构造了Schur向量的半经典相空间密度，将每个Schur向量关联至单个Planck元胞，并依据经典范数景观组织Schur态，以体现系统特征寿命，最后通过PT对称踢转子模型验证了该构造的普适性。
We investigate symmetric oscillators, and in particular their quantization, by employing semiclassical and quantum phase functions introduced in the context of Liouville-Green transformations of the Schrödinger equation. For anharmonic oscillators, first order semiclassical quantization is seldom accurate and the higher order expansions eventually break down given the asymptotic nature of the series. A quantum phase that allows in principle to retrieve the exact quantum mechanical quantization condition and wavefunctions is given along with an iterative scheme to compute it. The arbitrariness …
Decay Rates of Optical Modes Unveil the Island Structures in Mixed Phase Space
本研究通过计算非对称微腔中百万个光学本征模式，发现衰减率随波数演化的分立分支对应经典周期轨道，揭示衰减率可逐步解析混合相空间岛结构，并建立含多效应的半经典模型解释相关现象。
When one tries to take into account the non-trivial vacuum structure of Quantum Field Theory, the standard functional-integral tools such as generating functionals or transitional amplitudes, are often quite inadequate for such purposes. Here we propose a generalized generating functional for Green's functions which allows to easily distinguish among a continuous set of vacua that are mutually connected via unitary canonical transformations. In order to keep our discussion as simple as possible, we limit ourselves to Quantum Mechanics where the generating functional of Green's functions is con…
Chaos and anomalous transport in a semiclassical Bose-Hubbard chain
该研究针对半经典区域的Bose-Hubbard链，发现其长链仍存在混合相空间与强反常扩散，极长时间后过渡到正常扩散，反常输运由粒子初始分布决定，并用解析方法验证了结论。
We present a semiclassical prediction of regular-to-chaotic tunneling in systems with a mixed phase space, including the effect of a nonlinear resonance chain. We identify complex paths for direct and resonance-assisted tunneling in the phase space of an integrable approximation with one nonlinear resonance chain. We evaluate the resonance-assisted contribution analytically and give a prediction based on just a few properties of the classical phase space. For the standard map excellent agreement with numerically determined tunneling rates is observed. The results should similarly apply to ioni…
The boundary of the lemon billiards is defined by the intersection of two circles of equal unit radius with the distance 2B between their centers, as introduced by Heller and Tomsovic in Phys. Today 46 38 (1993). We study two classical and quantum lemon billiards, for the cases B = 0.1953, 0.083, which are mixed-type billiards with complex structure of phase space, without significant stickiness regions. A preliminary study of their spectra was published recently (Physics 1 1-14 (2021)). We calculate a great number $10^6$ of consecutive eigenstates and their Poincaré-Husimi (PH) functions, and…
Here we review the many aspects and distinct phenomena associated to quantum dynamics on general graph structures. For so, we discuss such class of systems under the energy domain Green's function ($G$) framework. This approach is particularly interesting because $G$ can be written as a sum over classical-like paths, where local quantum effects are taking into account through the scattering matrix amplitudes (basically, transmission and reflection amplitudes) defined on each one of the graph vertices. Hence, the {\em exact} $G$ has the functional form of a generalized semiclassical formula, wh…
Relaxation of few-body quantum systems can strongly depend on the initial state when the system's semiclassical phase space is mixed, i.e., regions of chaotic motion coexist with regular islands. In recent years, there has been much effort to understand the process of thermalization in strongly interacting quantum systems that often lack an obvious semiclassical limit. Time-dependent variational principle (TDVP) allows to systematically derive an effective classical (nonlinear) dynamical system by projecting unitary many-body dynamics onto a manifold of weakly-entangled variational states. We …
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