复杂势场量子弹球中疤痕态的量子化条件

量子疤痕是波函数在经典不稳定周期轨道周围反常凝聚的一种量子或波动现象.人们对疤痕态的量子化条件进行了大量研究,对深入理解半经典量子化起到了一定的促进作用.之前大部分研究工作主要集中在硬墙量子弹球上,即给定边界形状的无穷深量子势阱系统.本文研究具有光滑复杂势场的二维量子弹球系统,考察疤痕态的量子化条件及其重复出现的规律,得到了与硬墙弹球不一样的结果,对理解这类现象是一个有益的补充.这些结果将有助于理解具有无规长程杂质分布的二维电子系统的态密度谱和输运行为.     

Simulated quantum computation of global minima

Finding the optimal solution to a complex optimisation problem is of great importance in practically all fields of science, technology, technical design and econometrics. We demonstrate that a modified Grover's quantum algorithm can be applied to real problems of finding a global minimum using modest numbers of quantum bits. Calculations of the global minimum of simple test functions and Lennard-Jones clusters have been carried out on a quantum computer simulator using a modified Grover's algorithm. The number of function evaluations N reduced from O(N) in classical simulation to O(N ^1/2) in quantum simulation. We also show how the Grover's quantum algorithm can be combined with the classical Pivot method for global optimisation to treat larger systems.     

Hierarchical dimensional crossover of an optically-trapped quantum gas with disorder

Dimensionality serves as an indispensable ingredient in any attempt to formulate low-dimensional physics, and studying the dimensional crossover at a fundamental level is challenging. The purpose of this work is to study the hierarchical dimensional crossovers, namely the crossover from three dimensions (3D) to quasi-2D and then to 1D. Our system consists of a 3D Bose–Einstein condensate trapped in an anisotropic 2D optical lattice characterized by the lattice depths V₁ along the x direction and V₂ along the y direction, respectively, where the hierarchical dimensional crossover is controlled via V₁ and V₂. We analytically derive the ground-state energy, quantum depletion and the superfluid density of the system. Our results demonstrate the 3D-quasi-2D-1D dimensional crossovers in the behavior of quantum fluctuations. Conditions for possible experimental realization of our scenario are also discussed.     

Off shell dynamics of the O(3) NLS model beyond Monte Carlo and perturbation theory

The off-shell dynamics of the O(3) non-linear sigma model is probed in terms of spectral densities and two-point functions by means of the form factor approach. The exact form factors of the spin field, Noether current, EM tensor and the topological charge density are computed up to six particles. The corresponding n 6 particle spectral densities are used to compute the two-point functions, and are argued to deviate at most a few per mille from the exact answer in the entire energy range below 103 in units of the mass gap. To cover yet higher energies we propose an extrapolation scheme to arbitrary particle numbers based on a novel scaling hypothesis for the spectral densities. It yields candidate results for the exact two-point functions at all energy scales and allows us to exactly determine the values of two, previously unknown, non-perturbative constants.     

Tuning energy transfer efficiency in quantum dots mixture by controling donor/acceptor ratio

Improving the emission performance of colloidal quantum dots (QDs) is of paramount importance for their applications on light-emitting diodes (LEDs), displays and lasers. A highly promising approach is to tune the carrier recombination channels and lifetime by exploiting the energy transfer process. However, to achieve this precise emission optimization, quantitative modulation on energy transfer efficiency is highly desirable but still challenging. Here, we demonstrate a convenient approach to realize tunable energy transfer efficiency by forming QDs mixture with controllable donor/acceptor (D/A) ratio. With the mixing ratio ranging from 16/1 to 1/16, the energy transfer efficiency could be effectively tuned from near zero to ~70%. For the high mixing ratio of 16/1, acceptors obtain adequate energy supplied by closely surrounding donors, leading to~2.4-fold PL enhancement. While for the low mixing ratio, the ultrafast and efficient energy extraction process directly suppresses the multi-exciton and Auger recombination in the donor, bringing about a higher threshold. The facile modulation of emission performance by controllably designed mixing ratio and quantitatively tunable energy transfer efficiency will facilitate QD-based optoelectronic and photovoltaic applications.     

Evaluation of dynamical properties of open quantum systems using the driven Liouville-von Neumann approach: methodological considerations

Methodological aspects of using the driven Liouville-von Neumann (DLvN) approach for simulating dynamical properties of molecular junctions are discussed. As a model system we consider a non-interacting resonant level uniformly coupled to a single Fermionic bath. We demonstrate how a finite system can mimic the depopulation dynamics of the dot into an infinite band bath of continuous and uniform density of states. We further show how the effects of spurious energy resolved currents, appearing due to the approximate nature of the equilibrium state obtained in DLvN calculations, can be avoided. Several ways to approach the wide band limit, which is often adopted in analytical treatments, using a finite numerical model system are discussed including brute-force increase of the lead model bandwidth as well as efficient cancellation or direct subtraction of finite-bandwidth effect. These methodological considerations may be relevant also for other numerical schemes that aim to study non-equilibrium thermodynamics via simulations of open quantum systems.     

Comparing Two Definitions of Work for a Biological Quantum Heat Engine

Systems of photosynthetic reaction centres have been modelled as heat engines, while it has also been reported that the efficiency and power of such heat engines can be enhanced by quantum interference|a trait that has attracted much interest. We compare two definitions of the work of such a photosynthetic heat engine, i.e. definition A used by Weimer et al. and B by Dorfman et al. We also introduce a coherent interaction between donor and acceptor (CIDA) to demonstrate a reversible energy transport. We show that these two definitions of work can impart contradictory results, that is, CIDA enhances the power and efficiency of the photosynthetic heat engine with definition B but not with A. Additionally, we find that both reversible and irreversible excitation-energy transport can be described with definition A, but definition B can only model irreversible transport. As a result, we conclude that definition A is more suitable for photosynthetic systems than definition B.     

Variational quantum simulation of thermal statistical states on a superconducting quantum processer

Quantum computers promise to solve finite-temperature properties of quantum many-body systems, which is generally challenging for classical computers due to high computational complexities. Here, we report experimental preparations of Gibbs states and excited states of Heisenberg $XX$ and $XXZ$ models by using a 5-qubit programmable superconducting processor. In the experiments, we apply a hybrid quantum-classical algorithm to generate finite temperature states with classical probability models and variational quantum circuits. We reveal that the Hamiltonians can be fully diagonalized with optimized quantum circuits, which enable us to prepare excited states at arbitrary energy density. We demonstrate that the approach has a self-verifying feature and can estimate fundamental thermal observables with a small statistical error. Based on numerical results, we further show that the time complexity of our approach scales polynomially in the number of qubits, revealing its potential in solving large-scale problems.     

非马尔科夫耗散系统长时演化下的极限环振荡现象

本文研究结构化环境中非马尔科夫耗散系统在长时演化下可能出现的极限环振荡现象. 对于欧姆型谱密度环境中的二能级系统, 由于体系只允许一个束缚态模, 给定初态系统在Bloch空间的长时演化将收敛于一个极限环. 研究揭示了极限环半径与环心位置同环境谱密度函数间的关系. 对于多带光子晶体环境中的二能级系统, 由于其可以存在多个束缚态, 研究展现了系统在长时演化下可能出现的收敛于环面或周期或准周期的振荡行为. 有关环面的特征量与环境谱密度间的量化关系同样得以刻画. 论文随后讨论了两比特系统关联量在局域非马尔科夫耗散环境中长时演化可能出现的特征行为.     

Enhancing parameter precision of optimal quantum estimation by quantum screening

We propose a scheme of quantum screening to enhance the parameter-estimation precision in open quantum systems by means of the dynamics of quantum Fisher information.The principle of quantum screening is based on an auxiliary system to inhibit the decoherence processes and erase the excited state to the ground state.By comparing the case without quantum screening,the results show that the dynamics of quantum Fisher information with quantum screening has a larger value during the evolution processes.     

Theoretical study on the exciton dynamics of coherent excitation energy transfer in the phycoerythrin 545 light-harvesting complex

The experimental observation of long-lived quantum coherence in the excitation energy transfer(EET)process of the several photosynthetic light-harvesting complexes at low and room temperatures has aroused hot debate.It challenges the common perception in the field of complicated pigment molecular systems and evokes considerable theoretical efforts to seek reasonable explanations.In this work,we investigate the coherent exciton dynamics of the phycoerythrin 545(PE545)complex.We use the dissipation equation of motion to theoretically investigate the effect of the local pigment vibrations on the population transfer process.The result indicates that the realistic local pigment vibrations do assist the energy transmission.We demonstrate the coherence between different pigment molecules in the PE545 system is an essential ingredient in the EET process among various sites.The coherence makes the excitation energy delocalized,which leads to the redistribution of the excitation among all the chromophores in the steady state.Furthermore,we investigate the effects of the complex high-frequency spectral density function on the exciton dynamics and find that the high-frequency Brownian oscillator model contributes most to the exciton dynamic process.The discussions on the local pigment vibrations of the Brownian oscillator model suggest that the local heterogeneous protein environments and the effects of active vibration modes play a significant role in coherent energy transport.     

Selected topics of quantum computing for nuclear physics

Nuclear physics,whose underling theory is described by quantum gauge field coupled with matter,is fundamentally important and yet is formidably challenge for simulation with classical computers.Quantum computing provides a perhaps transformative approach for studying and understanding nuclear physics.With rapid scaling-up of quantum processors as well as advances on quantum algorithms,the digital quantum simulation approach for simulating quantum gauge fields and nuclear physics has gained lots of attention.In this review,we aim to summarize recent efforts on solving nuclear physics with quantum computers.We first discuss a formulation of nuclear physics in the language of quantum computing.In particular,we review how quantum gauge fields(both Abelian and non-Abelian)and their coupling to matter field can be mapped and studied on a quantum computer.We then introduce related quantum algorithms for solving static properties and real-time evolution for quantum systems,and show their applications for a broad range of problems in nuclear physics,including simulation of lattice gauge field,solving nucleon and nuclear structures,quantum advantage for simulating scattering in quantum field theory,non-equilibrium dynamics,and so on.Finally,a short outlook on future work is given.     

Matrix Algebras in Non-Hermitian Quantum Mechanics

In principle, non-Hermitian quantum equations of motion can be formulated using as a starting point either the Heisenberg's or the Schrödinger's picture of quantum dynamics. Here it is shown in both cases how to map the algebra of commutators, defining the time evolution in terms of a non-Hermitian Hamiltonian, onto a non-Hamiltonian algebra with a Hermitian Hamiltonian. The logic behind such a derivation is reversible, so that any Hermitian Hamiltonian can be used in the formulation of non-Hermitian dynamics through a suitable algebra of generalized (non-Hamiltonian) commutators.
These results provide a general structure (a template) for non-Hermitian equations of motion to be used in the computer simulation of open quantum systems dynamics.     

Intermolecular energy transfer in mixed laser dyes: photophysical properties of triplet states

Triplet-singlet energy transfer in laser dyes have been studied in EPA at 77K using N₂ laser as an excitation source. Phosphorescence of the donor (D) and the delayed fluorescence of the acceptor (A) and their lifetimes have been measured for coumarin 102 (D)-rhodamine B(A) and 9(10H)-acridone (D)-rhodamine 6G(A) dye systems as a function of acceptor concentration. These data yield energy transfer rate constants of ∼10³ dm³ mol⁻¹ s⁻¹ for the donor acceptor combinations, consistent with the Forster mechanism. The phosphorescence quantum efficiency and other spectral parameters are also reported.     

Excitation Dynamics in Chain-Mapped Environments

The chain mapping of structured environments is a most powerful tool for the simulation of open quantum system dynamics. Once the environmental bosonic or fermionic degrees of freedom are unitarily rearranged into a one dimensional structure, the full power of Density Matrix Renormalization Group (DMRG) can be exploited. Beside resulting in efficient and numerically exact simulations of open quantum systems dynamics, chain mapping provides an unique perspective on the environment: the interaction between the system and the environment creates perturbations that travel along the one dimensional environment at a finite speed, thus providing a natural notion of light-, or causal-, cone. In this work we investigate the transport of excitations in a chain-mapped bosonic environment. In particular, we explore the relation between the environmental spectral density shape, parameters and temperature, and the dynamics of excitations along the corresponding linear chains of quantum harmonic oscillators. Our analysis unveils fundamental features of the environment evolution, such as localization, percolation and the onset of stationary currents.     

Quantum Dynamics of Entanglement and Single Excitation Transfer in LH1-RC-Type Trimer

In this paper,we study quantum dynamics of entanglement and single excitation transfer(SET) in an LH1-RC-type trimer which can describe a basic unit cell in the LH1-RC complex in the photosynthetic process.It is shown that there exists a sudden change of entanglement at the critic point of quantum phase transition(QPT) of the system at low temperatures,the entanglement sudden change caused by the QPT is suppressed at higher temperatures.We investigate the influence of environment on entanglement and SET.We show the generation of the dephasing-assisted entanglement between a donor and an acceptor and the existence of the steady-state entanglement,and demonstrate the entanglement transfer from donor-donor entanglement to donor-acceptor entanglement in the dynamic evolution.We reveal the close relation between the SET probability and donor-acceptor entanglement.Especially,we find that the SET probability is proportional to the amount of donor-acceptor entanglement under certain conditions.     

He原子体系中偶极子响应的周期性量子相位调控的理论研究

基于激光诱导相位模型, 研究了周期性相位调控的He原子体系的光谱响应. 研究发现,周期性的相位调控会导致He原子吸收谱由单个孤立的洛伦兹线型转化为等间隔的“梳状”结构. “梳状”光谱的性质主要由原子系统和控制脉冲链的性质决定, 并给出了表征“梳状”光谱的理论公式. 该机理具有普遍适用性, 它可以应用到任意原子体系, 进而推广到任意波段, 并且为任意波段的脉冲整形提供了可能.     

Harnessing symmetry to control quantum transport

Controlling transport in quantum systems holds the key to many promising quantum technologies. Here we review the power of symmetry as a resource to manipulate quantum transport and apply these ideas to engineer novel quantum devices. Using tools from open quantum systems and large deviation theory, we show that symmetry-mediated control of transport is enabled by a pair of twin dynamic phase transitions in current statistics, accompanied by a coexistence of different transport channels. By playing with the symmetry decomposition of the initial state, one can modulate the importance of the different transport channels and hence control the flowing current. Motivated by the problem of energy harvesting, we illustrate these ideas in open quantum networks, an analysis that leads to the design of a symmetry-controlled quantum thermal switch. We review an experimental setup recently proposed for symmetry-mediated quantum control in the lab based on a linear array of atom-doped optical cavities, and the possibility of using transport as a probe to uncover hidden symmetries, as recently demonstrated in molecular junctions, is also discussed. Other symmetry-mediated control mechanisms are also described. Overall, these results demonstrate the importance of symmetry not only as an organizing principle in physics but also as a tool to control quantum systems.     

聚乙烯单链量子热输运的同位素效应

高分子导热材料的有效调控受到了日益广泛的关注.应用密度泛函理论(DFT)、中央插入延展(central insertion scheme,CIS)方法及非平衡格林函数(NEGF)理论,对包含432个原子、长18.533 nm的聚乙烯单链量子热输运的同位素效应进行了研究.计算结果表明,室温下长100 nm的纯12C聚乙烯单链的热导率理论上限高达314.1 W·m~(-1)·K~(-1);对于~(12)C聚乙烯单链,其他条件一定时,~(14)C掺杂引起的热导同位素效应比~(13)C更为显著;室温下纯~(12)C聚乙烯单链中~(14)C掺杂原子百分数为50%时同位素效应最显著,此时平均热导比未掺杂时下降了51%.这对探索聚乙烯材料热输运的同位素影响机理具有十分积极的意义.