Energy transfer as a random walk with long-range steps

We consider the incoherent energy transport in molecular crystals, where the transfer rates stem from Coulombic and exchange interactions. For substitutionally disordered lattices we present in a first passage model the excitation decay due to trapping by randomly distributed traps; the decay is related to the distribution of the number of distinct sites visited during the timet and is expressible through the cumulants of this distribution. The validity domains of approximate decay laws based on the first few cumulants are also discussed. We exemplify the findings for dipolar transfer rates between randomly distributed molecules on a square lattice, by comparing the random walk on the random system to its CTRW (continuous time random walk) counterpart.     

修饰态布居的选择性激发对无反转激光的作用

以三能级V型系统为例研究修饰态布居的选择性激发对无反转激光增益的作用. 当非 相干驱动场的频谱宽度远小于驱动场产生的修饰态能级的间距时，非相干驱动场只将一个修 饰态的布居抽运至激发态. 借助原子的衰减通道，系统中形成单向布居转移通道，从而建立 修饰态布居的选择性激发. 利用修饰态布居的选择性激发，可以摆脱裸态共振无反转激光的 三个限制: (1) 不再要求辅助的低频驱动跃迁比高频激光跃迁具有更高的衰减速率；(2) 显 著降低非相干激发速率的阈值；(3) 无反转激光的线性增益不再反比于相干驱动场的强 关键词： 修饰态布居的选择性激发 无反转激光增益 原子衰减速率 非相干激发阈值速率     

Finding tree symmetries using continuous-time quantum walk

Quantum walk, the quantum counterpart of random walk, is an important model and widely studied to develop new quantum algorithms. This paper studies the relationship between the continuous-time quantum walk and the symmetry of a graph, especially that of a tree. Firstly, we prove in mathematics that the symmetry of a graph is highly related to quantum walk. Secondly, we propose an algorithm based on the continuous-time quantum walk to compute the symmetry of a tree. Our algorithm has better time complexity O(N3) than the current best algorithm. Finally, through testing three types of 10024 trees, we find that the symmetry of a tree can be found with an extremely high efficiency with the help of the continuous-time quantum walk.     

Time-dependent photon correlations for incoherently pumped quantum dot strongly coupled to the cavity mode

The time dependence of correlations between the photons emitted from a microcavity with an embedded quantum dot under incoherent pumping is studied theoretically. Analytic expressions for the second-order correlation function g ⁽²⁾(t) are presented in strong and weak coupling regimes. The qualitative difference between the incoherent and coherent pumping schemes in the strong coupling case is revealed: under incoherent pumping, the correlation function demonstrates pronounced Rabi oscillations, but in the resonant pumping case, these oscillations are suppressed. At high incoherent pumping, the correlations decay monoexponentially. The decay time nonmonotonically depends on the pumping value and has a maximum corresponding to the self-quenching transition.     

Experimental demonstration of selective coherent population transfer via a continuum

We report the first experimental demonstration of coherent population transfer, induced by stimulated Raman adiabatic passage, via continuum states. Population is transferred from the metastable state 2s(1)S(0) to the excited state 4s(1)S(0) in helium atoms in a two-photon process mediated by coherent interaction with the ionization continuum. While incoherent techniques usually do not permit any population transfer in such a process, we show that stimulated Raman adiabatic passage allows significant population transfer to take place also via ultrafast decay channels.     

Continuous-time quantum walks on star graphs

In this paper, we investigate continuous-time quantum walk on star graphs. It is shown that quantum central limit theorem for a continuous-time quantum walk on star graphs for N-fold star power graph, which are invariant under the quantum component of adjacency matrix, converges to continuous-time quantum walk on K₂ graphs (complete graph with two vertices) and the probability of observing walk tends to the uniform distribution.     

Laser pulse sequence effects on selective ionization in three-level systems driven by coherent radiation fields

Using the time-dependent Schrödinger equation we investigate the effect of different pulse sequences of two lasers on selective ionization in three-level systems driven by coherent radiation fields. It is assumed that inhomogeneous broadening as well as the radiative decay can be neglected during the laser pulses. We demonstrate to what an extent the selectivity given by the absorption or relaxation process can be increased by coherent interaction during the first step and different timing of the incoherent interaction during the second step of a two-step photoionization. A comparison is made with the rate-equation results. We could show that utilization of coherent excitation for high ionization selectivity is most efficient if the two laser pulses do not overlap.     

Coherent population trapping in an ensemble of three-level atoms in the presence of cooperative relaxation

We derive equations that describe the dynamics of three-level atoms with a cascade system of levels interacting with two resonant coherent fields in conditions where cooperative relaxation dominates over incoherent spontaneous emission. We calculate the temporal dynamics of the values of the atomic populations. The possibility of coherent population trapping in the presence of the cooperative decay is established. Finally, we calculate the averaged population of the intermediate level as a function of detuning for different values of the coupling constant. Zh. éksp. Teor. Fiz. 112, 869–876 (September 1997)     

On the quantum CTRW approach

The concept of continuous-time random walk is generalized into the quantum approach using a completely positive map. This approach introduces in a phenomenological way the concept of disorder in the transport problem of a quantum open system. If the waiting-time of the continuous-time renewal approach is exponential we recover a semigroup for a dissipative quantum walk. Two models of non-Markovian evolution have been solved considering different types of waiting-time functions.     

State transfer on two-fold Cayley trees via quantum walks

Perfect state transfer(PST)has great significance due to its applications in quantum information processing and quantum computation.The main problem we study in this paper is to determine whether the two-fold Cayley tree,an extension of the Cayley tree,admits perfect state transfer between two roots using quantum walks.We show that PST can be achieved by means of the so-called nonrepeating quantum walk[Phys.Rev.A 89042332(2014)]within time steps that are the distance between the two roots;while both the continuous-time quantum walk and the typical discrete-time quantum walk with Grover coin approaches fail.Our results suggest that in some cases the dynamics of a discrete-time quantum walk may be much richer than that of the continuous-time quantum walk.     

Correlation functions of a time-continuous dissipative system with a strange attractor

We determine the exact decay of time correlation functions of a continuous-time chaotic system. In contrast to discrete-time chaotic systems where these correlations decay as a rule exponentially fast we find in our continuous-time system long-time tails well known from many-particle systems.     

Comparative study on the propagation performance of coherently combined and incoherently combined beams

Taking phase error, turbulent atmosphere, jitter, vacancy factor and tilt error into consideration, we propose a general propagation formula for both coherent and incoherent combined beams with different kinds of aberrations. Comparative study on the propagation performance of coherent and incoherent combined beams is presented. Beam propagation factor (BPF) defined as laser output power in a specified far-field bucket divided by the total output power radiating from the effective near-field exit aperture of the laser beam is introduced as the beam quality factor to give a quantitative study. It is revealed that the coherent combined beam has great advantage when propagating in free-space compared with the incoherent combined beams. However, the coherent combined beam is more sensitive to the environmental aberrations, and the beam quality degrades faster with an increase in the aberrations’ intensity. Scintillation index as the high moments of the combined intensity field is also studied. It is revealed that both coherent and incoherent combined beams exhibit less scintillations compared with a single-aperture beam, and the incoherent combined beam demonstrates better scintillations reducing the performance. Our methodology offers an all-round performance evaluation on the two kinds of laser systems.     

Approximate locality for quantum systems on graphs

In this Letter we make progress on a long-standing open problem of Aaronson and Ambainis [Theory Comput. 1, 47 (2005)]: we show that if U is a sparse unitary operator with a gap Delta in its spectrum, then there exists an approximate logarithm H of U which is also sparse. The sparsity pattern of H gets more dense as 1/Delta increases. This result can be interpreted as a way to convert between local continuous-time and local discrete-time quantum processes. As an example we show that the discrete-time coined quantum walk can be realized stroboscopically from an approximately local continuous-time quantum walk.     

Experimental observations of boundary conditions of continuous-time quantum walks

The continuous-time quantum walk(CTQW) is the quantum analogue of the continuous-time classical walk and is widely used in universal quantum computations. Here, taking the advantages of the waveguide arrays, we implement large-scale CTQWs on chips. We couple the single-photon source into the middle port of the waveguide arrays and measure the emergent photon number distributions by utilizing the fiber coupling platform. Subsequently, we simulate the photon number distributions of the waveguide arrays by considering the boundary conditions. The boundary conditions are quite necessary in solving the problems of quantum mazes.     

Cooperative population dynamics of an ensemble of Λ atoms in a bichromatic field

Equations are derived which describe the dynamics of three-level atoms with a Λ level scheme, interacting with two coherent resonance fields under conditions such that cooperative relaxation predominates over incoherent spontaneous emission. A numerical calculation of the temporal dynamics of the values of the atomic populations is performed. It is shown that coherent population trapping in the presence of cooperative decay is possible. The quantities characterizing this phenomenon are calculated—the width of the black line and the transition time to coherent trapping in this scheme. Zh. Tekh. Fiz. 68, 19–24 (June 1998)     

Detection of elements with coherent reflectivity inside a multiscatterers incoherent media

In this paper we use the Spatial Compound method to detect a long element with coherent reflectivity within a vast incoherent media. We consider the case in which the echographics temporal signals of the coherent element, and of the scatterers of the media in which it is hidden are of the same order of amplitude. We calculate analytically the spatial coherent function of the acoustic pressure signals considering space-temporal averages. We find experimentally that when there is a coherent long scatter this function is broader (in the spatial direction along the scatter) than when there is only incoherent media. By selecting short temporal windows of analysis we show the possibility of detecting the depth where the coherent element is located.     

On the Relationship Between Continuous- and Discrete-Time Quantum Walk

Quantum walk is one of the main tools for quantum algorithms. Defined by analogy to classical random walk, a quantum walk is a time-homogeneous quantum process on a graph. Both random and quantum walks can be defined either in continuous or discrete time. But whereas a continuous-time random walk can be obtained as the limit of a sequence of discrete-time random walks, the two types of quantum walk appear fundamentally different, owing to the need for extra degrees of freedom in the discrete-time case. In this article, I describe a precise correspondence between continuous- and discrete- time quantum walks on arbitrary graphs. Using this correspondence, I show that continuous-time quantum walk can be obtained as an appropriate limit of discrete-time quantum walks. The correspondence also leads to a new technique for simulating Hamiltonian dynamics, giving efficient simulations even in cases where the Hamiltonian is not sparse. The complexity of the simulation is linear in the total evolution time, an improvement over simulations based on high-order approximations of the Lie product formula. As applications, I describe a continuous-time quantum walk algorithm for element distinctness and show how to optimally simulate continuous-time query algorithms of a certain form in the conventional quantum query model. Finally, I discuss limitations of the method for simulating Hamiltonians with negative matrix elements, and present two problems that motivate attempting to circumvent these limitations.     

非相干多分量空间双稳态孤子

非相干多分量光束在高阶非线性介质中可以形成双稳态孤子.利用相干密度法首次得到了非相干多分量双稳态孤子的解析表达式.介质的高阶非线性决定了双稳态孤子的存在范围和峰值强度.得到了光束的截止波长，最小宽度和介质的非线性条件，并研究了非相干多分量光束中相干分量的传输演化.利用稳定性条件，严格证明了非相干多分量双稳态孤子在介质中可以保持稳定传播. 关键词： 非相干多分量光束 空间孤子 双稳态     

Coherence effect on the measurement of optical fiber nonlinear coefficient

We investigated a source coherence effect on the measurement of the optical fiber nonlinear coefficient by use of dual optical frequencies. We produced coherent and incoherent dual optical frequencies with a self-heterodyne Mach-Zehnder interferometer with and without a delay line, respectively. Our measurement results included both an electrostrictive and a Kerr nonlinear coefficient because of the low modulation frequency. Comparing the coherent case with an incoherent case, we obtained a nonlinear coefficient that was more than 3% higher in the coherent case.     

饱和对数非线性介质中非相干孤子碰撞对相干性的改善

基于相干密度理论，数值地研究了饱和对数非线性支持的部分非相干亮孤子对的相互作用.研究表明，两个非相干亮孤子碰撞不仅能增大碰撞区的光强，还可以大大改善部分非相干光束的相干性.同时还研究了非相干性对孤子碰撞的影响，非相干性不仅抑制了孤子间的相干作用如吸引、排斥和能量交换，同时还由于非相干叠加作用而引入了弱的相互吸引. 关键词： 非相干性 饱和对数非线性 空间光孤子