Assessing non-Markovian quantum dynamics

We investigate what a snapshot of a quantum evolution--a quantum channel reflecting open system dynamics--reveals about the underlying continuous time evolution. Remarkably, from such a snapshot, and without imposing additional assumptions, it can be decided whether or not a channel is consistent with a time (in)dependent Markovian evolution, for which we provide computable necessary and sufficient criteria. Based on these, a computable measure of "Markovianity" is introduced. We discuss how the consistency with Markovian dynamics can be checked in quantum process tomography. The results also clarify the geometry of the set of quantum channels with respect to being solutions of time (in)dependent master equations.     

Effect of collisions on the resonance fluorescence spectrum

The effect of collisions on the resonance fluorescence spectrum of two-level atoms excited by monochromatic resonance radiation is studied. Analysis is performed for systems where the Doppler broadening is small compared to the collision frequency (high buffer gas pressures) and for the general case where collisions arbitrarily change the phase of the radiation-induced dipole moment (from completely interrupted to completely unaffected phase of the dipole moment). Both at a relatively low and at a high excitation radiation intensity, the resonance fluorescence spectrum is shown to depend on whether the two-level system is closed or open. This is especially true for the narrow unshifted Rayleigh scattering line. It is shown that, although the absorption line is homogeneously broadened, the resonance fluorescence spectrum exhibits a clearly pronounced anisotropy. In a direction close to the direction of propagation of the excitation radiation, the Rayleigh scattering line is maximally narrowed. Under certain conditions (that can easily be created in experiments), the width of this line is proportional to the diffusion coefficient of atoms interacting with the radiation.     

Cooperative effects in the two atom resonance fluorescence spectrum

We consider the resonance fluorescence spectrum of two similar interacting atoms in the limit of high photon densities. The excitation spectrum of the symmetric modes contains two sidebands in addition to the usual three peaks which are analogous to those of the isolated atom. These two new sidebands are due entirely to the cooperative behaviour of the two atoms and vanish when the atoms are far apart. The energy shifts and spectral widths for these two sidebands are two and five times larger than those for the isolated atom respectively. The probability of occurrence of these sidebands depends on the parameters V_AB/Ω and γ²₀/Ω², where V_AB is the dipole-dipole interaction energy, γ₀ is the spontaneous emission probability and Ω is the Rabi frequency. The asymmetric broadening of both sidebands depends on the parameter γ₀/Ω. The possibility to measure the dipole-dipole energy through the observation of these sidebands is discussed.     

Theoretical study of the continuous evolution of a resonance fluorescence spectrum towards a resonance Raman spectrum

A theoretical model is proposed to interpret the continuous transition from a resonance fluorescence spectrum to a resonance Raman spectrum, recently observed when the scattering molecule is progressively perturbed by its environment. An analytical expression for the resonant scattering tensor is given, which describes the evolution of the resonance fluorescence spectrum of a diatomic molecule perturbed by a foreign gas. The variations of the intensities and polarizations are quantitatively studied in the case of iodine, and it is shown how the resonance fluorescence spectrum is progressively transformed into a resonance Raman spectrum when the pressure of the perturbing gas is increased.     

Dynamical learning of non-Markovian quantum dynamics

We study the non-Markovian dynamics of an open quantum system with machine learning.The observable physical quantities and their evolutions are generated by using the neural network.After the pre-training is completed,we fix the weights in the subsequent processes thus do not need the further gradient feedback.We find that the dynamical properties of physical quantities obtained by the dynamical learning are better than those obtained by the learning of Hamiltonian and time evolution operator.The dynamical learning can be applied to other quantum many-body systems,non-equilibrium statistics and random processes.     

Theory for frequent measurements of spontaneous emissions in a non-Markovian environment: Beyond the Lorentzian spectrum

The measurement-result-conditioned evolution of a system(e.g., an atom) with spontaneous emissions of photons is described by the quantum trajectory(QT) theory. In this work we generalize the associated QT theory from an infinitely wide bandwidth Markovian environment to the finite bandwidth non-Markovian environment. In particular, we generalize the treatment for an arbitrary spectrum, which is not restricted by the specific Lorentzian case. We rigorously prove the general existence of a perfect scaling behavior jointly defined by the bandwidth of the environment and the time interval between successive photon detections.For a couple of examples, we obtain analytic results to facilitate the QT simulations based on the Monte-Carlo algorithm. For the case where the analytical result is not available, a numerical scheme is proposed for practical simulations.     

Theory for frequent measurements of spontaneous emissions in a non-Markovian environment: Beyond the Lorentzian spectrum

We consider the special case that the dark matter(DM) candidate is not detected in direct-detection programs when the experimental sensitivity reaches the neutrino flux background. In such circumstance the DM searches at the colliders impose constraints on the DM relic abundance if the DM candidate is a WIMPs type. Specifically, we consider the triplet(quintet and septet) DMs in the framework of minimal DM model and explore the potential of discovering the DM candidate in the mono-jet, mono-photon and vector boson fusion channels at the Large Hadron Collider(LHC) and future 100 TeV hadron collider. If the DM candidate in such a scenario is discovered at the LHC, then additional DM candidates are needed to explain the observed relic abundance.On the other hand, null results in those DM searching programs at the colliders give rise to lower limits of DM relic abundance.     

An efficient non-Markovian theory of non-equilibrium dynamics

A direct interaction (DI) approximation-like theory is developed. It effectively truncates the time history integrals of DI by explicitly allowing for non-Gaussian correlations. Numerical comparisons with DI for the case of the Zakharov equations are presented.     

Different indicators for Markovian and non-Markovian dynamics

The Markovianity/non-Markovianity of two different systems are discussed by means of the quantum speed limit time and quantum Fisher information. The first system is described by a central mass particle interacts locally with its surrounding particles, while the second and third models consist of a single qubit interacts with a non-detuning Lorentzian cavity and with a thermal reservoir, respectively. For the first model, the large distance between the central particle and the surrounding particles is guaranty for a fixed quantum speed limit, while the driving time plays the central role on the fixed behavior of the quantum speed limit time. Due to the stable behavior of the quantum speed limit time and the quantum Fisher information, the exchange information between the systems and their surrounding is limited. The distance between the central mass particle and its surrounding particle plays the main role on predicating the Markovianity/non-Markovianity. For the second system the driving time is an important parameter that control on the Markovianity/non-Markovianity behavior. Finally the third model proves that non-Markovianity dynamic may increase the speed and the sensitivity of the open system.     

Photon statistics and dynamics of fluorescence resonance energy transfer

We report high time-resolution measurements of photon statistics from pairs of dye molecules coupled by fluorescence resonance energy transfer (FRET). In addition to quantum-optical photon antibunching, we observe photon bunching on a time scale of several nanoseconds. We show by numerical simulation that configuration fluctuations in the coupled fluorophore system could account for minor deviations of our data from predictions of basic F?rster theory. With further characterization we believe that FRET photon statistics could provide a unique tool for studying DNA mechanics on time scales from 10(-9)-10(-3) s.     

Delineating incoherent non-Markovian dynamics using quantum coherence

We introduce a method of characterization of non-Markovianity using coherence of a system interacting with the environment. We show that under the allowed incoherent operations, monotonicity of a valid coherence measure is affected due to non-Markovian features of the system–environment evolution. We also define a measure to quantify non-Markovianity of the underlying dynamics based on the non-monotonic behavior of the coherence measure. We investigate our proposed non-Markovianity marker in the behavior of dephasing and dissipative dynamics for one and two qubit cases. We also show that our proposed measure captures the back-flow of information from the environment to the system and compatible with well known distinguishability criteria of non-Markovianity.     

Exact solution for a non-Markovian dissipative quantum dynamics

We provide the exact analytic solution of the stochastic Schr?dinger equation describing a harmonic oscillator interacting with a non-Markovian and dissipative environment. This result represents an arrival point in the study of non-Markovian dynamics via stochastic differential equations. It is also one of the few exactly solvable models for infinite-dimensional systems. We compute the Green's function; in the case of a free particle and with an exponentially correlated noise, we discuss the evolution of Gaussian wave functions.     

Velocity spectrum for non-Markovian Brownian motion in a periodic potential

Non-Markovian Brownian motion in a periodic potential is studied by means of an electronic analogue simulator. Velocity spectra, the Fourier transforms of velocity autocorrelation functions, are obtained for three types of random force, that is, a white noise, an Ornstein—Uhlenbeck process, and a quasimonochromatic noise. The analogue results are in good agreement both with theoretical ones calculated with the use of a matrix-continued-fraction method, and with the results of digital simulations. An unexpected extra peak in the velocity spectrum is observed for Ornstein-Uhlenbeck noise with large correlation time. The peak is attributed to a slow oscillatory motion of the Brownian particle as it moves back and forth over several lattice spaces. Its relationship to an approximate Langevin equation is discussed.     

Entanglement dynamics of multiqubit system in Markovian and non-Markovian reservoirs

We study the entanglement dynamics of three qubits in contact with independent Markovian or non-Markovian reservoirs. The qubits are prepared in two types of GHZ-like or W-like states distinguished by initial excited-state populations. Though belonging to the same GHZ or W class of entanglement, the states with different initial excitations exhibit strikingly different dynamics. In addition, we show that the non-Markovian reservoirs can recover the multiqubit entanglement at instantaneous points or after a finite interval of entanglement disappearance. We also investigate the protection of multiqubit entanglement by the control of excitation emission via the detuning.     

The fluorescence spectrum of a three-level atom in two-photon resonance with the laser field

The influence of the two-photon resonant interaction on the fluoresence spectrum of a three-level atom is studied. The levels in questions are the 1S, 2P and 3S states of a two-electron atom. It is shown, that in the case of exact two-photon resonance between the 1S–3S transition and the laser field, and when the 2P state is out of resonance, the fluorescence spectrum contains 4 lines. The properties of these lines are discussed.     

Comparison between non-Markovian dynamics with and without rotating wave approximation

In the limit of weak coupling between a system and its reservoir,we derive the time-convolutionless(TCL) nonMarkovian master equation for a two-level system interacting with a zero-temperature structured environment with no rotating wave approximation(NRWA).By comparing the dynamics with RWA,we demonstrate the impact of RWA on the system dynamics,as well as the effects of non-Markovianity on the preservation of atomic coherence,squeezing,and entanglement.     

Effect of non-Markovian dynamics on the violation of tripartite Bell inequalities

The effect of the non-Markovian dynamics on the violation of Bell inequalities for a generic tripartite entangled state has been investigated. Our results imply that the dynamics of Bell violation depends not only on the coupling strengths between the system and the environment but also on the rotation angles. Under both the strong and weak couplings, the Bell nonlocality can be eliminated by the decoherence induced by the environment in finite time when the rotation angles take certain values.     

单光子调制频谱用于量子点荧光寿命动力学的研究

本文开展了基于单光子调制频谱测量量子点荧光寿命动力学特性的研究.在脉冲激光激发下,对探测到的量子点单光子荧光信号进行频谱分析以获得荧光调制频谱,研究发现特征频谱信号幅值与荧光寿命之间存在确定的非线性对应关系.这种单光子调制频谱方法能有效消除背景噪声和单光子探测器暗计数的影响,用于分析量子点荧光寿命动力学特性时在准确度以及时间分辨率方面都较目前普遍采用的荧光衰减曲线寿命拟合方法呈现出明显优势:当涨落误差为5%时,寿命测量准确度提高了一个数量级;当涨落误差和偏离误差均为5%时,对动力学测量效率以及时间分辨率提高了四倍以上.因此单光子调制频谱可以作为获取量子点在短时间尺度内激发态动力学信息的一种有效技术手段.