Enhancement of feasibility of macroscopic quantum superposition state with the quantum Rabi-Stark model

We propose an efficient scheme to generate a macroscopical quantum superposition state with a cavity optomechanical system, which is composed of a quantum Rabi-Stark model coupling to a mechanical oscillator. In a low-energy subspace of the Rabi-Stark model, the dressed states and then the effective Hamiltonian of the system are given. Due to the coupling of the mechanical oscillator and the atom-cavity system, if the initial state of the atom-cavity system is one of the dressed states, the mechanical oscillator will evolve into a corresponding coherent state. Thus, if the initial state of the atom-cavity system is a superposition of two dressed states, a coherent state superposition of the mechanical oscillator can be generated. The quantum coherence and their distinguishable properties of the two coherent states are exhibited by Wigner distribution. We show that the Stark term can enhance significantly the feasibility and quantum coherence of the generated macroscopic quantum superposition state of the oscillator.     

Controlling Entropic Uncertainty in the Presence of Quantum Memory by Non-Markovian Effects and Atom-Cavity Couplings

Based on the time-convolutionless master-equation approach,the entropic uncertainty in the presence of quantum memory is investigated for a two-atom system in two dissipative cavities.We find that the entropic uncertainty can be controlled by the non-Markovian effect and the atom-cavity coupling.The results show that increasing the atom-cavity coupling can enlarge the oscillating frequencies of the entropic uncertainty and can decrease the minimal value of the entropic uncertainty.Enhancing the non-Markovian effect can reduce the minimal value of the entropic uncertainty.In particular,if the atom-cavity coupling or the non-Markovian effect is very strong,the entropic uncertainty will be very close to zero at certain time points,thus Bob can minimize his uncertainty about Alice's measurement outcomes.     

原子在弱相干场光纤耦合腔系统中的纠缠特性

研究由两个相同的二能级原子分别处于用单模光纤耦合的两弱相干光场系统的共生纠缠特性, 通过数值计算研究了光纤模-腔模与原子-腔模的耦合强度比、弱相干光场的强度和两光场相对相位差等因素对系统纠缠演化的影响. 结果表明: 两腔中的两原子之间、两光场之间和每个腔中的原子与光场之间的纠缠随时间呈现周期或准周期性演化, 两腔场之间的纠缠与腔中的两原子的纠缠可以相互转换, 与两原子之间和两光场之间的纠缠相比, 每个腔中光场与原子之间的纠缠随时间变化的周期缩短. 光纤模-腔模与原子-腔模的耦合强度比与两腔中光场相位差对系统纠缠的影响很大, 较小的光纤模-腔模与原子-腔模的耦合强度之比可以获得较大的系统纠缠度. 关键词： 弱相干场 光纤耦合腔 耦合强度 量子纠缠     

光纤耦合双光学腔系统的相干动力学

为了研究量子相干性在腔量子电动力学系统中的动力学和分布特性,基于两个各自捕获原子系综的光学腔建立了双光学腔系统,腔与腔之间由光纤耦合.利用相对熵度量的量子相干性,引入量子相干非平衡性的概念,分析了系统中相干动力学和光纤-腔耦合强度对相干性分布的影响.结果表明:在强耦合极限下,光纤-腔耦合强度的增加有利于保持两腔中的原子的整体相干性;光纤-腔耦合强度、原子-腔耦合强度以及原子数三个参数之间满足特定条件时,腔内的原子相干性可以传输至另一个腔.考虑腔、光纤及原子都存在耗散的情形,对比了不同耗散速率和非耗散情形下的相干性演化,发现耗散使得耦合双腔系统的相干性以及各个腔中的原子相干性发生衰减.     

Entanglement Properties in the System of Three Atoms Trapped in Three Distant Cavities Connected by Two Optical Fibers

This paper discusses the entanglement dynamics of a coupled cavity quantum electrodynamics (CQED) setup, which comprises three two-level atoms resonantly interacting with three cavities that are coupled by two optical fibers. The influences of atom-cavity coupling constant on the entanglement between atoms and that between cavities are discussed. The results obtained from the numerical method show that the entanglement between non-adjacent atoms or that between adjacent cavities has a nonlinear relation with increasing of the atom-cavity coupling coefficient. On the other hand, the entanglement between non-adjacent cavities is strengthened and the entanglement between adjacent atoms is weakened with increasing of atom-cavity coupling constant.     

等同双原子Jaynes-Cummings模型的量子特性及系统光谱

在全量子理论的背景下提出两个二能级原子分别与一单模腔场相互作用的系统模型,利用量子主方程和数值模拟计算等方法,研究该体系中腔场平均光子数、Mandel's Q因子及二阶量子相关度在非稳态时的变化规律。此外,对体系中原子及腔场中光谱结构进行了分析。结果表明:减小腔场耗散系数,增大原子间耦合系数,体系量子特性愈加明显。体系光谱呈现出Mollow三重峰结构,且原子辐射谱强度远大于腔场辐射谱强度。当原子跃迁频率与腔场跃迁频率为近共振时,Mollow峰值为三峰中最大值。此外,增大原子与腔场间耦合系数,可增大原子光谱的中峰强度;而增大腔场光谱的中峰强度,则需减小原子与腔场间耦合系数。     

Dynamical Measurement's Uncertainty in the Curved Space‐Time

The dynamical characteristics of measurement's uncertainty are investigated under two modes of Dirac field in the Garfinkle–Horowitz–Strominger dilation space‐time. It shows that the Hawking effect induced by the thermal field would result in an expansion of the entropic uncertainty with increasing dilation‐parameter value, as the systemic quantum coherence reduces, reflecting that the Hawking effect could undermine the systemic coherence. Meanwhile, the intrinsic relationship between the uncertainty and quantum coherence is obtained, and it is revealed that the uncertainty's bound is anti‐correlated with the system's quantum coherence. Furthermore, it is illustrated that the systemic mixedness is correlated with the uncertainty to a large extent. Via the information flow theory, various correlations including quantum and classical aspects, which can be used to form a physical explanation on the relationship between the uncertainty and quantum coherence, are also analyzed. Additionally, this investigation is extended to the case of multi‐component measurement, and the applications of the entropic uncertainty relation are illustrated on entanglement criterion and quantum channel capacity. Lastly, it is declared that the measurement uncertainty can be quantitatively suppressed through optimal quantum weak measurement. These investigations might pave an avenue to understand the measurement's uncertainty in the curved space‐time.     

Full counting statistics as a probe of quantum coherence in a side-coupled double quantum dot system

We study theoretically the full counting statistics of electron transport through side-coupled double quantum dot (QD) based on an efficient particle-number-resolved master equation. It is demonstrated that the high-order cumulants of transport current are more sensitive to the quantum coherence than the average current, which can be used to probe the quantum coherence of the considered double QD system. Especially, quantum coherence plays a crucial role in determining whether the super-Poissonian noise occurs in the weak inter-dot hopping coupling regime depending on the corresponding QD-lead coupling, and the corresponding values of super-Poissonian noise can be relatively enhanced when considering the spins of conduction electrons. Moreover, this super-Poissonian noise bias range depends on the singly-occupied eigenstates of the system, which thus suggests a tunable super-Poissonian noise device. The occurrence-mechanism of super-Poissonian noise can be understood in terms of the interplay of quantum coherence and effective competition between fast-and-slow transport channels.     

Influences of control coherence and decay coherence on optical bistability in a semiconductor quantum well

We discuss the influences of two different types of mechanisms of quantum coherence on optical bistability in a semiconductor quantum well structure.In the first mechanism,only quantum coherence induced by the resonant coupling of a strong control laser is considered.In the second mechanism,the decay coherence is taken into account under the condition where the control field is weak.In two different cases,optical bistability can be obtained through choosing appropriate physical parameters.Our studies show quantum coherence makes the optical nonlinear effect of the system become stronger,which takes an important role in the process of generating optical bistability.A semiconductor quantum well with flexibility and easy integration in design could potentially be exploited in real solid-state devices.     

Manipulation of nonreciprocal unconventional photon blockade in a cavity-driven system composed of an asymmetrical cavity and two atoms with weak dipole-dipole interaction

We present a work of manipulating collective unconventional photon blockade (UCPB) and nonreciprocal UCPB (NUCPB) in a cavity-driven system composed of an asymmetrical single-mode cavity and two interacting identical two-level atoms (TLAs). When the atoms do not interact directly, the frequency and intensity restrictions of collective UCPB can be specified, and a giant NUCPB exists due to the splitting of optimal atom-cavity coupling strength in proper parameter regime. However, if a weak atom-atom interaction which provides a new and feeble quantum interference pathway to UCPB is taken into account, two restrictions of UCPB are combined complexly, which are rigorous to be matched simultaneously. Due to the push-and-pull effect induced by weak dipole-dipole interaction, the UCPB regime is compressed more or less. NUCPB is improved as a higher contrast is present when the two complex UCPB restrictions are matched, while it is suppressed when the restrictions are mismatched. In general, whether NUCPB is suppressed or promoted depends on its working parameters. Our findings show a prospective access to produce giant quantum nonreciprocity by a couple of weakly interacting atoms.     

有心结构耦合腔系统中的几何量子失谐（英文）

引入了由N+1个单模腔构成的有心结构耦合腔物理模型.其中,中心腔起耦合器作用,其余N个腔通过N条光纤与中心腔耦合,并且每个腔囚禁1个二能级原子.在系统激发数等于1的情况下,给出了系统态矢的演化规律,研究了两原子间和两腔场间的几何量子失谐.通过数值计算,讨论了耦合腔数目和原子与腔场间耦合强度对几何量子失谐的影响.研究结果表明:随着腔数目的增加两原子间和两腔场间的几何量子失谐都减弱;随着原子与腔场间耦合系数增大,两原子间几何量子失谐减弱,但两腔场间几何量子失谐却加强.     

Freezing quantum coherence with weak measurement

We show how to optimally protect quantum states and freeze coherence under incoherent channels using a quantum weak measurement and quantum measurement reversal. In particular, we present explicit formulas for the conditions for freezing quantum coherence in a given quantum state.     

Amplifying and freezing of quantum coherence using weak measurement and quantum measurement reversal

We analyze universal conditions where the l_1 norm and relative entropy of coherence are amplified and frozen under identical bit-flip channels;that is,using pre-measurements(quantum weak measurements or quantum measurement reversals) on the systems before undergoing local bit-flip channels.With the option of quantum weak measurements or quantum measurement reversals,the measurement strength and the success probability are all determined by the initial state of the quantum system.     

Preparation of Six-Atom Cluster State via Cavity Quantum Electrodynamics

A cavity quantum electrodynamics scheme for preparing a six-atom cluster state is proposed. In the scheme, the atom-cavity detuning is much bigger than the atom-cavity coupling strength and the time needed to complete the whole procedure is much shorter than the Rydberg-atom lifespan. Thus the scheme is insensitive to the cavity decay and the atom radiation.     

Entanglement Protection for Two-Qubit in a Non-Markovian Common Bath

In this paper, we propose a scheme to protect quantum entanglement and coherence from a non-Markovian noisy environment. By applying two quantum weak measurements before and after sending the quantum state into the noisy channel, the quantum state can be “pushed” closer to a decoherence free state thus suffer less decoherence in the time evolution. After the time evolution the second weak measurement can partially retrieve the original information encoded in the quantum system. Our study is based on a non-Markovian dynamic equation which allows us to investigate the impact of the memory effect on the performance of the protection scheme. We analyze several factors that may affect the protection efficiency. The results suggest that two measurement strengths should be chosen in a linear relation but the ratio is not one. Besides, we also show the memory effect can drastically improve the protection efficiency.     

Dynamics of super-quantum discord and direct control with weak measurement in open quantum system

Super-quantum discord(SQD) with weak measurement is regarded as a kind of quantum correlation in quantum information processing. We compare and analyze the dynamical evolutions of SQD, quantum discord(QD), and quantum entanglement(QE) between two qubits in the correlated dephasing environmental model. The results indicate that(i) owing to the much smaller influence of weak measurement on the coherence of the system than that of von Neumann projection measurement, SQD with weak measurement is larger than QD, and(ii) dynamical evolution of QD or QE monotonically goes to zero with time, while SQD monotonically tends to a stable value and a freezing phenomenon occurs. The stable value after freezing mainly depends on the measurement strength and the purity of the initial quantum state.     

Multiple-scale analysis of open quantum systems

In this work, we present a multiple-scale perturbation technique suitable for the study of open quantum systems, which is easy to implement and in few iterative steps allows us to find excellent approximate solutions. For any time-local quantum master equation, whether markovian or non-markovian, in Lindblad form or not, we give a general procedure to construct analytical approximations to the corresponding dynamical map and, consequently, to the temporal evolution of the density matrix. As a simple illustrative example of the implementation of the method, we study an atom-cavity system described by a dissipative Jaynes-Cummings model. Performing a multiple-scale analysis we obtain approximate analytical expressions for the strong and weak coupling regimes that allow us to identify characteristic time scales in the state of the physical system.     

Enhancement of the Quantum Coherence in a Hierarchical Environment System by Weak Measurement and Weak Measurement Reversal

The quantum coherence of a two-qubit system in which each qubit is coupled to its own hierarchical environment is studied. The effect of the hierarchical environment is explored in order to improve the coherence effectively. It is discovered that the dynamics of coherence can be manipulated by the number N of the second-layer cavities ($m_n$), the coupling strengths Ω₀, Ω, κ, respectively for the qubit-cavity (m₀), nearest-neighbor, and m₀-$m_n$ cavities. It depends also on the decay rate Γ of second-layer cavities. Furthermore, an effective scheme for enhancing coherence is proposed by using weak measurement and weak measurement reversal. The explicit conditions of the optimal measurement strengths to improve the coherence comparatively are derived. It is seen that the final stable value of the coherence is independent of the number N of second-layer cavities but is related to the weak measurement strength m when the coupling strength between two nearest-neighbor cavities is taken into account.     

超高精细度微光学腔共振频率及有效腔长的精密测量

超高精细度微共振器是实现原子或者其他偶极子与腔强耦合作用的基本部分, 在腔量子电动力学(QED)、弱光非线性效应及微光学器件研究中扮演着重要的角色. 微腔基本参数的精密测量最终可以确定腔与原子的耦合系数、腔场衰减率, 对决定系统的动力学特性具有重要的意义. 但是由于超高精细度光学微腔本身的构造和多层镀膜的特点, 高精度地确定其共振频率及有效腔长存在一定困难. 本文结合修正的多层介质膜模型, 实验上完成了膜层为37层的超高精细度光学微腔在不同共振频率下有效腔长的精密测量, 获得了超高精细度光学微腔的共振频率及波长; 理论计算分析与实验测量结果相符, 对纵模间隔的测量精度误差低于0.004 nm, 较为修正前提高了约两个量级. 同时给出了对应不同模式数下, 光波渗入到介质中的深度. 该方法可望应用到其他微共振器的精密测量中. 关键词： 光学微腔 高精细度 共振频率     

Quantum Transfer Energy and Nonlocal Correlation in a Dimer with Time-Dependent Coupling Effect

The presence of coherence phenomenon due to the interference of probability amplitude terms, is one of the most important features of quantum mechanics theory. Recent experiments show the presence of quantum processes whose coherence provided over suddenly large interval-time. In particular, photosynthetic mechanisms in light-harvesting complexes provide oscillatory behaviors in quantum mechanics due to quantum coherence. In this work, we investigate the coherent quantum transfer energy for a single-excitation and nonlocal correlation in a dimer system modelled by a two-level atom system with and without time-dependent coupling effect. We analyze and explore the required conditions that are feasible with real experimental realization for optimal transfer of quantum energy and generation of nonlocal quantum correlation. We show that the enhancement of the probability for a single-excitation transfer energy is greatly benefits from the combination of the energy detuning and time-dependent coupling effect. We investigate the presence of quantum correlations in the dimer using the entanglement of formation. We also find that the entanglement between the donor and acceptor is very sensitive to the physical parameters and it can be generated during the coherent energy transfer. On the other hand, we study the dynamical behavior of the quantum variance when performing a measurement on an observable of the density matrix operator. Finally, an interesting relationship between the transfer probability, entanglement and quantum variance is explored during the time evolution in terms of the physical parameters.