与双模场依赖强度耦合下多光子通道中原子比 特周期量子回声产生和调控

旋波近似条件下,运用全量子理论研究了与双模相干光场依赖强度耦合多光子通道中原子比特周期量子回声的产生和控制. 采用数值计算的方法,讨论了双模相干光场平均光子数分布形式、分布范围及原子跃迁时吸收(或发射)的光子数k对原子比特态保真度演化的影响,获得了产生和控制原子比特周期量子回声的系统参量;根据纠缠理论,分析了原子比特态保真度演化与原子约化熵演化的关联. 结果表明:在k=1的双光子过程中,调控光场平均光子数呈对称或不对称分布,当它的取值在一定范围内,原子比特保持良好的相干性和保真度,产生周期量子回声; 对于k≥2的多光子过程,原子比特与双模相干光场始终处于最大纠缠,因此导致了原子比特始终处于部分失真状态,不产生周期量子回声. 本研究揭示了周期量子回声产生的物理实质是原子比特与光场周期性退纠缠.     

双模场与原子相互作用中的量子纠缠和内禀退相干

通过求解系统的Milburn方程，研究了两能级原子与双模SU(1,1)相干态光场发生相互作用系统中，原子与场的纠缠及双模SU(1,1)相干态场的模间纠缠随时间的演化问题，讨论了内禀退相干、双模光子数差等对纠缠度的影响.结果表明，存在内禀退相干时，随着时间的演化，场-原子纠缠逐渐减小到一个确定值，而模间纠缠逐渐增大到一个确定值，两者演化的最终值只取决于双模光子数差和平均光子数，而与内禀退相干因子无关. 关键词： Milburn理论 SU(1 1)相干态 量子约化熵 量子相对熵     

非旋波近似下Λ型三能级原子与相干态光场的量子纠缠

在非旋波近似下,利用相干态正交化展开方法,对相干态光场与Λ型三能级原子相互作用的量子场熵进行了精确求解.利用量子熵理论讨论了耦合强度、平均光子数以及初始时刻原子处于不同的能级对量子纠缠的影响.数值计算的结果表明:当初始时刻原子处于激发态时,量子纠缠在较短的时间内就能演化到最大值,随着平均光子数的增大,纠缠演化的周期性逐渐明显;原子初始时刻处于三个能级的叠加态会使初始阶段量子纠缠显著降低;与旋波近似下的结果不同的是,随着耦合强度以及平均光子数的增加,非旋波项的贡献显著增强,使得量子纠缠演化曲线出现小锯齿状的振荡.     

非旋波近似下Λ型三能级原子与相干态光场的量子纠缠

在非旋波近似下,利用相干态正交化展开方法,对相干态光场与Λ型三能级原子相互作用的量子场熵进行了精确求解.利用量子熵理论讨论了耦合强度、平均光子数以及初始时刻原子处于不同的能级对量子纠缠的影响.数值计算的结果表明：当初始时刻原子处于激发态时,量子纠缠在较短的时间内就能演化到最大值,随着平均光子数的增大,纠缠演化的周期性逐渐明显|原子初始时刻处于三个能级的叠加态会使初始阶段量子纠缠显著降低|与旋波近似下的结果不同的是,随着耦合强度以及平均光子数的增加,非旋波项的贡献显著增强,使得量子纠缠演化曲线出现小锯齿状的振荡.     

Schrǒdinger—cat态光场与耦合双原子相互作用系统的量子场熵演化

利用全量子理论，研究了Schrǒdinger-cat态光场与耦合双能级原子相互作用系统在非共振情况下的量子场熵演化特性．结果表明，场一原子系统的量子场熵演化特性不仅与初始平均光子数no、场与原子的耦合强度g、原子与原子之间的耦合强度ga以及失谐量△有着密切的关系，而且与原子的初始状态有关．     

双模相干光场与原子依赖强度耦合双光子过程的纠缠演化

分别应用量子约化熵和量子相对熵研究了双模相干光场与原子依赖强度耦合双光子过程中原子与光场之间及光场的双模之间纠缠演化.结果表明,通过选择系统初态参数、控制原子与光场的作用时间,可以调控原子与光场间的纠缠和光场的模间纠缠.可增加或减弱双模相干光场间纠缠,但无法使双模相干光场完全退纠缠.强场条件下,可周期性获得光场与原子的EPR态和可分离态.     

双模相干光场与原子依赖强度耦合双光子过程的纠缠演化

分别应用量子约化熵和量子相对熵研究了双模相干光场与原子依赖强度耦合双光子过程中原子与光场之间及光场的模之间纠缠演化.结果表明,通过选择系统初态参数、控制原子与光场的作用时间,可以调控原子与光场间的纠缠和光场的模间纠缠.可增加或减弱双模相干光场间纠缠,但无法使双模相干光场完全退纠缠.强场条件下,可周期性获得光场与原子的EPR态和可分离态.     

Schrdinger-cat态光场与耦合双原子相互作用系统的量子场熵演化

利用全量子理论,研究了Schrdinger-cat态光场与耦合双能级原子相互作用系统在非共振情况下的量子场熵演化特性.结果表明,场—原子系统的量子场熵演化特性不仅与初始平均光子数n0、场与原子的耦合强度g、原子与原子之间的耦合强度ga以及失谐量Δ有着密切的关系,而且与原子的初始状态有关.     

相干耦合腔场中量子纠缠信息交换传递机理研究

提出了相干多模腔场-相干耦合原子相互作用系统的物理模型.利用全量子理论,研究了该系统中多光子相互作用过程的演化性质,结果表明：在多模腔场与相干耦合多原子相互作用过程中,多光子纠缠态与多原子纠缠态可以周期性的相互转换,在此过程中,同时可以实现纠缠信息的交换传递.通过对原子保真度的数值计算,给出了纠缠信息交换传递的图示说明.并进一步揭示出纠缠信息交换传递的一般特征.目前所报道的研究结果仅是本文所得普遍性结果在各种不同情况下的特例.     

微波调制的里德堡原子集体量子跳跃

对于一个三能级原子体系,原子的两个基态能级通过微波耦合起来,其中一个基态能级可被激发到里德堡态,从而可观察量子跳跃现象.本文采用量子轨线方法研究了微波调制的里德堡原子集体量子跳跃.研究结果表明,微波耦合基态能级可以提高光子关联,增强光子聚束效应,即使较少的原子中也可以观察到系统在高里德堡占据数态和低里德堡占据数态之间的切换.这一结果为将来进一步研究里德堡自旋晶格中的多体动力学提供了新思路.     

Quantum many‐body phenomena in coupled cavity arrays

The increasing level of experimental control over atomic and optical systems gained in recent years has paved the way for the exploration of new physical regimes in quantum optics and atomic physics, characterised by the appearance of quantum many‐body phenomena, originally encountered only in condensed‐matter physics, and the possibility of experimentally accessing them in a more controlled manner. In this review article we survey recent theoretical studies concerning the use of cavity quantum electrodynamics to create quantum many‐body systems. Based on recent experimental progress in the fabrication of arrays of interacting micro‐cavities and on their coupling to atomic‐like structures in several different physical architectures, we review proposals on the realisation of paradigmatic many‐body models in such systems, such as the Bose‐Hubbard and the anisotropic Heisenberg models. Such arrays of coupled cavities offer interesting properties as simulators of quantum many‐body physics, including the full addressability of individual sites and the accessibility of inhomogeneous models.     

Dynamics and quantum entanglement of two-level atoms in de Sitter spacetime

In the framework of open quantum systems, we study the internal dynamics of both freely falling and static two-level atoms interacting with quantized conformally coupled massless scalar field in de Sitter spacetime. We find that the atomic transition rates depend on both the nature of de Sitter spacetime and the motion of atoms, interestingly the steady states for both cases are always driven to being purely thermal, regardless of the atomic initial states. This thermalization phenomenon is structurally similar to what happens to an elementary quantum system immersed in a thermal field, and thus reveals the thermal nature of de Sitter spacetime. Besides, we find that the thermal baths will drive the entanglement shared by the freely falling atom (the static atom) and its auxiliary partner, a same two-level atom which is isolated from external fields, to being sudden death, and the proper time for the entanglement to be extinguished is computed. We also analyze that such thermalization and disentanglement phenomena, in principle, could be understood from the perspective of table-top simulation experiment.     

Dynamical aspects in the optical bloch equations

In quantum optics, some models are considered to describe many aspects of the dynamics of atoms coupled to an electromagnetic field (laser). The simplest atomic model is of course the two-level-atom which is governed by the Bloch optical equations. In general this system is solved in the steady state or by using some approximations. An extended analytic approach is considered for this coupled equations. The separation approach of coupled differential equations is always possible with a sequence of special transformation into nonlinear differential equations. The conditions that permit an exact solution of three coupled systems are extracted in a natural manner. The case of sodium atom moving along the axis of a standing-wave is investigated in some details.     

Nanoscale superconducting quantum bits

Various physical systems were proposed for quantum information processing. Among those nanoscale devices appear most promising for integration in electronic circuits and large-scale applications. We discuss Josephson junction circuits in two regimes where they can be used for quantum computing. These systems combine intrinsic coherence of the superconducting state with control possibilities of single-charge circuits. In the regime where the typical charging energy dominates over the Josephson coupling, the low-temperature dynamics is limited to two states differing by a Cooper-pair charge on a superconducting island. In the opposite regime of prevailing Josephson energy, the phase (or flux) degree of freedom can be used to store and process quantum information. Under suitable conditions the system reduces to two states with different flux configurations. Several qubits can be joined together into a register. The quantum state of a qubit register can be manipulated by voltage and magnetic field pulses. The qubits are inevitably coupled to the environment. However, estimates of the phase coherence time show that many elementary quantum logic operations can be performed before the phase coherence is lost. In addition to manipulations, the final state of the qubits has to be read out. This quantum measurement process can be accomplished using a single-electron transistor for charge Josephson qubits, and a d.c.-SQUID for flux qubits. Recent successful experiments with superconducting qubits demonstrate for the first time quantum coherence in macroscopic systems.     

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.     

Coherence control for photonic logic gates via Y-configuration double-control quantum interferences

Destructive and constructive quantum interferences exhibited in a four-level Y-configuration double-control atomic system are suggested. It is shown that the probe transition (driven by the probe field) can be manipulated by the quantum interferences between two control transitions (driven by the control fields) of the four-level system. The atomic vapor is opaque (or transparent) to the probe field if the destructive (or constructive) quantum interference between the control transitions emerges. The optically sensitive responses due to double-control quantum interferences can be utilized to realize some quantum optical and photonic devices such as the logic-gate devices, e.g., the NOT, OR, NOR and EXNOR gates.     

Quantum information capsule in multiple-qudit systems and continuous-variable systems

Quantum correlations in an entangled many-body system are capable of storing information. Even when the information is injected by a local unitary operation to the system, the entanglement delocalizes it. In a recent study on multiple-qubit systems, it is shown that a virtual qubit defined in the correlation space plays a role of perfect storage of delocalized information, which is called a quantum information capsule (QIC). To enhance the capacity of quantum information storage, it is crucial to formulate the cases for multiple-qudit systems and continuous-variable (CV) systems. We analytically prove that it is possible to construct a QIC for general write operations of the systems. It turns out that the extension to quantum field theory is achievable. For Gaussian states, we explicitly construct a QIC for shift write operations. We analyze the time-evolution of QIC in a CV system to demonstrate the diffusion of information in entangled pure states.     

Quantum counterpart of measure synchronization: A study on a pair of Harper systems

Measure synchronization is a well-known phenomenon in coupled classical Hamiltonian systems over last two decades. Here, synchronization in a pair of coupled Harper systems is investigated both in classical and quantum contexts. It seems that the concept of measure synchronization is restricted in the classical limit as it involves with the phase space. We show the quantum counterpart of the synchronization in a pair of coupled quantum kicked Harper chains. In the quantum context, the coupling occurs between two spins chains via a time and site dependent potential. We use the average interaction energy between the participating systems as an order parameter in both the contexts to establish a connection between the classical and the quantum scenarios. Besides, we also study the entanglement between the chains and difference between the average bare energies in the quantum context. Interestingly, all such indicators suggest a connection between the MS transition in classical maps and a phase transition in quantum spin chains.     

Rotating wave approximation and entropy

This Letter studies composite quantum systems, like atom-cavity systems and coupled optical resonators, in the absence of external driving by resorting to methods from quantum field theory. Going beyond the rotating wave approximation, it is shown that the usually neglected counter-rotating part of the Hamiltonian relates to the entropy operator and generates an irreversible time evolution. The vacuum state of the system is shown to evolve into a generalized coherent state exhibiting entanglement of the modes in which the counter-rotating terms are expressed. Possible consequences at observational level in quantum optics experiments are currently under study.     

A survey on HHL algorithm: From theory to application in quantum machine learning

The Harrow-Hassidim-Lloyd (HHL) algorithm is a method to solve the quantum linear system of equations that may be found at the core of various scientific applications and quantum machine learning models including the linear regression, support vector machines and recommender systems etc. After reviewing the necessary background on elementary quantum algorithms, we provide detailed account of how HHL is exploited in different quantum machine learning (QML) models, and how it provides the desired quantum speedup in all these models. At the end, we briefly discuss some of the remaining challenges ahead for HHL-based QML models and related methods.