BEC Dynamics in a Double‐Well with Interferometric Feedback

The feedback control scheme for a Bose‐Einstein condensate (BEC) in a double‐well trapping potential located in one arm of Mach‐Zehnder interferometer (MZI) is investigated. The off‐resonant light beam performs the phase probing in one of the wells, thus creating information about the number of atoms in this well. The parameters of the trapping potential are controlled via a feedback loop based on the measured output of the MZI. The problem is analyzed in the framework of master equations for hybrid quantum‐classical systems. Significant modifications of the stationary distribution of atoms over the wells are predicted. These distributions can effectively be controlled by the tunable phase shift in the other arm of the MZI.     

Quantum dynamics of an impurity-doped Bose-Einstein condensate system

We study the quantum dynamics of an impurity-doped Bose–Einstein condensate (BEC) system. We show how to generate the macroscopic quantum superposition states (MQSSs) of the BEC by the use of projective measurements on impurity atoms. It is found that the nonclassicality of MQSSs can be manipulated by changing the number of the impurities and their interaction with the BEC. It is shown that the BEC matter-wave field exhibits a collapse and revival phenomenon which reveals the quantum nature of the BEC matter-wave field. We investigate the micro-macro entanglement between the impurities and the BEC, and find enhancement of the micro-macro entanglement induced by the initial quantum coherence of the impurity atoms.     

Strong-coupling polarons in dilute gas Bose-Einstein condensates

A neutral impurity atom immersed in a dilute Bose-Einstein condensate (BEC) can have a bound ground state in which the impurity is self-localized. In this polaronlike state, the impurity distorts the density of the surrounding BEC, thereby creating the self-trapping potential minimum. We describe the self-localization in a strong-coupling approach.     

Thermalization of Isolated Bose‐Einstein Condensates by Dynamical Heat Bath Generation

If and how an isolated quantum system thermalizes despite its unitary time evolution is a long‐standing, open problem of many‐body physics. The eigenstate thermalization hypothesis (ETH) postulates that thermalization happens at the level of individual eigenstates of a system's Hamiltonian. However, the ETH requires stringent conditions to be validated, and it does not address how the thermal state is reached dynamically from an initial non‐equilibrium state. We consider a Bose‐Einstein condensate (BEC) trapped in a double‐well potential with an initial population imbalance. We find that the system thermalizes although the initial conditions violate the ETH requirements. We identify three dynamical regimes. After an initial regime of undamped Josephson oscillations, the subsystem of incoherent excitations or quasiparticles (QP) becomes strongly coupled to the BEC subsystem by means of a dynamically generated, parametric resonance. When the energy stored in the QP system reaches its maximum, the number of QPs becomes effectively constant, and the system enters a quasi‐hydrodynamic regime where the two subsystems are weakly coupled. In this final regime the BEC acts as a grand‐canonical heat reservoir for the QP system (and vice versa), resulting in thermalization. We term this mechanism dynamical bath generation (DBG).     

Phase‐Dependent Quantum Correlation in a Cavity‐Atom System

A scheme to manipulate quantum correlation between output lights of a cavity‐atom system by phase control is proposed. A driving‐field phase is introduced which has a similar value with that of building up quantum correlation in a Hanbury–Brown–Twiss setup. A closed‐loop phase is formed to improve quantum coherence by phase‐dependent electromagnetically induced transparency. The closed‐loop phase has been utilized to realize quantum correlation and even quantum entanglement in the atomic system of previous work. With these two phases, a steady and maximum quantum correlation has been obtained in the scheme here. Moreover, the maximum quantum correlation is free to decoherence of this cavity‐atom system. The study on field‐intensity correlation (quantum correlation) has potential applications on correlated imaging, image encryption transmission, and the improvement of noise resistance in a quantum network.     

半无限深势阱中自旋相关玻色-爱因斯坦凝聚体的量子反射与干涉

玻色-爱因斯坦凝聚体与势垒或势阱的量子反射及干涉是考察宏观物质波奇特物性的最有效途径之一.利用传播子方法和基于冷原子实验广泛采用的飞行时间吸收成像方案,研究自旋相关玻色-爱因斯坦凝聚体在半无限深势阱中的反射和干涉演化动力学,得到了自旋相关的凝聚体波函数的严格解析解.结果表明,当自旋相关光晶格关闭后,非局域于不同格点中相同自旋态的物质波在自由膨胀过程中发生量子干涉,形成了对比度明显的干涉条纹.与此同时,扩张的自旋相关物质波包与半无限深势阱壁相遇发生量子反射,反射波与入射波产生二重干涉,在密度分布两边对称的局部位置出现剧烈的振荡,干涉条纹表现出显著的调制效应.分析讨论了自旋态、相干输运距离和相对相位等因素对干涉条纹的影响.该研究有助于促进对自旋相关凝聚体宏观量子特性的认识,为深入检验自旋相关光晶格中凝聚体干涉的理论模型和物理机理提供依据和新方案.     

原子光学讲座第二讲量子原子光学

近年来,有关玻色一爱因斯坦凝聚（BEC）及其量子光学性质的理论与实验研究得到了飞速发展,并取得了一系列重大进展,从而形成了一门原子光学的新分支学科——“量子原子光学”．文章重点介绍了量子原子光学的研究内容、实验结果及其最新进展,主要包括BEC实验研究的重大进展、原子量子态的实验制备、原子激光的产生及其最新进展、BEC凝聚体或原子激光的相干性和费米原子气体的量子简并等．     

Three‐Phonon and Four‐Phonon Interaction Processes in a Pair‐Condensed Fermi Gas

We study the interactions among phonons and the phonon lifetime in a pair‐condensed Fermi gas in the BEC‐BCS crossover in the collisionless regime. To compute the phonon‐phonon coupling amplitudes we use a microscopic model based on a generalized BCS Ansatz including moving pairs, which allows for a systematic expansion around the mean field BCS approximation of the ground state. We show that the quantum hydrodynamic expression of the amplitudes obtained by Landau and Khalatnikov apply only on the energy shell, that is for resonant processes that conserve energy. The microscopic model yields the same excitation spectrum as the Random Phase Approximation, with a linear (phononic) start and a concavity at low wave number that changes from upwards to downwards in the BEC‐BCS crossover. When the concavity of the dispersion relation is upwards at low wave number, the leading damping mechanism at low temperature is the Beliaev‐Landau process 2 phonons ? 1 phonon while, when the concavity is downwards, it is the Landau‐Khalatnikov process 2 phonons ? 2 phonons. In both cases, by rescaling the wave vectors to absorb the dependence on the interaction strength, we obtain a universal formula for the damping rate. This universal formula corrects and extends the original analytic results of Landau and Khalatnikov [ZhETF 19 , 637 (1949)] for the 2?2 processes in the downward concavity case. In the upward concavity case, for the Beliaev 1? 2 process for the unitary gas at zero temperature, we calculate the damping rate of an excitation with wave number q including the first correction proportional to q ⁷ to the q ⁵ hydrodynamic prediction, which was never done before in a systematic way.     

Electronic Properties of Strained Double‐Weyl Systems

The effects of strains on the low‐energy electronic properties of double‐Weyl phases are studied in solids and cold‐atom optical lattices. The principal finding is that deformations do not couple, in general, to the low‐energy effective Hamiltonian as a pseudoelectromagnetic gauge potential. The response of an optical lattice to strains is simpler, but still only one of the several strain‐induced terms in the corresponding low‐energy Hamiltonian can be interpreted as a gauge potential. Most interestingly, the strains can induce a nematic order parameter that splits a double‐Weyl node into a pair of Weyl nodes with the unit topological charges. The effects of deformations on the motion of wavepackets in the double‐Weyl optical lattice model are studied. It is found that, even in the undeformed lattices, the wavepackets with opposite topological charges can be spatially split. Strains, however, modify their velocities in a very different way and lead to a spin polarization of the wavepackets.     

Effects of localized impurity of trap on characteristics of two Bose-Einstein condensates

The characteristics of solitons with a localized impurity in Bose-Einstein condensates (BECs) are investigated with numerical simulations of the Gross-Pitaevskii (GP) equation, the effects of the impurity on BEC solitons are discussed, and the atom population transferring ratios between the two BECs as time goes on are analyzed. It is found that population transfer depends on the impurity strength and the parameters of the system of BECs.     

玻色-爱因斯坦凝聚体中的怪波操控

利用Darboux变换法, 解析地研究了玻色-爱因斯坦凝聚体(BEC)中的怪波. 结果表明: 当谱参数等于非线性系数时, BEC中形成一种新型的单洞怪波; 而当谱参数小于非线性系数时, BEC中出现双洞怪波. 进一步地, 怪波的出现位置可通过调节周期性势阱的驱动频率和强度来控制. 此外, 随着原子间相互作用的减小, 怪波的最高幅度也随之降低. 相关结果可为预防怪波的危害提供帮助.     

玻色-爱因斯坦凝聚体中的怪波操控

利用Darboux变换法, 解析地研究了玻色-爱因斯坦凝聚体(BEC)中的怪波. 结果表明: 当谱参数等于非线性系数时, BEC中形成一种新型的单洞怪波; 而当谱参数小于非线性系数时, BEC中出现双洞怪波. 进一步地, 怪波的出现位置可通过调节周期性势阱的驱动频率和强度来控制. 此外, 随着原子间相互作用的减小, 怪波的最高幅度也随之降低. 相关结果可为预防怪波的危害提供帮助.     

玻色-爱因斯坦凝聚体中的怪波操控

利用Darboux变换法,解析地研究了玻色-爱因斯坦凝聚体(BEC)中的怪波.结果表明:当谱参数等于非线性系数时,BEC中形成一种新型的单洞怪波;而当谱参数小于非线性系数时,BEC中出现双洞怪波.进一步地,怪波的出现位置可通过调节周期性势阱的驱动频率和强度来控制.此外,随着原子间相互作用的减小,怪波的最高幅度也随之降低.相关结果可为预防怪波的危害提供帮助.     

硅纳米结构晶体管中与杂质量子点相关的量子输运

在小于10 nm的沟道空间中,杂质数目和杂质波动范围变得十分有限,这对器件性能有很大的影响.局域纳米空间中的电离杂质还能够展现出量子点特性,为电荷输运提供两个分立的杂质能级.利用杂质原子作为量子输运构件的硅纳米结构晶体管有望成为未来量子计算电路的基本组成器件.本文结合安德森定域化理论和Hubbard带模型对单个、分立和耦合杂质原子系统中的量子输运特性进行了综述,系统介绍了提升杂质原子晶体管工作温度的方法.     

Water‐Dispersible Silicon‐Quantum‐Dot‐Containing Micelles Self‐Assembled from an Amphiphilic Polymer

The dispersion of silicon quantum dots (Si QDs) in water has not been established as well as that in organic solvents. It is now demonstrated that the excellent dispersion of Si QDs in water with photoluminescence (PL) quantum yields (QYs) comparable to those for hydrophobic Si QDs can be realized by combining the processes of hydrosilylation and self‐assembly. Hydrogen‐passivated Si QDs are initially hydrosilylated with 1‐dodecence. The toluene solution of the resulting dodecyl‐passivated Si QDs is mixed with the water solution of the amphiphilic polymer of Pluronic F127 to form an emulsion. Dodecyl‐passivated Si QDs are encapsulated in the micelles self‐assembled from F127 in the emulsion. The size of the Si‐QD‐containing micelles may be tuned in the range from 10 to 100 nm. Although self‐assembly in the emulsion causes the PL QY of Si QDs to decrease, after a few days of storage in ambient conditions, Si QDs encapsulated in the water‐dispersible micelles exhibit recovered PL QYs of ≈24% at the PL wavelength of ≈680 nm. The intensity of the PL from Si QDs encapsulated in the water‐dispersible micelles is >90% of the original value after 60 min ultraviolet illumination, indicating excellent photostability.     

Excitation picture of an interacting Bose gas

Atomic Bose–Einstein condensates (BECs) can be viewed as macroscopic objects where atoms form correlated atom clusters to all orders. Therefore, the presence of a BEC makes the direct use of the cluster-expansion approach–lucrative e.g. in semiconductor quantum optics–inefficient when solving the many-body kinetics of a strongly interacting Bose. An excitation picture is introduced with a nonunitary transformation that describes the system in terms of atom clusters within the normal component alone. The nontrivial properties of this transformation are systematically studied, which yields a cluster-expansion friendly formalism for a strongly interacting Bose gas. Its connections and corrections to the standard Hartree–Fock–Bogoliubov approach are discussed and the role of the order parameter and the Bogoliubov excitations are identified. The resulting interaction effects are shown to visibly modify number fluctuations of the BEC. Even when the BEC has a nearly perfect second-order coherence, the BEC number fluctuations can still resolve interaction-generated non-Poissonian fluctuations.     

Controlled Manipulation with a Bose--Einstein Condensates N-Soliton Train under the Influence of Harmonic and Tilted Periodic Potentials

A model of the perturbed complex Toda chain （PCTC） to describe the dynamics of a Bose-Einstein condensate （BEC） N-soliton train trapped in an applied combined external potential consisting of both a weak harmonic and tilted periodic component is first developed. Using the developed theory, the BEC N-soliton train dynamics is shown to be well approximated by 4N coupled nonlinear differential equations, which describe the fundamental interactions in the system arising from the interplay of amplitude, velocity, centre-of-mass position, and phase. The simplified analytic theory allows for an efficient and convenient method for characterizing the BEC N-soliton train behaviour. It further gives the critical values of the strength of the potential for which one or more localized states can be extracted from a soliton train and demonstrates that the BEC N-soliton train can move selectively from one lattice site to another by simply manipulating the strength of the potential.     

Completely Subradiant Multi‐Atom Architectures Through 2D Photonic Crystals

Following recent advances in the manipulation of atoms trapped near 1D waveguides and proposals to use surface acoustic waves on piezoelectric substrates for the same purpose, the potential of two‐dimensional platforms is shown. Directional emission of atoms near photonic crystal slabs with square symmetry is used, in the ideal case, to build perfect subradiant states of 2 distant atoms, possible in 2D only for finite lattices with perfectly reflecting boundaries. These allow the design of massively parallel 1D arrays of atoms above a single crystal, useful for multi‐port output of nonclassical light, by exploiting destructive interference of guided resonance modes. Directionality of the emission is shown to be present whenever a linear iso‐frequency manifold is present in the dispersion relation of the crystal. Multi‐atom radiance properties can be predicted from a simple cross‐talk coefficient of a master equation, in good agreement with exact atom‐crystal dynamics, showing its predictive power. Departing from the ideal theoretical case, possible experimental issues in photonic crystal implementations are also discussed, and an outlook of other relevant modern platforms for 2D propagation of excitations is given.     

Normal‐Mode Splitting in a Weakly Coupled Electromechanical System with a Mechanical Modulation

Normal‐mode splitting (NMS) is one of the most significant manifestations of strongly coupled systems. Here, NMS is shown to occur in a weakly coupled electromechanical system. In this system, an exponentially enhanced electromechanical coupling is obtained by periodically modulating the electromechanical interaction and the mechanical spring constant. Under the mechanical modulation, an obvious NMS can occur in the fluctuation spectra of the mechanical oscillator's displacement and the output field. Besides, the Stokes and anti‐Stokes fields can also display an obvious NMS. Interestingly, the Stokes field can be changed from full absorption to significant amplification, and the anti‐Stokes field can also be enhanced significantly. Another novel feature of the results is that the intensity between the two peaks in NMS is almost zero, which means that the two peaks can be well resolved in experiments. This work presents an effective method for the generation of a well‐resolved NMS in a weakly coupled system.     

Electron Interference in a Double‐Dopant Potential Structure

Herein, an analysis of interference effects as a result of the electron evolution within a coherent transport medium is presented, offering a double‐dopant Coulomb potential structure. Injection of coherent electron states into the structure is used to investigate the effects on the current transport behavior within the quantum Wigner phase space picture. Quantum effects are outlined by using classical simulation results as a reference frame. The utilized signed particle approach inherently provides a seamless transition between the classical and quantum domain. Based on this the occurring quantum effects caused by the non‐locality of the action of the quantum potential, leading to spatial resonance, can be indentified. The resulting interference patterns enable novel applications in the area of entangletronics.