Casimir-Polder interaction of atoms with magnetodielectric bodies

A general theory of the Casimir-Polder interaction of single atoms with dispersing and absorbing magnetodielectric bodies is presented, which is based on QED in linear, causal media. Both ground-state and excited atoms are considered. Whereas the Casimir-Polder force acting on a ground-state atom can conveniently be derived from a perturbative calculation of the atom-field coupling energy, an atom in an excited state is subject to transient force components that can only be fully understood by a dynamical treatment based on the body-assisted vacuum Lorentz force. The results show that the Casimir-Polder force can be influenced by the body-induced broadening and shifting of atomic transitions — an effect that is not accounted for within lowest-order perturbation theory. The theory is used to study the Casimir-Polder force of a ground-state atom placed within a magnetodielectric multilayer system, with special emphasis on thick and thin plates as well as a planar cavity consisting of two thick plates. It is shown how the competing attractive and repulsive force components related to the electric and magnetic properties of the medium, respectively, can — for sufficiently strong magnetic properties — lead to the formation of potential walls and wells.     

Casimir-Polder interaction of excited media

The potential of long-range interaction between two dissimilar atoms, one of which is excited, drops as 1/R ₂ with the distance for the Casimir-Polder limit of large distances in comparison with the wave-length of atom transitions (E.A. Power and T. Thirunamachandran, Phys. Rev. A 51, 3660 (1995)). It is shown that such a dependence, obtained with the help of perturbation technique, results in a divergence for the interaction potential between an excited atom and a medium of dilute gas. We develop a nonperturbative method based upon quantum Green’s functions (Yu. Sherkunov, Phys. Rev. A 72, 052703 (2005)) to calculate the interaction potential for an excited atom and a ground-state atom embedded in a dielectric medium, taking into account the absorption of photons in the dielectric medium. The exponential suppression of the interaction between the atoms is demonstrated. The force acting on an excited atom near the interface of dilute gas medium is calculated. The result is no more divergent. The force between gas media in Casimir-Polder regime is calculated as well. The text was submitted by the author in English.     

The excited state model and an elementary act of softening of glassy solids

A new elementary act of the glass-liquid transition is proposed, and the softening criterion is calculated for different classes of glassy systems. The transition of an amorphous substance from a glassy solid state to a liquid state is explained by defreezing of the process of excitation of the kinetic unit responsible for viscous flow when the average energy of thermal lattice vibrations becomes equal to or higher than the work of the ultimate elastic deformation of the interatomic bond, which corresponds to the maximum of the quasi-elastic force. The excitation of the kinetic unit is considered to mean its critical displacement from the equilibrium position, which corresponds to the maximum of the attractive force between particles. The kinetic unit (an atom or a group of atoms) capable of being displaced over a critical distance is referred to as an excited atom, and the approach under consideration is termed the excited state model. The nature of excited atoms in silicate glasses and amorphous polymers is discussed. The nature of fluctuation holes in liquids and glasses is considered in the excited state model. The known Frenkel exponential formula for the hole concentration acquires the meaning of the probability of excitation of the kinetic unit responsible for viscous flow. The elementary act of softening of glasses can serve as a molecular mechanism of their plastic deformation.     

Small atomic displacements recorded in bismuth by the optical reflectivity of femtosecond laser-pulse excitations

Subtle atomic motion in a Bi crystal excited by a 35 fs-laser pulse has been recovered from the transient reflectivity of an optical probe measured with an accuracy of 10(-5). Analysis shows that a novel effect reported here-an initial negative drop in reflectivity-relates to a delicate coherent displacement of atoms by the polarization force during the pulse. We also show that reflectivity oscillations with a frequency coinciding with that of cold Bi are related to optical phonons excited by the electron temperature gradient through electron-phonon coupling.     

Geometric resonance cooling of polarizable particles in an optical waveguide

In the radiation field of an optical waveguide, the Rayleigh scattering of photons is shown to result in a strongly velocity-dependent force on atoms. The pump field, which is injected in the fundamental branch of the waveguide, is favorably scattered by a moving atom into one of the transversely excited branches of propagating modes. All fields involved are far detuned from any resonances of the atom. For a simple polarizable particle, a linear friction force coefficient comparable to that of cavity cooling can be achieved.     

Casimir-Polder力对左手材料板附近的原子的动力学作用

本文研究了初始处于激发态的两能级原子在左手材料附近运动时Casimir-Polder力对原子动力学的影响. 左手材料有两个的作用: 一是在距离界面波长区域内提供了较强的Casimir-Polder共振力, 二是在这一范围原子的自发辐射受到抑制, 延长了作用时间. 这两种效应使得依靠原子自发辐射这一过程中的Casimir-Polder力能对原子的运动学产生影响, 并将一定初速度的原子排斥远离界面. 由于原子偶极矩的取向会影响Casimir-Polder力的性质, 因此对于某些初始条件(初速度和初始位置), 不同偶极矩取向的原子有不同的运动学结果, 会被吸引到界面或反射出去, 从而对具有不同偶极矩方向的原子进行筛选. 当然由于Casimir-Polder力很小, 能够反射的初速度也很小, 但是已经可以反抗极低温的热涨落, 我们的理论预估值约为15 μupK. 如果和其他约束手段同时作用, 便能对原子的动力学产生更为有利的控制.     

Modification of radiation pressure due to cooperative scattering of light

Cooperative spontaneous emission of a single photon from a cloud of N atoms modifies substantially the radiation pressure exerted by a far-detuned laser beam exciting the atoms. On one hand, the force induced by photon absorption depends on the collective decay rate of the excited atomic state. On the other hand, directional spontaneous emission counteracts the recoil induced by the absorption. We derive an analytical expression for the radiation pressure in steady-state. For a smooth extended atomic distribution we show that the radiation pressure depends on the atom number via cooperative scattering and that, for certain atom numbers, it can be suppressed or enhanced. Cooperative scattering of light by extended atomic clouds can become important in the presence of quasi-resonant light and could be addressed in many cold atoms experiments.     

The Mott Problem in One Dimension

In the α decay of a nucleus, the tracks left in the medium by the α particle are linear, even though its initial wave function is spherically symmetric. Understanding this quantum phenomenon has been called “the Mott problem”, ever since Mott’s fundamental paper on the subject (Mott in Proc. R. Soc. London Ser. A 126:79 1929). Here we study a one dimensional version of the Mott problem. The particle emitted in the decay is represented as a superposition of waves, one traveling to the left, the other to the right. The atoms with which the particle interacts are modeled as two level systems. The wave equation obeyed by the particle is taken to be the massless Dirac equation. For a certain space-time structure for the particle-atom interaction, it is possible to derive an explicit space-time solution for the entire system, for an arbitrary number of atoms. In the one dimensional solution, the coherent superposition of right and left-moving wave packets leaves behind tracks of excited atoms. The Mott problem on the nature of the tracks left behind is addressed using the reduced density matrix, defined by taking the trace over all particle degrees of freedom. It is found that the reduced density matrix is the incoherent sum of two terms, one involving excited atoms only on the right; the other involving excited atoms only on the left, implying that tracks will show excited atoms on one side or the other. In one dimension, tracks which involve excited atoms exclusively on one side or the other are the analog of straight tracks in three dimensions.     

Interaction enhanced imaging of individual Rydberg atoms in dense gases

We propose a new all-optical method to image individual Rydberg atoms embedded within dense gases of ground state atoms. The scheme exploits interaction-induced shifts on highly polarizable excited states of probe atoms, which can be spatially resolved via an electromagnetically induced transparency resonance. Using a realistic model, we show that it is possible to image individual Rydberg atoms with enhanced sensitivity and high resolution despite photon-shot noise and atomic density fluctuations. This new imaging scheme could be extended to other impurities such as ions, and is ideally suited to equilibrium and dynamical studies of complex many-body phenomena involving strongly interacting particles. As an example we study blockade effects and correlations in the distribution of Rydberg atoms optically excited from a dense gas.     

Nonlinear interaction of laser modes

The interaction of N atoms each with 3 levels at random lattice sites with a set of cavity modes is considered. The optical transition between the lowest two atomic levels is taken into account explicitely assuming a Lorentzian line shape, whereas the third level just serves for the pumping process. If homogeneous inversion of the atoms is assumed, only one coherent mode oscillates in the steady state. It is the one being closest to the atomic resonance and having highestQ. If, however, in the next approximation a mode-dependent depletion of the excited atomic states is taken into account, with increasing pumping rate several modes may oscillate simultaneously. The behaviour of two such modes is treated in detail and it is shown, that one obtains a stable configuration. Using a higher approximation the nonlinear interaction between these two modes brought about by the amplifying material is studied in detail. As a special result one obtains a repulsion of the frequencies of the modes as a function of pumping power in accordance with gaslaser experiments. Quantum noise effects are neglected throughout the present paper.     

Correlated photon emission from multiatom Rydberg dark States

We consider three-level atoms driven by two resonant light fields in a ladder scheme where the upper level is a highly excited Rydberg state. We show that the dipole-dipole interactions between Rydberg excited atoms prevents the formation of single particle dark states and leads to strongly correlated photon pairs from atoms separated by distances large compared to the emission wavelength. For a pair of atoms, this enables realization of an efficient photon-pair source with on average one pair every 30 μs.     

Many-body interband tunneling as a witness of complex dynamics in the Bose-Hubbard model

A perturbative model is studied for the tunneling of many-particle states from the ground band to the first excited energy band, mimicking Landau-Zener decay for ultracold, spinless atoms in quasi-one-dimensional optical lattices subjected to a tunable tilting force. The distributions of the computed tunneling rates provide an independent and experimentally accessible signature of the regular-chaotic transition in the strongly correlated many-body dynamics of the ground band.     

Peculiarities of transport phenomena in highly excited gases

Highly excited gases consisting of a considerable amount (nonequilibrium) of excited atoms on the electron quantum level may have completely different physical properties in comparison with the gas of the same nonexcited atoms. The strong resonant dipole–dipole interaction between differently excited atoms associated with exchange of excitation may result in formation of the metastable quasi-molecules. We evaluate the lifetime of these quasi-molecules that may have the anomalous cross-section for high quantum states of the excited atoms. Large scattering cross-sections result in decreasing of the diffusion and heat conductivity coefficient in the highly excited gases.     

Cobmecтhaя диффuзия фop;oh;ob и khcy;acтиц b пoлuБecкoheчhom пpocтpahcтbe—I pacпpeдeлehиe boзБuждehhьч aтomob b пoлuБecкoheчhom пpocтpahcтbe

The excited atom distribution produced by the simultaneous action process, namely, excitation transfer by radiation in a spectral line and spce movement of excited atoms, is considered. A kinetic equation describing these process is analysed. For steady-state conditions, an adympototic analytical solutio id obtained. This solution describes the concentration distribution of excited atoms for a plane geometry in a region which is a distance exceeding the effective free path away from the surface limiting the volume.The influence on the general solution of either the excitation transfer process id found as a function of the parameters involved.     

The effect of spatial dispersion and boundaries of a medium on resonance radiation transfer

An integral equation is derived for the spectral density of excited atoms using optically dense bounded dispersion media as an example. It is found that the inclusion of the thermal motion of atoms and of the effects due to the existence of a boundary brings about a nonlocal correlation between the concentration of excited particles and the intensity of electromagnetic field in the medium. It is demonstrated that, when the spatial dispersion of permittivity and the boundary effects are disregarded, the obtained result transforms to the well-known Biberman-Holstein equation. The problem on the spectral intensity of radiation of a heated half-space is also investigated.     

Collective electronically excited states of a uniform chain of atoms

Electronically excited states of finite uniform chains of atoms were considered taking into account the influence of the continuous energy spectrum. Traditional quantum-chemical methods for calculating two-electron transitions between neighboring chain atoms were combined with the asymptotic theory of interactions between excited atoms and neutral particles and the mathematical apparatus of the theory of multiple scattering for taking into account intercenter transitions in an ensemble of interacting centers. Recurrence equations for describing energy zones containing symmetrical and antisymmetric excited state levels of chains with an arbitrary length were obtained. Depending on system parameters, different modes of the distribution of the electron density of collective excited states were possible. At a certain ratio between level shifts and exchange integral values, excited states with a uniform electron density distribution over all chain nodes could form for certain solutions. This was a fortuitous circumstance caused by the influence of the continuous spectrum. Such states appeared at small principal quantum number n values, they were similar to one-electron excitations of the type of Frenkel excitons, when an electron was localized near its Coulomb center. These conditions were rapidly disturbed as n increased, and one-electron excitations of a linear molecule were formed in the system (that is, limiting excitations of the type of Wannier-Mott excitons did not form).     

Influence of the radiationless transitions on an intensity of the optical radiation emitted by excited atoms ejected from solids by an ion beam

The excited atoms of the target material are ejected during an ion bombardment of solids. These atoms belong to one of two velocity groups — fast or slow. The fast atoms arise in binary collisions of bombarding ions with target atoms and the slow ones are knocked out as a result of a sputtering process. Excited atoms flying off the surface intersect the solid-vacuum boundary and can transfer their excitation energy in the radiationless transitions mainly of the resonance ionization type. The probability of this process depends strongly on the electronic energy level structure of solids and a velocity of ejected atoms. On this statement our method of the electronic energy level structure of solids study is based by means of investigation of excited atoms velocity spectrum. On the results of paper [4] we remark that in their experimental conditions the surface of the lithium target was apparently strongly oxidized. Using our method and results of paper [4] we can estimate the energy width of the conduction band of Li₂O to 0.4 eV. In general the cascade corrections to the mean life times of excited atoms may be important and one can take them into account. The detailed analysis of the influence of the cascade corrections to the mean life times of upper excited states of some TiI lines (λλ 5210 Å, 5064 Å, 4682 Å, 3981 Å, 4533 Å, 4856 Å) was carried out. It was found that in the case when our method was applied to determination of the work function of metallic titanium the cascade corrections either are negligible or not necessary.     

Population dynamics of excited atoms in non-Markovian environments at zero and finite temperature

The population dynamics of a two-atom system, which is in two independent Lorentzian reservoirs or in two independent Ohmic reservoirs respectively, where the reservoirs are at zero temperature or finite temperature, is studied by using the time-convolutionless master-equation method. The influences of the characteristics and temperature of a non-Markovian environment on the population of the excited atoms are analyzed. We find that the population trapping of the excited atoms is related to the characteristics and the temperature of the non-Markovian environment. The results show that, at zero temperature, the two atoms can be effectively trapped in the excited state both in the Lorentzian reservoirs and in the Ohmic reservoirs. At finite temperature, the population of the excited atoms will quickly decay to a nonzero value.     

Zum Nachweis von änderungen im angeregten Zustand eines Atoms durch Beobachten der folgenden änderungen im Grundzustand

Radio frequency transitions in the excited state of Rb atoms are observed by measuring the transmitted light in a resonance cell experiment. The influence of additional radio frequency transitions simultaneously performed between the Zeeman sub-levels of the ground state is studied. Methods for observing signals of the excited states without using fluorescent light as detector are discussed concerning the magnetic deflection of optical oriented atoms and also the influence of recoil of the scattered light quants on the direction of an atomic beam.     

Two level metal vapor lasers with thermal creation of population inversion

We present a new technique for direct conversion of thermal energy into coherent radiation. The near-surface layer of evaporated excited atoms of rare earth metals is proposed to be used as a converter. There is an inverted population in this layer. Operation principles of two level lasers on the transitions to the ground state using Eu are considered. The fact of detachment of Sm atoms in the excited 4f(N-1)5d6s2 state during the thermal evaporation process has been proved experimentally.