Atomic ensemble

Here we present first results of ab-initio LDA-MD studies of crystalline and liquid K-Sb alloys[51]. The liquid alloys are modelled by 64-atom ensembles of appropriate... 

Geometrical effects, related to the number and geometrical arrangement of the surface metal atoms participating in the formation of the essential surface intermediates of the reaction in question. For these, number of atoms (ensemble size) appeared to be particularly crucial. 

Maroun F, Ozanam F, Magnussen OM, Behm RJ. 2001. The role of atomic ensembles in the reactivity of bimetallic electrocatalysts. Science 293 1811-1814. 

One must distinguish between macroscopic and microscopic imperfections existing on a real surface. Macroscopic imperfections are perturbations of the periodic structure covering a region of dimensions considerably greater than the lattice constant. They include cracks on the surface of the crystal, pores, and various macroscopic inclusions. We shall not deal with such imperfections here. Microscopic imperfections are perturbations of dimensions of the order of a crystallc raphic cell. Microscopic imperfections include vacancies in the surface layer of the crystal, foreign atoms or lattice atoms on the surface, different groups of such atoms (ensembles), etc. We shall limit ourselves to a consideration of this kind of imperfection. 

Petrashen , A.G., Rebane, V.N. and Rebane, T.K. (1973). Relaxation of electron multipole moments and collisional depolarization of resonance fluorescence of an atomic ensemble in the state with an inner quantum number j = 2, Optika i Spektroskopiya, 35, 408-416. [Opt. Spectrosc. (USSR), 35, 240-244]. 

For any given catalytic reaction the active surface area is normally only a small fraction of the area of the active component (active phase). The term active sites is often applied to the sites effective for a particular heterogeneous catalytic reaction. The terms active site and active centre are often used as synonyms, but active centre may also be used to describe an ensemble of sites at which a catalytic reaction takes place. There is evidence that the centres required for some catalytic reactions are composed of a collection of several metal atoms (ensemble). This appears to be the case for such reactions as, for example, hydrogenolysis, hydrogenation of CO, and certain deuterium-exchange processes with hydrocarbons. 

The new mechanistic routes opened by the introduction of a catalyst to the reaction mixture typically start with the adsorption of the reactants. Because of the heterogeneous nature of the surface of the catalyst, both the adsorption and the subsequent conversion reactions may take place preferentially at particular ensembles of surface sites, often called active sites or active centers. An atomic ensemble may become active because of a specific structural arrangement of those atoms. Alternatively, the electronic properties of metal atoms may also influence the adsorption and activation of reactants. Typically, the performance of catalytic active sites depends on both structural and electronic effects. 

The oxidative addition of the OH (SH) group may occur with zero-valent metal atoms/ensembles having M—M bonds ... 

It may be argued that the low reactivity of the alloy with respect to tin is due to the fact that the bimetallic system does not have adsorption sites with two or three adjacent tin atoms ( ensemble effects [31,32]). But the differences in reactivity... 

This part addresses mesoscopic systems as diverse as SQUIDS, Bose condensates, trapped atomic ensembles, electronic inerferometers and micromasers. It shows that coherence and entanglement can persist in such systems ... 

Keywords Electromagnetically induced transparency, Raman scattering, atomic ensembles,... 

These pulse shapes can be quantitatively understood by examining the propagation of the Stokes and anti-Stokes fields through an optically dense atomic ensemble. As shown in Fig. 2 B, the results of this analysis are in an excellent agreement with experimental observations. We theoretically model the medium as N three-level A-type atoms as shown in Fig. 1 A. The two laser fields are treated classically, whereas the generation and propagation of one transverse mode of the Stokes (anti-Stokes) quantum field is described by an effective one-dimensional slowly varying operator s z, t) ( s(z. t)), which corresponds to the vacuum field at the entrance of the cell z = 0. Our model allows us to treat a small number of transverse spatial modes which evolve... 

Since we are primarily interested in the conditional quantum state of the atomic ensemble, we can use the density-matrix formalism to calculate the fidelity with which we create the desired atomic state, Ins ) ... 

Starting from the gap Hamiltonian (33) and the interactions with the reservoirs (34) and (35), eliminating the reservoir degrees of freedom within the standard Bom-Markov approach we derive a master equation for the reduced density operator of the atomic ensemble. The calculation is lengthy but straight forward. Disregarding level shifts caused by the bath interaction we find for the populations in states cj ) the following density matrix equations in the interaction picture ... 

One may envision extensions of the present approach to matter teleportation [Opatrny 2001] and quantum computation based on continuous variables [Braunstein 1998 (a) Lloyd 1998 Lloyd 1999], Such extensions may involve the coupling of entangled atomic ensembles in optical lattices by photons carrying quantum information. 

Here the authors consider the possibility of inferring such statistical characteristics from the spectral features of probe photons or particles that are scattered by the density fluctuations of trapped atoms, notably in optical lattices, in two hitherto unexplored scenarios, (a) The probe is weakly (perturbatively) scattered by the local atomic density corresponding to the random occupancy of different lattice sites, (b) The probe is multiply scattered by an arbitrary (possibly unknown a priori) multi-atom distribution in the lattice. The highlight of the analysis, which is based on this random matrix approach, is the prediction of a semicircular spectral lineshape of the probe scattering in the large-fluctuation limit of trapped atomic ensembles. Thus far, the only known case of quasi-semicircular lineshapes in optical scattering has been predicted [Akulin 1993] and experimentally verified [Ngo 1994] in dielectric microspheres with randomly distributed internal scatterers. 

In this paper we explore the design of microwave-based structures that can enhance the interaction of electromagnetic fields with cold-atom ensembles, leading to novel sensing modalities based on the quantum-mechanical behavior of these systems In particular, we discuss electromagnetically-induced transparency in a single uncondensed cold-atom cloud, and a two-cloud version of a SQUID, where the clouds are EEC s and take the place of the weakly coupled superconductors. 

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