Spin correlations in a pair of atoms induced by spontaneous emission of photons

An analysis is presented of the state that arises after photons have been spontaneously emitted by a pair of spatially separated excited two-level atoms with spin-1/2 ground and excited states. Selection of possible decay scenarios conditioned on the helicities of the photons (even on the helicity of the one emitted first) makes it possible to reveal ground-state spin-projection correlations between atoms. The correlations are due to quantum interference between alternative scenarios (the atom that has emitted a particular photon cannot be identified). The correlations obtained by the chosen selection method are classical.     

Atomic ordering in amorphous systems

A theoretical method is proposed for describing the atomic ordering in multicomponent amorphous systems. It accounts for both two-particle and multiparticle correlations between atoms. A configurational Hamiltonian allows for the existence of multiparticle interaction potentials. A crystal with an arbitrary number of various structural defects is assumed to model the amorphous state. These defects are regarded as new kinds of atoms. The present work is based on the multiparticle-entropy method.     

Detection of spatial correlations in an ultracold gas of fermions

Spatial correlations are observed in an ultracold gas of fermionic atoms close to a Feshbach resonance. The correlations are detected by inducing spin-changing rf transitions between pairs of atoms. We observe the process in the strongly interacting regime for attractive as well as for repulsive atom-atom interactions and both in the regime of high and low quantum degeneracy. The observations are compared with a two-particle model that provides theoretical predictions for the measured rf transition rates.     

Spontane Strahlung eines Zwei-Atom-Systems

The method of “many-centers-quantization” developed in a foregoing paper [1] is used to solve the decay problem of two atoms with various initial states. Especially the angular distributions and angular correlations are given.     

Transport properties of a random binary side-coupled chain

We investigate the transport properties of a random binary side-coupled chain by using the transfer-matrix technique. It is found that there are resonant states in the systems with short-range correlations between the host chain atoms and the side-coupled atoms. The analytic expressions for the extended states are also presented in the systems with the side couplings between like atoms and between unlike atoms.     

Creation of quantum correlations via the initial classical-mixed states

Using the pseudomode method, we theoretically analyze the creation of quantum correlations between two two-level dipole-dipole interacting atoms coupled with a common structured reservoir with different coupling strengths. Considering certain classes of initial separable-mixed states, we demonstrate that the sudden birth of atomic entanglement as well as the generation of stationary quantum correlations occur. Our results also suggest a possible way to control the occurrence time of entanglement sudden birth and the stationary value of quantum correlations by modifying the initial conditions of states, the dipole-dipole interaction, and the relative coupling strength. These results are helpful for the experimental engineering of entanglement and quantum correlations.     

Delay of Spatial Quantum Correlations in Magnetic Nanostructures

Simultaneous quantum correlations between two spins in magnetic nanostructures are considered in the model of a linear chain of a finite number of atoms with exchange interaction between electron spins of neighboring atoms in the framework of the Heisenberg ferromagnetism theory. We assume that in the initial state, the spins of all chain atoms except the first two are oriented along the same direction. The spins of the first two atoms are flipped. Due to the exchange interaction, this initial state generates a spin flip wave along the chain. The expressions obtained for nonstationary quantum amplitudes of the flip probability waves for an even number of spins can be used for calculating quantum correlations between two spins separated by a large distance in a chain. Numerical calculations of the spin correlator reveal that the correlation between two spins in the chain occurs with a delay on the order of the time of propagation of the exchange interaction along the spin chain. After the delay, the spin correlation amplitude abruptly increases followed by subsequent oscillatory temporal behavior.     

Quantum Correlation of Two Entangled Atoms Interacting with the Binomial Optical Field

Quantum correlations of two atoms in a system of two entangled atoms interacting with the binomial optical field are investigated. In eight different initial states of the two atoms, the influence of the strength of the dipole-dipole interaction, probabilities of a the Bernoulli trial and particle number of the binomial optical field on the temporal evolution of the geometrical quantum discord between two atoms are discussed. The result shows that two atoms always exist the correlation for different parameters. In addition, when and only when the two atoms are initially in the maximally entangled state, the temporal evolution of geometrical quantum discord is not affected by the parameters, and always keep in the degree of geometrical quantum discord that is a fixed value.     

Testing a Bell-type inequality with a micromaser

We propose a phase-sensitive micromaser setup to demonstrate experimentally a violation of a Bell-type inequality. The interaction of atoms with the cavity field produces entanglement between the atoms and the cavity photons and therefore also between the atoms. We derive a Bell-type inequality for the atom-atom correlations and show that it can be violated not only in an idealized model but also under realistic circumstances when various sources of additional randomness are accounted for. Among them are the energy dissipation in the resonator and the Poissonian arrival statistics of the atoms.Prof. F.P. Schäfer on the occasion of his 65th birthday.     

Elastic scattering of slow positrons on atoms

The results of calculations of the elastic scattering cross section of positrons on noble gas and alkali atoms are presented. The calculations are performed within the one-electron Hartree-Fock approximation with multielectron correlations in the so-called random phase approximation with exchange taken into account. Virtual positronium formation is taken into account and proved to be very important. Arguments are presented that the positron polarization potential is repulsive for alkali atoms. The results obtained are in a reasonable agreement with experiment and with some previously reported calculations.     

Controlling and detecting spin correlations of ultracold atoms in optical lattices

We report on the controlled creation of a valence bond state of delocalized effective-spin singlet and triplet dimers by means of a bichromatic optical superlattice. We demonstrate a coherent coupling between the singlet and triplet states and show how the superlattice can be employed to measure the singlet-fraction employing a spin-blockade effect. Our method provides a reliable way to detect and control nearest-neighbor spin correlations in many-body systems of ultracold atoms. Being able to measure these correlations is an important ingredient in studying quantum magnetism in optical lattices. We furthermore employ a SWAP operation between atoms which are part of different triplets, thus effectively increasing their bond-length. Such a SWAP operation provides an important step towards the massively parallel creation of a multiparticle entangled state in the lattice.     

Quantum correlations among superradiant Bose-Einstein condensate atoms

Quantum correlations among atoms in superradiant Bose-Einstein condensates are discussed. It is shown that atoms in the superradiant atomic condensate can exhibit continuous variable quantum entanglement analogous to Einstein-Podolsky-Rosen (EPR)-type quantum correlations. Comparison to quantum entanglement in the Dicke model in thermal equilibrium is provided.     

Probing pair-correlated fermionic atoms through correlations in atom shot noise

Pair-correlated fermionic atoms are created through dissociation of weakly bound molecules near a magnetic-field Feshbach resonance. We show that correlations between atoms in different spin states can be detected using the atom shot noise in absorption images. Furthermore, using time-of-flight imaging we have observed atom pair correlations in momentum space.     

Modeling migration of adsorbed atoms in dense multicomponent adsorbed layers

Within the framework of a simple (i.e., ignoring lateral interactions) lattice model, surface migration of adsorbed atoms of a crystalline substance A is considered for the case in which the surface is densely covered by the adsorbed atoms of a noncrystallized substance B. Simulation of the migration, performed by means of the Monte-Carlo method, demonstrates a sharp decrease in the root-mean-square displacement of the adsorbed atoms of A with an increase in the degree of covering by adsorbed atoms of B in the case where these adsorbed atoms exhibit weak migration mobility. This effect is associated with strong correlations in the directions of individual jumps of an adsorbed atom of A. The results of the simulation are applied to estimate the influence of the degree of covering of a surface with an impurity on the temperature at which a transition is accomplished from an nascent to a stepwise-layered mechanism of growth in silicon in molecular-beam epitaxy. V. D. Kuznetsov Siberian Institute for Technical Physics. Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 89–94, June, 1998.     

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.     

Measurement of one-particle correlations and momentum distributions for trapped 1D gases

Van Hove's theory of scattering of probe particles by a macroscopic target is generalized so as to relate the differential cross section for atomic ejection via stimulated Raman transitions to one-particle momentum-time correlations and momentum distributions of 1D trapped gases. This method is well suited to probing the longitudinal momentum distributions of 1D gases in situ, and examples are given for bosonic and fermionic atoms.     

Angular correlation studies of heavy-particle impact excitation of atoms

A review is given of recent developments of angular correlation studies of heavy-particle impact excitation of atoms and molecules. After a brief discussion of the basic principles of experimental methods a theory of measurement of angular correlations from heavy-particle atom collisions is outlined in the following way. By applying angular correlation measurements a subensemble from the collision processes is selected which is described by state and scattering parameters. These parameters (scattering amplitudes and their phases, alignment and orientation, state multipoles and coherence parameters) can be connected with the experimental angular correlation data. Results from heavy-particle angular correlations have been discussed in separate sections with ions or atoms as projectiles; subsections are dealing with charge exchange excitation, excitation of autoionizing states, direct and simultaneous excitation of target and projectile particles. The angular correlation parameters are very sensitive to the detailed excitation mechanisms and quasi-molecular parameters in heavy-particle atom collisions. The majority of data available so far were obtained from simple projectile-target systems such as one-electron atom systems and are rare gas atoms. Attention is drawn to possible spin-orbit effects in the analysis of angular correlations from collisions between very heavy atomic particles.     

Prise en compte des corrélations diélectroniques dans les petits amas par la méthode de Gutzwiller

We apply a variational Gutzwiller method to the analysis of the influence of the electronic correlations on small aggregates properties. We first describe the general method then we check it by comparison to the results given by an “exact” solution of the Hubbard Hamiltonian for 4 or 5 atoms. We also examine the behaviour of the cluster stabilities when the number of atoms varies. One knows that, for U = 0, the stablest ions have an odd number of atoms but that this property disappears when U = ∞. We obtain here, as expected, an intermediate result.     

Uncondensed atoms in the regime of velocity-selective coherent population trapping

We consider the model of a Bose condensate in the regime of velocity-selective coherent population trapping. As a result of interaction between particles, some fraction of atoms is outside the condensate, remaining in the coherent trapping state. These atoms are involved in brief events of intense interaction with external resonant electromagnetic fields. Intense induced and spontaneous transitions are accompanied by the exchange of momenta between atoms and radiation, which is manifested as migration of atoms in the velocity space. The rate of such migration is calculated. A nonlinear kinetic equation for the many-particle statistical operator for uncondensed atoms is derived under the assumption that correlations of atoms with different momenta are insignificant. The structure of its steady-state solution leads to certain conclusions about the above-mentioned migration pattern taking the Bose statistics into consideration. With allowance for statistical effects, we derive nonlinear integral equations for frequencies controlling the migration. The results of numerical solution of these equations are represented in the weak interatomic interaction approximation.