arXiv subjects

Hassan Safari

Publications and source records attributed to Hassan Safari.

13 recordsLinked to original sources

arXiv2024

Dynamics of a two-level atom in the presence of a medium-assisted thermal field

In this paper the time evolution of a two-level atom in the presence of medium-assisted thermal field is explored through which, the formula of decay rate of an excited atom is generalized in two aspects. The obtained formula applies for the thermal electromagnetic field as well as the presence of arbitrary arrangement of magneto-electric media. In order to be general with respect to the material environment, the Green's function approach is used. It is seen that the non-zero temperature contributes to the decay rate via an additive term that is equal to the zero-temperature result multiplied by two times of photon number at atomic transition frequency.

physics.atom-ph↗

arXiv2020

Exact dynamics of concurrence-based entanglement in a system of four spin-1/2 particles on a triangular ladder structure

Motivated by the ability of triangular spin ladders to implement quantum information processing, we propose a type of such systems whose Hamiltonian includes the XX Heisenberg interaction on the rungs and DzyaloshinskiiMoriya (DM) coupling over the legs. In this work, we discuss how tuning the magnetic interactions between elements of a nanomagnetic cell of a triangular ladder which contains four qubits influences on the dynamical behavior of entanglement shared between any pairs of the system. In this work, we make use of concurrence for monitoring entanglement. It is realized that the generation of quantum W states is an important feature of the present model when the system evolves unitarily with time. In general, coincidence with the emergence of W states, the concurrences of all pairs are equal to $N/2$, where N is the number of system's qubits. We also obtain the precise relationship between the incidence of such states and the value of DM interaction as well as the time of entanglement transfer. Finally, by studying the two-point quantum correlations and expectation values of different spin variables, we find that xx and yy correlations bring the entanglement to a maximum value for W states, whereas for these states, zz correlation between any pairs completely quenches. Our results reveal that although $S^{tot}_{z}$ does not commute with the system's Hamiltonian, its expectation value remains constant during time evolution which is a generic property of quantum W states.

quant-ph↗

arXiv2019

Medium-assisted van der Waals dispersion interactions involving chiral molecules

The van der Waals dispersion interaction between two chiral molecules in the presence of arbirary magnetoelectric media is derived using perturbation theory. To be general, the molecular polarisabilities are assumed to be of electric, paramagnetic and diamagnetic natures and the material environment is considered to possess a chiral electromagnetic response. The derived formulas of electric-chiral, paramagnetic-chiral, diamagnetic-chiral and chiral-chiral interaction potentials when added to the previously obtained contributions in literature, form a complete set of dispersion interaction formulas. We present them in a unified form making use of electric--magnetic duality. As an application, the case of two anisotropic molecules in free space is considered where we drive the retarded and non-retarded limits with respect to intermolecular distance.

quant-ph↗

arXiv2017

Cavity-QED interactions of two correlated atoms

We consider the resonant van der Waals interaction between two correlated identical two-level atoms (at least one of which being excited) within the framework of macroscopic cavity quantum electrodynamics in linear, dispersing and absorbing media. The interaction of both atoms with the body-assisted electromagnetic field of the cavity is assumed to be strong. Our time-independent evaluation is based on an extended Jaynes-Cummings model. For a system prepared in a superposition of its dressed states, we derive the general form of the van der Waals forces, using a Lorentzian single mode approximation. We demonstrate the applicability of this approach by considering the case of a planar cavity and showing the position-dependence of Rabi oscillations. We also show that in the limiting case of weak coupling, our results reproduce the perturbative ones, for the case where the field is initially in vacuum state while the atomic state is in a superposition of two correlated states sharing one excitation.

quant-ph↗

arXiv2017

Enhanced chiral discriminatory van der Waals interactions mediated by chiral surfaces

We predict a discriminatory interaction between a chiral molecule and an achiral molecule which is mediated by a chiral body. To achieve this, we generalize the van der Waals interaction potential between two ground-state molecules with electric, magnetic and chiral response to non-trivial environments. The force is evaluated using second-order perturbation theory with an effective Hamiltonian. Chiral media enhance or reduce the free interaction via many-body interactions, making it possible to measure the chiral contributions to the van der Waals force with current technology. The van der Waals interaction is discriminatory with respect to enantiomers of different handedness and could be used to separate enantiomers. We also suggest a specific geometric configuration where the electric contribution to the van der Waals interaction is zero, making the chiral component the dominant effect.

quant-ph↗

arXiv2014

Body-assisted van der Waals interaction between excited atoms

We present a formula for the body-assisted van der Waals interaction potential between two atoms, one or both being prepared in an excited energy eigenstate. The presence of arbitrary arrangement for material environment is taken into account via the Green function. The resulting formula supports one of two conflicting findings recorded. The consistency of our formula is investigated by applying it for the case of two atoms in free space and comparing the resulting expression with the one found from the limiting Casimir-Polder potential between an excited atom and a small dielectric sphere.

quant-ph↗

arXiv2012

Body-assisted dispersion potentials of diamagnetic atoms

We derive Casimir-Polder and van der Waals potentials of one or two atoms with diamagnetic properties in an arbitrary environment of magnetoelectric bodies. The calculations are based on macroscopic quantum electrodynamics and leading-order perturbation theory. For the examples of an atom and a perfect mirror and two atoms in free space we show that diamagnetic dispersion potentials have the same sign as their electric counterparts, but can exhibit quite different distance dependences.

quant-ph↗

arXiv2008

Van-der-Waals potentials of paramagnetic atoms

We study single- and two-atom van der Waals interactions of ground-state atoms which are both polarizable and paramagnetizable in the presence of magneto-electric bodies within the framework of macroscopic quantum electrodynamics. Starting from an interaction Hamiltonian that includes particle spins, we use leading-order perturbation theory for the van der Waals potentials expressed in terms of the polarizability and magnetizability of the atom(s). To allow for atoms embedded in media, we also include local-field corrections via the real-cavity model. The general theory is applied to the potential of a single atom near a half space and that of two atoms embedded in a bulk medium or placed near a sphere, respectively.

quant-ph↗

arXiv2008

Dispersion forces and duality

We formulate a symmetry principle on the basis of the duality of electric and magnetic fields and apply it to dispersion forces. Within the context of macroscopic quantum electrodynamics, we rigorously establish duality invariance for the free electromagnetic field in the presence of causal magnetoelectrics. Dispersion forces are given in terms of the Green tensor for the electromagnetic field and the atomic response functions. After discussing the behavior of the Green tensor under a duality transformation, we are able to show that Casimir forces on bodies in free space as well as local-field corrected Casimir-Polder and van der Waals forces are duality invariant.

quant-ph↗

arXiv2008

Interatomic van der Waals potential in the presence of a magneto-electric sphere

On the basis of a general formula obtained earlier via fourth-order erturbation theory within the framework of macroscopic quantum electrodynamics, the van der Waals potential between two neutral, unpolarized, ground-state atoms in the presence of a homogeneous, dispersing and absorbing magnetoelectric sphere is studied. When the radius of the sphere becomes sufficiently large, the result asymptotically agrees with that for two atoms near a planar interface. In the opposite limit of a very small sphere, the sphere can effectively be regarded as being a third ground-state atom, and the nonadditive three-atom van der Waals potential is recovered. To illustrate the effect of a sphere of arbitrary radius, numerical results are presented for the triangular arrangement where the atoms are at equidistance from the sphere, and for the linear arrangement where the atoms and the sphere are aligned along a straight line. As demonstrated, the enhancement or reduction of the interaction potential in the presence of purely electric or magnetic spheres can be physically understood in terms of image charges.

quant-ph↗

arXiv2007

Two-atom van der Waals interaction between polarizable/magnetizable atoms near magneto-electric bodies

The van der Waals potential of two atoms in the presence of an arbitrary arrangement of dispersing and absorbing magnetodielectric bodies is studied. Starting from a polarizable atom placed within a given geometry, its interaction with a second polarizable/magnetizable atom is deduced from its Casimir-Polder interaction with a weakly polarizable/magnetizable test body. The general expressions for the van der Waals potential hence obtained are illustrated by considering first the case of two atoms in free space, with special emphasis on the interaction between (i) two polarizable atoms and (ii) a polarizable and a magnetizable atom. Furthermore, the influence of magnetodielectric bodies on the van der Waals interaction is studied in detail for the example of two atoms placed near a perfectly reflecting plate or a magnetodielectric half space, respectively.

quant-ph↗

arXiv2007

Microscopic origin of Casimir-Polder forces

We establish a general relation between dispersion forces. First, based on QED in causal media, leading-order perturbation theory is used to express both the single-atom Casimir-Polder and the two-atom van der Waals potentials in terms of the atomic polarizabilities and the Green tensor for the body-assisted electromagnetic field. Endowed with this geometry-independent framework, we then employ the Born expansion of the Green tensor together with the Clausius-Mosotti relation to prove that the macroscopic Casimir-Polder potential of an atom in the presence of dielectric bodies is due to an infinite sum of its microscopic many-atom van der Waals interactions with the atoms comprising the bodies. This theorem holds for inhomogeneous, dispersing, and absorbing bodies of arbitrary shapes and arbitrary atomic composition on an arbitrary background of additional magnetodielectric bodies.

quant-ph↗

arXiv2006

Body-assisted van der Waals interaction between two atoms

Using fourth-order perturbation theory, a general formula for the van der Waals potential of two neutral, unpolarized, ground-state atoms in the presence of an arbitrary arrangement of dispersing and absorbing magnetodielectric bodies is derived. The theory is applied to two atoms in bulk material and in front of a planar multilayer system, with special emphasis on the cases of a perfectly reflecting plate and a semi-infinite half space. It is demonstrated that the enhancement and reduction of the two-atom interaction due to the presence of a perfectly reflecting plate can be understood, at least in the nonretarded limit, by using the method of image charges. For the semi-infinite half space, both analytical and numerical results are presented.

quant-ph↗