Separated subsystems in four-level atom

Robert J. Bettles

Dmitriy V. Kupriyanov, Igor M. Sokolov, A. V. Slavgorodskii

In this paper we consider the optical coupling of two four-level atoms via cooperative Raman scattering of correlated photons of the nondegenerate spontaneous parametric radiation. The Raman scattering of twin photons incident on the atoms is additionally stimulated by coherent classical radiation in such a way that it is slightly off resonant for each atom considered independently but resonant for the joint system. We discuss the special excitation conditions when the amplitude of the cooperative stimulated Raman scattering from quantum to classical modes becomes greater than the normal transition rates associated with independent Raman scattering. In certain conditions weak entanglement between the Zeeman sublevels in the ground states of these atoms can appear. The small admixture of entanglement in the system of two atoms leads to strong entanglement between transverse macroscopic spin fluctuations of two spatially separated atomic ensembles coupled by such quantum light. The proposed mechanism of entanglement is relevant to the ground state spin subsystem of two distant macroscopic and optically thin atomic ensembles. Such an entangled state can be stored for a long time and used as a basic Einstein-Podolsky-Rosen system in different applications pursuing the goal of spreading quantum information schemes in atomic subsystems.

LU Dao-ming

In this paper, we consider the situation that four identical two-level atoms are separately trapped in separated tetrahedral structure single-mode optical cavities, which are placed at the vertices of a tetrahedron and are coupled by four fibres. Each atom resonantly interacts with cavity via a one-photon hopping. The evolution of the state vector of the system is given by solving the Schrödinger equation when the total excitation number of the system equals one. Negativity is adopted to quantify the degree of entanglement between two subsystems. The entanglement dynamics between atoms and between cavities is studied. The influences of atom-cavity coupling coefficient on the entanglement between atoms and that between cavities are discussed. The results obtained using the numerical method show that the atom–atom entanglement and the cavity–cavity entanglement are all strengthened with increase of atom-cavity coupling coefficient.

Dieter Suter

Harold J. Metcalf, P. van der Straten

Oladunjoye A. Awoga, A. N. Ikot, Eno E. Ituen +1 more

Abstract We consider a four-level system with two subsystems coupled by weak interaction. The system is in thermal equilibrium. The thermodynamics of the system, namely internal energy, free energy, entropy and heat capacity, are evaluated using the canonical density matrix by two methods. First by Kronecker product method and later by treating the subsystems separately and then adding the evaluated thermodynamic properties of each subsystem. It is discovered that both methods yield the same result, the results obey the laws of thermodynamics and are the same as earlier obtained results. The results also show that each level of the subsystems introduces a new degree of freedom and increases the entropy of the entire system. We also found that the four-level system predicts a linear relationship between heat capacity and temperature at very low temperatures just as in metals. Our numerical results show the same trend.