Entanglement Generalization in Coupled Harmonic Oscillators
  A general and in principle exact approach for the continuous variable entanglement in a system of coupled harmonic oscillators in contact with a thermal bath is formulated. This allows a generalization to describe entanglement's existence between two sites in any system of this kind. It is employed a method of simultaneous diagonalization of two quadratic forms to obtain the uncoupled quantum Hamiltonian. Making use of the transformations that uncouple the system, the covariance matrix for two positions is constructed, and through the positive partial transpose criterion (PPT-criterion), the condition that determines the existence or not of quantum correlation between the sites is obtained. 
Danyer P\'erez Ad\'an, Fernando Guzm\'an Martinez, Oscar Rodr\'iguez   Hoyos
quant-ph