  Based on total variance of a pair of Einstein-Podolsky-Rosen (EPR) type
operators, the generalized EPR entangled states in continuous variable systems
are defined. We show that such entangled states must correspond with two-mode
squeezing states whether these states are Gaussian or not and whether they are
pure or not. With help of the relation between the total variance and the
entanglement, the degree of such entanglement is also defined. Through
analyzing some specific cases, we see that this method is very convenient and
easy in practical application. In addition, an entangled state with no
squeezing is studied, which reveals that there certainly exist something
unknown about entanglement in continuous variable systems.

Quantum Physics