  We propose a reliable entanglement measure for a two-mode squeezed thermal
state of the quantum electromagnetic field in terms of its Bures distance to
the set of all separable states of the same kind. The requisite Uhlmann
fidelity of a pair of two-mode squeezed thermal states is evaluated as the
maximal transition probability between two four-mode purifications. By applying
the Peres-Simon criterion of separability we find the closest separable state.
This enables us to derive an insightful expression of the amount of
entanglement. Then we apply this measure of entanglement to the study of the
Braunstein-Kimble protocol of teleportation. We use as input state in
teleportation a mixed one-mode Gaussian state. The entangled state shared by
the sender (Alice) and the receiver (Bob) is taken to be a two-mode squeezed
thermal state. We find that the properties of the teleported state depend on
both the input state and the entanglement of the two-mode resource state. As a
measure of the quality of the teleportation process, we employ the Uhlmann
fidelity between the input and output mixed one-mode Gaussian states.
