  We investigate the properties of quantum entanglement of two-mode squeezed
states interacting with linear baths with general gain and loss parameters. By
explicitly solving for \rho from the master equation, we determine analytical
expressions of eigenvalues and eigenvectors of \rho^{T_A} (the partial
transposition of density matrix \rho). In Fock space, \rho^{T_A} is shown to
maintain a block diagonal structure as the system evolves. In addition, we
discover that the decoherence induced by the baths would break the degeneracy
of \rho^{T_A}, and leads to a novel set of eigenvectors for the construction of
entanglement witness operators. Such eigenvectors are shown to be
time-independent, which is a signature of robust entanglement of two-mode
squeezed states in the presence of noise.
