  Nonclassicality, entanglement, as well as dimensionality of a noisy twin beam
are determined using characteristic function of the beam written in the Fock
basis. One-to-one correspondence between the negativity quantifying
entanglement and the nonclassicality depth is revealed. Twin beams, which are
either entangled or nonclassical (independent of their entanglement), are
observed only for the limited degrees of noise that degrades their quantumness.
Dimensionality of the twin beam quantified by the participation ratio is
compared with the dimensionality of entanglement determined from the
negativity. The partition of the degrees of freedom of the twin beam into those
related to entanglement and to noise is suggested. Both single-mode and
multimode twin beams are analyzed. Weak nonclassicality based on
integrated-intensity quasidistributions of multimode twin beams is studied.
Relation of the model to the experimental twin beams is discussed.
