On singular Bosonic linear channels

Properties of Bosonic linear (quasi-free) channels, in particular, Bosonic Gaussian channels with two types of degeneracy are considered. The first type of degeneracy can be interpreted as existence of noise-free canonical variables (for Gaussian channels it means that $\det\alpha=0$). It is shown that this degeneracy implies existence of (infinitely many) "direct sum decompositions" of Bosonic linear channel, which clarifies reversibility properties of this channel (described in arXiv:1212.2354) and provides explicit construction of reversing channels. The second type of degeneracy consists in rank deficiency of the operator describing transformations of canonical variables. It is shown that this degeneracy implies existence of (infinitely many) decompositions of input space into direct sum of orthogonal subspaces such that the restriction of Bosonic linear channel to each of these subspaces is a discrete classical-quantum channel.