Qubit-qunit decomposition of quantum channel
  Problem of classification of parallel quantum channels for information transfer is studied by method of ladder operators.   Detailed compared to http://arxiv.org/abs/quant-ph/0702076 presentation of method of ladder operators is given. Examples of paired channel and qubit-qunit pair are added. Relations between eigenvalues of density matrices of composite channel and its subchannels determining presence/absence of entanglement of arbitrary mixed state of channel are obtained. 
Constantin V. Usenko