Comparison of quantum channels

Anna Jencova

Published 2015-12-22Version 1

For a pair of quantum channels with the same input space, we show that the possibility of approximation of one channel by post-processings of the other can be characterized by comparing the success probabilities for ensembles obtained as outputs for any ensemble on the input space coupled with an ancilla. In particular, this yields the randomization criterion for quantum statistical experiments, with a clear operational interpretation. The results are obtained using some properties of the diamond norm and its dual, which are of independent interest.