Quantum intersection and union.

In information theory, we often use intersection and union of the typical sets to analyze various communication problems. However, in the quantum setting it is not very clear how to construct a measurement which behaves analogous to intersection and union of the typical sets. In this work, we construct a projection operator which behaves very similar to intersection and union of the typical sets. Our construction relies on the Jordan's lemma. Using this construction we study the problem of communication over authenticated classical-quantum channels and derive its capacity. As another application of our construction, we study the problem of quantum asymmetric composite hypothesis testing. Further, we also prove a converse for the quantum binary asymmetric hypothesis testing problem which is arguably very similar in spirit to the converse given in the Thomas and Cover book for the classical version of this problem.