Dealing with unknown quantum operations

Abstract: In the context of quantum communications between two parties (here Alice and Bob), Bob's lack of knowledge about the communications channel can affect the purity of the states that he receives. The operation of applying an unknown unitary transformation to a state, thus reducing its purity, is called "twirling". As twirling affects the states that Bob receives, it also affects his perception of the operations that Alice applies to her states. In this work we find that not every operation is representable after a twirling, we show the minimal requirement for this to be possible, and we identify the correct form of the "twirled" operations.