  In this paper we present the quantum control attack on quantum key
distribution systems. The cornerstone of the attack is that Eve can use unitary
(polar) decomposition of her positive-operator valued measure elements, which
allows her to realize the feed-forward operation (quantum control), change the
states in the channel after her measurement and impose them to Bob. Below we
consider the general eavesdropping strategy and the conditions those should be
satisfied to provide the attack successfully. Moreover we consider several
types of the attack, each of them is based on a different type of
discrimination. We also provide the example on two non-orthogonal states and
discuss different strategies in this case.
