  The axis compensation is a procedure in which the sender and the receiver
compensate the axes of their transmitter and detector so that the bit sequence
can be transmitted more reliably. We show the optimal axis compensations
maximizing the key generation rate for unital channels. We consider the case in
which only Bob is allowed to compensate his axis, and the case in which both
Alice and Bob are allowed to compensate their axes. In the former case, we show
that we should utilize the mismatched measurement outcomes in the channel
estimation phase. In the latter case, we show that we do not have to utilize
the mismatched measurement outcomes in the channel estimation phase.
