The uncertainty of quantum channels in terms of variance

By use of the generalized variance for any operator (not necessarily Hermitian), we introduce the uncertainty of quantum channels as the sum of the generalized variances for the Kraus operators of quantum channels and prove that it satisfies several desirable properties. Then, we establish two trade-off relations between the uncertainty of quantum channels and the entanglement fidelity which is introduced by Schumacher (Phys. Rev. A 54: 2614, 1996) and quantifies how well the channel preserves the entanglement between the input system and the auxiliary system for purification. Finally, we illustrate the uncertainty of quantum channels through some typical examples.