channel Ψ is that is, it is the capacity of Ψ with respect to the ideal channel on the classical one-bit system . Similarly the quantum capacity of Ψ is where the reference system is now the one qubit system . Channel fidelity Another measure of how well a quantum channel preserves information is called channel fidelity, and it arises from fidelity of quantum states. Bistochastic quantum channel A bistochastic quantum channel is a quantum channel which is unital, i.e. . See also No-communication theorem Amplitude damping channel References M. Keyl and R. F. Werner, How to Correct Small Quantum Errors, Lecture Notes in Physics Volume 611, Springer, 2002. . Quantum information theory