 Coherent information

For a channel output $Q$ and complementary environment output $E$, coherent information is $I_c(\Lambda ,\rho )=S(Q)-S(E)=-H(R|Q)$ for a purification reference $R$.

Coherent information is a concept derived from quantum information theory, particularly in the context of quantum communication and quantum error correction. It describes a specific type of information that can be transmitted or processed coherently through a quantum channel, taking advantage of the unique properties of quantum mechanics, such as superposition and entanglement. In classical information theory, information is typically concerned with bits—units that can exist in one of two states (0 or 1).