The coherent information is a quantity used in quantum communication theory. It is a property of a quantum state and a quantum channel Λ; intuitively, it attempts to describe how much of the quantum information in the state will remain after the state goes through the channel. In this sense, it is analogous to the classical mutual information of classical information theory. As the classical mutual information gives the capacity of a classical channel, the coherent information gives the capacity of a quantum channel. The coherent information is written I(A⟩B).
The coherent information is defined as I(A⟩B) ≡ S(B) − S(AB) where S(B) is the Von Neumann entropy of the output state ρB and S(AB) is the entropy of the channel environment (equivalently, the joint entropy of the output state, and the purification ρA) of the input state. One may think of inputting part of a pure state through the channel. The part which remains at the input is ρA, and the channel produces ρB at the output.
The coherent information, when maximised over input states ρA, gives the quantum capacity of the quantum channel. This is known as the LSD Theorem after Lloyd, Shor and Devetak. The quantity is not additive, and one must go to large block size (i.e. many quantum uses of the channel).
The coherent information, gives also the one way distillable entanglement when maximised over local operations performed by Alice, and if maximised over LOCC operations, it gives the two-way distillable entanglement.
- Nielsen, Michael A. and Isaac L. Chuang (2000). Quantum Computation and Quantum Information, Cambridge University Press, ISBN 0-521-63503-9
- Nielsen, Michael A. and Benjamin Schumacher (1996). Quantum data processing and error correction. Physical Review A., 54 (4), 2629-2635.