We prove a new result limiting the amount of accessible information in a quantum channel. This generalizes Kholevo's theorem and implies it as a simple corollary. Our proof uses the strong subadditivity of the von Neumann entropy functional $S\left(\rho \right)$ and a specific physical analysis of the measurement process. The result presented here has application in information obtained from “weak” measurements, such as those sometimes considered in quantum cryptography.

• Received 26 October 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.3452