Limitation on the amount of accessible information in a quantum channel

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(rho) 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.