  When quantum states are used to send classical information, the receiver
performs a measurement on the signal states. The amount of information
extracted is often not optimal due to the receiver's measurement scheme and
experimental apparatus. For quantum non-demolition measurements, there is
potentially some residual information in the post-measurement state, while part
of the information has been extracted and the rest is destroyed. Here, we
propose a framework to characterize a quantum measurement by how much
information it extracts and destroys, and how much information it leaves in the
residual post-measurement state. The concept is illustrated for several
receivers discriminating coherent states.
