Information encoded in non-orthogonal quantum states cannot be duplicated, nor amplified, and in general at is only partly recoverable. The most efficient way of retrieving it is not a direct uquantum measurement” (as defined by von Neumann), but an indirect method similar to heterodyne detection in communications engineering. The mathematical representation of this process requires the introduction of a positive operator valued measure. The optimization of these measures is not yet fully undersiood. An interesting and potentially important application of quantum information is its use an cryptography.