  We present a quantum information theory that allows for a consistent
description of entanglement. It parallels classical (Shannon) information
theory but is based entirely on density matrices (rather than probability
distributions) for the description of quantum ensembles. We find that quantum
conditional entropies can be negative for entangled systems, which leads to a
violation of well-known bounds in Shannon information theory. Such a unified
information-theoretic description of classical correlation and quantum
entanglement clarifies the link between them: the latter can be viewed as
``super-correlation'' which can induce classical correlation when considering a
tripartite or larger system. Furthermore, negative entropy and the associated
clarification of entanglement paves the way to a natural information-theoretic
description of the measurement process. This model, while unitary and causal,
implies the well-known probabilistic results of conventional quantum mechanics.
It also results in a simple interpretation of the Kholevo theorem limiting the
accessible information in a quantum measurement.
