  Based on the concept of many-letter theory, an observable is defined
measuring the raw quantum information content of single messages. A general
characterization of quantum codes using the Kraus representation is given.
Compression codes are defined by their property of decreasing the expected raw
information content of a given message ensemble. Lossless quantum codes, in
contrast to lossy codes, provide the retrieval of the original data with
perfect fidelity. A general lossless coding scheme is given that translates
between two quantum alphabets. It is shown that this scheme is never
compressive. Furthermore, a lossless quantum coding scheme, analog to the
classical Huffman scheme but different from the Braunstein scheme, is
implemented, which provides optimal compression. Motivated by the concept of
lossless quantum compression, an observable is defined that measures the core
quantum information content of a particular message with respect to a given a
priori message ensemble. The average of this observable yields the von Neumann
entropy.
