QUANTUM CODING THEOREM FOR MIXED STATES

We prove a theorem for coding mixed-state quantum signals. For a class of coding schemes, the von Neumann entropy S of the density operator describing an ensemble of mixed quantum signal states is shown to be e qual to the number of spin-1/2 systems necessary to represent the sign al faithfully. This generalizes previous works on coding pure quantum signal states and is analogous to the Shannon's noiseless coding theor em of classical information theory. We also discuss an example of a mo re general class of coding schemes which beat the limit set by our the orem.