Symbol h_2

The passage around this formula

There is another number in the same pair of states. The Holevo quantity of the equal-prior pure ensemble is $\chi_H(T)= h_2\!\left(\frac{1+V(T)}{2}\right)$. It is the von Neumann entropy of the average conditional state because each conditional state is pure. Holevo’s theorem makes this quantity an upper bound on classical information accessible through a quantum measurement, not a guarantee that an arbitrary one-copy receiver attains it [ 13 ] . Fuchs and Caves developed ensemble-dependent bounds that make the gap between a state’s information content and a receiver’s accessible information explicit [ 15 ] .

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