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Equation 13 · Part 5 · How Fast Can a Horizon Learn Which Path You Took?

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χH(T)=h2 ⁣(1+V(T)2).\chi_H(T)= h_2\!\left(\frac{1+V(T)}{2}\right).
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What this part means

The expressions on both sides represent the same quantity under the stated assumptions.

Its job in the formula

The equals sign connects the complete expression on the left with the complete expression on the right. Both sides must have compatible units.

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 χH(T)=h2 ⁣(1+V(T)2)\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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Learn the underlying idea

An equals sign says that the expression on its left and the expression on its right have the same value under the stated definitions and assumptions.

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Sources cited in the surrounding passage

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