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One has to trace out the subsystem A not B Tag: Reverted |
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== Definition ==
Given a bipartite quantum state <math>\rho^{AB}</math>, the entropy of the joint system AB is <math>S(AB)_\rho \ \stackrel{\mathrm{def}}{=}\ S(\rho^{AB})</math>, and the entropies of the subsystems are <math>S(A)_\rho \ \stackrel{\mathrm{def}}{=}\ S(\rho^A) = S(\mathrm{tr}
By analogy with the classical conditional entropy, one defines the conditional quantum entropy as <math>S(A|B)_\rho \ \stackrel{\mathrm{def}}{=}\ S(AB)_\rho - S(B)_\rho</math>.
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