Strong subadditivity of quantum entropy
= Strong subadditivity of quantum entropy
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= Strong subadditivity of Von Neumann entropy
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For a tripartite <density operator> $\rho_{ABC}$,
$$
S(\rho_{ABC})+S(\rho_B)\leq S(\rho_{AB})+S(\rho_{BC}).
$$
Equivalently, conditioning on an additional quantum system cannot increase <quantum conditional entropy>.