The [[conditional quantum entropy]] is an [[entropy measure]] used in [[quantum information theory]]. It is a generalization of the [[conditional entropy]] of [[classical information theory]]. The conditional quantum entropy measures the [[Quantum_statistical_mechanics#Von Neumann entropy|von Neumann entropy]] of a [[quantum state]] <math>\rho</math>, if we have already [[measurement in quantum computation|measured]] the value of a second quantum state <math>\sigma</math>. This conditional entropy is written <math>S(\rho|\sigma)</math>, or <math>H(\rho|\sigma)</math>, depending on the notation being used for the von Neumann entropy.
For the remainder of the article, we use the notation <math>S(\rho)</math> for the von Neumann entropy.
