Squashed entanglement, also called CMI entanglement (CMI can be pronounced "see me"), is an information theoretic measure of quantum entanglement for a bipartite quantum system. If is the density matrix of a system composed of two subsystems and, then the CMI entanglement of system is defined by
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,Eq.(1)
where is the set of all density matrices for a tripartite system such that . Thus, CMI entanglement is defined as an extremum of a functional of . We define, the quantum Conditional Mutual Information (CMI), below. A more general version of Eq.(1) replaces the ``min" (minimum) in Eq.(1) by an ``inf" (infimum). When is a pure state, in agreement with the definition of entanglement of formation for pure states. Here is the Von Neumann entropy of density matrix .
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