State-merging

In quantum information theory, quantum state merging is the transfer of a quantum state when the receiver already has part of the state. The process optimally transfers partial information using entanglement and classical communication. It allows for sending information using an amount of entanglement given by the conditional quantum entropy, ${\displaystyle H(A|B)\,=\,H(AB)-H(B)\,.}$ with ${\displaystyle H(A)}$ the Von Neumann entropy, ${\displaystyle H(A):=-Tr\rho _{A}\log \rho _{A}}$. It thus provides an operational meaning to this quantity.

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