In quantum mechanics, and especially quantum information processing, the entropy exchange of a quantum operation acting on the density matrix of a system is defined as

where is the von Neumann entropy of the system and a fictitious purifying auxiliary system after they are operated on by . Here,

and

where in the above equation acts on leaving unchanged.

References

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  • Nielsen, Michael A.; Chuang, Isaac L. (2010). Quantum Computation and Quantum Information (2nd ed.). Cambridge: Cambridge University Press. ISBN 978-1-107-00217-3. OCLC 844974180.