  The fascinating concept of coherent quantum absorber - which can absorb any
photon emitted by another system while maintaining entanglement with that
system - has found diverse implications in open quantum system theory and
quantum metrology. This work generalizes the concept by proposing the so-called
reversal conditions for the two systems, in which a "reverser" coherently
reverses any effect of the other system on a field. The reversal conditions are
rigorously boiled down to concise formulas involving the Petz recovery map and
Kraus operators, thereby generalizing as well as streamlining the existing
treatments of coherent absorbers.
