Quantum reversal: a general theory of coherent quantum absorbers

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.