Kastella, K., & Conti, R. S. (2011). Signal enhancement and background suppression using interference and entanglement. Physical Review A, 83(1), 014302.
We describe two-photon absorption processes excited by entangled pairs but not by nonentangled pairs of the same energy and polarization. Photon states are selected for destructive interference in the nonentangled process between different sequences of absorption via multiple intermediate states. A nonzero entangled absorption cross section is obtained by varying the entanglement time and pair delay parameters. Detailed energy and polarization requirements are derived for Rb 5S1/2−5D3/2 transitions. Applications to quantum steganography, quantum illumination, and quantum key distribution are discussed.