Theory of entangled two-photon emission/absorption [E2P- EA] between molecules

Entangled photons are of great interest as promising candidates for numerous applications in quantum information, quantum communication and quantum sensing. We present a comprehensive study of the theory of entangled two-photon emission/absorption (E2P-EA) between a many-level cascade donor and a many-level acceptor (which could be quantum dots or molecules) using second-order perturbation theory. We analyze how dipole orientation, radiative lifetime, energy detuning between intermediate states, separation distance, and entanglement time impact the E2P-EA rate. Our study demonstrates that there are quantum interference effects in the E2P-EA rate expression that lead to oscillations in the rate as a function of entanglement time. Moreover, we show that the E2P-EA rate can be comparable to one-photon emission/absorption (OP-EA) when donor and acceptor are within a few nm.