Experimental test of the entanglement enhancement in two-photon fluorescence

Entangled photons with time-energy correlations are predicted to have improved sensitivity compared with classical light in two-photon fluorescence, which is promising for lowering the phototoxicity in biomedical imaging. While the mechanism has been demonstrated in atoms and crystals, the effectiveness in biological molecules has been under debate. Here we report the experimental results, including both positive and negative ones, in various samples. While entanglement induced enhancement in the sum frequency generation of a nonlinear crystal was confirmed, we observe no such enhancement in the two-photon nonlinear process of various molecules. However, we observed classical two-photon fluorescence excited by nW light in rare earth element doped core-shell structure nano-crystals, which can also be used in sensitive biomedical imaging. Our results provide guidance in characterizing the effect of entanglement in two-photon processes.