Challenges in fluorescence-detected entangled two-photon-absorption experiments: exploring the low-to-high-gain squeezing regimes

The rate of two-photon absorption of time-frequency-entangled photon pairs has been the subject of much study for its potential to enable quantum-enhanced molecular spectroscopy and imaging. We closely replicated recent experiments that reportedly observed such enhancement and have found that in the low-photon-flux regime the signal is below detection threshold. Using an optical parametric down-conversion photon-pair source that can be varied from the low-gain spontaneous regime to the high-gain squeezing regime, we observe two-photon absorption with a molecular sample in solution for the high-gain regime but not for the low-gain regime. The observed rates are consistent with theoretical predictions and indicate that time-frequency photon entanglement does not yet provide a practical means to enhance spectroscopy or imaging with current techniques.