A signal enhancement of an order of magnitude was observed at low light levels using biphotons for sum-frequency generation compared to coherent light. We will discuss issues of applying biphotons to two-photon excited fluorescence.
We closely replicated and extended a recent experiment ("Spatial properties of entangled two-photon absorption," Phys. Rev. Lett. 129, 183601, 2022) that reportedly observed enhancement of two-photon absorption rates in molecular samples by using time-frequency-entangled photon pairs, and we found that in the low-flux regime, where such enhancement is theoretically predicted in-principle, the two-photon fluorescence signal is below detection threshold using current state-of-the-art methods. The results are important in the context of efforts to enable quantum-enhanced molecular spectroscopy and imaging at ultra-low optical flux. 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 observed two-photon-induced fluorescence in the high-gain regime but in the low-gain regime any fluorescence was below detection threshold. We supplemented the molecular fluorescence experiments with a study of nonlinear-optical sum-frequency generation, for which we are able to observe the low-to-high-gain crossover, thereby verifying our theoretical models and experimental techniques. The observed rates (or lack thereof) in both experiments are consistent with theoretical predictions and with our previous experiments, and indicate that time-frequency photon entanglement does not provide a practical means to enhance in-solution molecular two-photon fluorescence spectroscopy or imaging with current techniques.
Abstract We propose a quantum-enhanced plasma diagnostic based on squeezed states of light. Squeezed light can exhibit super-Poissonian photon statistics, leading to enhanced multiphoton absorption cross-sections compared to classical light. This effect enables improved sensitivity for two-photon excitation of high-energy atomic transitions such as ground-state excitation, and results in increased absorbed power and fluorescence in a plasma. We consider two methods of generating squeezed states and evaluate their advantages and limitations in the context of plasma absorption diagnostics. By comparing with and extending previous theoretical work, we predict an enhancement of the absorption signal by up to seven orders of magnitude at low intensities (10 1 W/m 2 ), with diminishing enhancement persisting up to high intensities (10 10 W/m 2 ). These results suggest that squeezed-light sources offer a viable pathway toward quantum-enhanced plasma diagnostics.
Single-photon detection (SPD) technologies have been applied to underwater optical imaging to overcome the strong attenuation of seawater.However, external photon noise, resulting from the natural light, hinders their further applications due to the extreme sensitivity of SPD and a weakly received optical signal.In this work, we performed noise-resistant underwater correlated biphoton imaging (CPI) to partly solve the influence of the external noise, through a home-built super-bunching laser generated by the stochastic nonlinear interaction between a picosecond laser and a photonic crystal fiber.Compared with a coherent laser, the probabilities of generated bundle N-photons (N ≥ 2) of the super-bunching laser have been enhanced by at least one order of magnitude, enabling CPI under weak light intensity.We experimentally demonstrated CPI with reasonable imaging contrast under the noise-to-signal ratio (NSR) up to 10 3 , and the noise-resistant performance has been improved by at least two orders of magnitude compared to that of the single-photon imaging technology.We further achieved underwater CPI with good imaging contrast under NSR of 150, in a glass tank with a length of 10 m with Jerlov type III water (an attenuation coefficient of 0.176 m -1 ).These results break the limits of underwater imaging through classical coherent lasers and may offer many enhanced imaging applications through our super-bunching laser, such as long-range target tracking and deep-sea optical exploration under noisy environments.