Limitations of Entangled Two-Photon Absorption detection

TL;DR

This work provides a rate-based, black-box framework to quantify the sensitivity of entangled two-photon absorption measurements by modeling signal and noise components and expressing detectability in Göppert-Mayer units. It compares two measurement schemes—separation and attenuation—deriving explicit lower bounds on the TPA cross-section and identifying optimal strategies to enhance detection. The authors show separation-based approaches typically outperform attenuation, and demonstrate that practical improvements such as time gating, dispersion management, and dark-count suppression can enable detections in several existing setups. The framework offers a direct, quantitative path to optimize ETPA experiments and compare results across diverse experimental configurations.

Abstract

We introduce a method for determining the sensitivity of any given Entangled Two-Photon Absorption (ETPA) measurement. By modeling all signal and noise contributions to the measurement, we derive a single numerical value that describes the sensitivity of the ETPA measurement in Göppert-Mayer units. This allows us to directly compare vastly different experimental approaches and, determine whether ETPA will be detectable under the given conditions. Therefore, we can quantify the effect of any change to a given experimental apparatus and identify the ideal optimization pathway.

Figures (3)

• Figure 1: Experimental configuration for Signal and Background measurements in both schemes discussed here. Entangled photon pairs are created by pumping a PDC crystal and are sent to a two-photon resonant absorber (ABS). The resulting fluorescence is then detected by a single-photon-sensitive detector. A setup for measuring the signal $S_{sep}$ in the separation scheme is shown in (a1). The background measurement $B_{sep}$ is obtained by delaying the idler beam by $\Delta \tau$ (a2). In the attenuation scheme, an attenuator $\eta$ is placed either in the pump beam to obtain the signal measurement $S_{att}$ (b1) or in the signal and idler beams to measure the background $B_{att}$ (b2).
• Figure 2: Comparing the attenuation method \ref{['eq:sig_att']} (blue) with different attenuation values $\eta$ to the separation method \ref{['eq:sig_sep_det']}. The shaded area indicates possible ETPA detection using Rhodamine 6G at 1064 nm as the target. The separation method outperforms the attenuation method up to the case of very high flux, where the two methods converge if $\eta$ is chosen optimally. The markers represent the parameter sets used in the attenuation method measurements published in Landes20212Landes2024Tabakaev2022. The full set of parameters can be found in table \ref{['tab:other_people']}.
• Figure 3: Our model applied to published experimental data (black). We optimized the sensitivity by selecting the optimal method (blue) and, in the cases of pulsed excitation, introduced time-gated detection (yellow). Additionally, we assume Fourier-limited Gaussian photon pairs (green), reaching the sensitivity goal in cases e and f. Finally, we eliminate detector dark counts (red) and also reach the goal in cases a and c.