Hot-Band Absorption Can Mimic Entangled Two-Photon Absorption

TL;DR

This study demonstrates that hot-band absorption can produce signals similar to entangled two-photon absorption, potentially leading to overestimates of quantum efficiency enhancements in experiments.

Contribution

It reveals that hot-band absorption can mimic entangled two-photon absorption signals, challenging previous interpretations of quantum enhancement measurements.

Findings

HBA produces signals with linear power dependence.

HBA can cause overestimation of quantum advantage in E2PEF.

HBA is significant under typical E2PEF experimental conditions.

Abstract

It has been proposed that entangled two-photon absorption (E2PA) can be observed with up to 10 orders of magnitude lower photon flux than its classical counterpart. However, there is a significant controversy regarding the magnitude of this quantum enhancement in excitation efficiency. We investigated the fluorescence signals from Rhodamine 6G and LDS798 excited with a CW laser or an entangled photon pair source at 1060 nm. We observed a signal that originates from hot-band absorption (HBA), which is one-photon absorption from thermally-populated vibrational levels of the ground electronic state. This mechanism, which has not been previously discussed in the context of E2PA, produces a signal with a linear power dependence, as would be expected for entangled two-photon excited fluorescence (E2PEF). For the typical conditions under which E2PEF measurements are performed, contributions from the HBA process could lead to a several orders-of-magnitude overestimate of the quantum advantage for excitation efficiency.