Hot-Band Absorption Can Mimic Entangled Two-Photon Absorption

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.

Spatial properties of entangled two-photon absorption

“…The fitting was performed by a least-squares method, resulting in the coefficient of determination R 2 = 0.997 for the linear fit and root mean squared error (RMSE) of 0.12 for the quadratic one. A linear dependence in the case of laser beam attenuation, and quadratic in the case of SPDC beam attenuation, confirmed that the measured signal was due to ETPA and not caused by direct detection of scattered pump, down-converted photons or single-photon absorption of any type [21]. It also constitutes one of the most robust demonstrations of genuine ETPA [23][24][25].…”

“…ETPA signature -While ETPA is more efficient with respect to photon flux than TPA, the signal-to-noise ratio (SNR) of experiments relying on entangled photon sources is typically low. It is therefore important to ensure that the detected signals are indeed produced by the ETPA process and not arising from, pump leakage, hot-band single-photon absorption [21] or any other single-photon process. The risk of this misattribution comes from the indistinguishability of fluorescence signals, produced by these events that would have the same type of (linear) dependence on the input flux.…”

Single-Photon Scattering Can Account for the Discrepancies among Entangled Two-Photon Measurement Techniques

“…The measurements in this paper are performed on a virtual state-mediated two-photon excitable molecule. Recently, theoretical and experimental measurements have emphasized the importance of resonant or near-resonant real states to achieve large two-photon-entangled interactions. ,, The results of this paper indicate that caution must be taken when measuring entangled two-photon events because single-photon scattering and other reported single-photon processes such as hot band absorption will mimic linear absorption at low powers. Only by characterizing the entangled state, instead of relying solely on intensity measurements, can a definitive conclusion be reached.…”

“…band absorption 52 will mimic linear absorption at low power. Only by characterizing the entangled state, instead of relying solely on intensity measurements, can a definitive conclusion be reached.…”

Spatial Properties of Entangled Two-Photon Absorption

“…ETPA signature.-While ETPA is more efficient with respect to photon flux than TPA, the signal-to-noise ratio (SNR) of experiments relying on entangled photon sources is typically low. It is therefore important that the detected signals are indeed produced by the ETPA process and not arising from pump leakage, hot-band single-photon absorption [23], or any other single-photon process. The risk of this misattribution comes from the indistinguishability of fluorescence signals, produced by these events that would have the same type of (linear) dependence on the input flux.…”