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arXiv:2012.06736 · Physical Review Research

Experimental feasibility of molecular two-photon absorption with isolated time-frequency-entangled photon pairs

Tiemo S. Landes, Markus Allgaier, Sofiane Merkouche, Brian J. Smith, Andrew H. Marcus, Michael G. Raymer

Published Aug 13, 2021• 73 citations
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Abstract

Entangled photon pairs have promised to deliver a substantial quantum advantage for two-photon absorption spectroscopy. However, recent work has challenged the previously reported magnitude of quantum enhancement in two-photon absorption. We present a measurement of molecular absorption driven by isolated photon pairs using sum-frequency generation in a nonlinear optical crystal as a calibration device. We thereby establish an upper bound on the enhancement for entangled two-photon absorption in Rhodamine 6G, which lies well below previously reported values.

DOI
10.1103/physrevresearch.3.033154

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Entangled photon pairs have promised to deliver a substantial quantum advantage for two-photon absorption spectroscopy. However, recent work has challenged the previously reported magnitude of quantum enhancement in two-photon absorption. We present a measurement of molecular absorption driven by isolated photon pairs using sum-frequency generation in a nonlinear optical crystal as a calibration device. We thereby establish an upper bound on the enhancement for entangled two-photon absorption in Rhodamine 6G, which lies well below previously reported values.
The authors present an experimental study of entangled two-photon absorption in solvated rhodamine 6G, providing strong evidence that the orders-of-magnitude increases in two-photon-absorption efficiency by using entangled light reported in previous studies cannot be explained by the community's current understanding of the process. Thus, the sought-after advantages of using two-photon absorption of time-frequency-entangled photon pairs as a practical tool for enhancing molecular spectroscopy and biological imaging remains elusive.
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.
We present an ongoing experiment to investigate the potential enhancement of the two-photon absorption (TPA) rate of an atom using time–frequency-entangled photon pairs, a process known as entangled two-photon absorption (ETPA). There has been significant debate in the literature regarding the existence and magnitude of such enhancement. Previous studies have primarily focused on ETPA in molecular fluorophores, which are relevant for biological imaging applications but typically exhibit relatively low two-photon cross sections, often not exceeding 10⁻⁵⁰ cm⁴s/photon. In this work, we sacrifice broad applicability for experimental practicality, and instead probe ETPA in rubidium vapor, which has a comparatively large two-photon cross section (∼10⁻¹⁸ cm⁴s/photon) relative to most molecular fluorophores used in prior studies. We have developed and characterized an entangled photon-pair source, and have observed two-photon absorption in rubidium vapor. We have also reproduced previous theoretical models of ETPA. Preliminary results are presented, along with a discussion of the future direction of the experiment.
While two-photon absorption (TPA) and other forms of nonlinear interactions of molecules with isolated time-frequency-entangled photon pairs (EPP) have been predicted to display a variety of fascinating effects, their potential use in practical quantum-enhanced molecular spectroscopy requires close examination. This paper presents a detailed theoretical study of quantum-enhanced TPA by both photon-number correlations and spectral correlations, including an account of the deleterious effects of dispersion. While such correlations in EPP created by spontaneous parametric down conversion can increase the TPA rate significantly in the regime of extremely low optical flux, we find that for typical molecules in solution this regime corresponds to such low TPA event rates as to be unobservable in practice. Our results support the usefulness of EPP spectroscopy in atomic or other narrow-linewidth systems, while questioning the efficacy of such approaches for broadband systems including molecules in solution.