Experimental study on the effects of photon-pair temporal correlations\n in entangled two-photon absorption
Samuel Corona-Aquino, Omar Calderón-Losada, Mayte Y. Li-Gómez, Héctor Cruz-Ramírez, Violeta Álvarez-Venicio, María del Pilar Carreón, Roberto de J. León‐Montiel, Alfred B. U’Ren
Entangled two-photon absorption (ETPA) has recently become a topic of lively debate, mainly due to the apparent inconsistencies in the experimentally-reported ETPA cross sections of organic molecules. In this work, we provide a thorough experimental study of ETPA in the organic molecules Rhodamine B (RhB) and Zinc Tetraphenylporphirin (ZnTPP). The goal of this contribution is twofold: on one hand, it seeks to reproduce the results of previous experimental reports and, on the other, it aims to determine the effects of different temporal correlations -- introduced as a controllable time-delay between the photons to be absorbed -- on the strength of the ETPA signal. In our experiment, the samples are excited by entangled pairs produced by type-I SPDC, with a spectral distribution centered at 810 nm. Surprisingly, and contrary to what was expected, the time delay did not produce in our experiments any systematic change in the cross-sections when monitoring the ETPA signal using a transmission measurement scheme. As a plausible cause of this unexpected result, we argue that the photon-pair flux, typically-used in these experiments, is not sufficient to promote the two-photon absorption process in these molecules. This suggests that the actual absorption cross-section values are lower than those previously reported, and therefore do not lead to a measurable ETPA effect for the transmission method.
Entangled two-photon absorption (ETPA) has recently become a topic of lively debate, mainly due to the apparent inconsistencies in the experimentally-reported ETPA cross sections of organic molecules. In this work, we provide a thorough experimental study of ETPA in the organic molecules Rhodamine B (RhB) and Zinc Tetraphenylporphirin (ZnTPP). The goal of this contribution is twofold: on one hand, it seeks to reproduce the results of previous experimental reports and, on the other, it aims to determine the effects of different temporal correlations -- introduced as a controllable time-delay between the photons to be absorbed -- on the strength of the ETPA signal. In our experiment, the samples are excited by entangled pairs produced by type-I SPDC, with a spectral distribution centered at 810 nm. Surprisingly, and contrary to what was expected, the time delay did not produce in our experiments any systematic change in the cross-sections when monitoring the ETPA signal using a transmission measurement scheme. As a plausible cause of this unexpected result, we argue that the photon-pair flux, typically-used in these experiments, is not sufficient to promote the two-photon absorption process in these molecules. This suggests that the actual absorption cross-section values are lower than those previously reported, and therefore do not lead to a measurable ETPA effect for the transmission method.
Entangled two-photon absorption (ETPA) has recently become a topic of lively debate, mainly due to the apparent inconsistencies in the experimentally reported ETPA cross sections of organic molecules obtained by a number of groups. In this work, we provide a thorough experimental study of ETPA in the organic molecules Rhodamine B (RhB) and zinc tetraphenylporphirin (ZnTPP). Our contribution is 3-fold: first, we reproduce previous results from other groups; second, we on the one hand determine the effects of different temporal correlations─introduced as a controllable temporal delay between the signal and idler photons to be absorbed─on the strength of the ETPA signal, and on the other hand, we introduce two concurrent and equivalent detection systems with and without the sample in place as a useful experimental check; third, we introduce, and apply to our data, a novel method to quantify the ETPA rate based on taking into account the full photon-pair behavior rather than focusing on singles or coincidence counts independently. Through this experimental setup we find that, surprisingly, the purported ETPA signal is not suppressed for a temporal delay much greater than the characteristic photon-pair temporal correlation time. While our results reproduce the previous findings from other authors, our full analysis indicates that the signal observed is not actually due to ETPA but simply to linear losses. Interestingly, for higher RhB concentrations, we find a two-photon signal that, contrary to expectations, likewise does not correspond to ETPA.
By controlling quantum interactions of molecules and light, new technology in quantum information science promises to outperform classical-based technology in both function and efficiency. In particular, chemists stand to gain much advantage from quantum sensing techniques, allowing them to measure new spectroscopic signals that classical techniques cannot do, control photochemical reactions in new ways, and even measure nonlinear interactions with a simple push of a button. Much of the current work in using entangled photons for spectroscopy has focused on the unique physics of the photons. There is still much to be learned about how the molecule’s structure influences quantum spectroscopic signals and leaves its signature. In this dissertation, the unique ways that organic chromophores interact with entangled photons from spontaneous parametric down-conversion (SPDC) has been analyzed in detail. In Chapter 3, I calculate entangled two-photon absorption (ETPA) cross-sections for a few diatomic molecules to show how the varying dipole properties of the molecules affect the ETPA cross-section oscillation vs the entanglement time, Te. In Chapter 4, new theory shows that the excited state linewidth from ETPA excitation is extremely narrow compared to classical TPA excitation. I show that the reason for the ETPA linewidth being so narrow is due to the large arrival time uncertainty of the entangled photon pair, which makes the first photons’ absorption time in the molecule largely uncertain and effectively narrows the excited state linewidth. In Chapter 5, I used Type-II SPDC to prove, for the first time, that two halogenated anesthetic ethers, sevoflurane and isoflurane, have the ability to target and interact with quantum particles by showing that they interact with 800 nm entangled photons but not 800 nm classical photons. In Chapter 6, I build an ETPA experimental setup that uses CW laser pumping of Type-I SPDC, which is shown to significantly improve the accuracy, signal-to-noise ratio (SNR), and limit of detection of ETPA while also making the experiment more compact and cost-effective. I also show how to quantify the degree of frequency entanglement using the Schmidt decomposition of the joint frequency spectrum and propose how the Schmidt modes can be tuned to control photochemical reactions. Contrary to prior belief in the literature, I show experimentally in Chapter 7 that ETPA with Type-I SPDC increases linearly with increasing Te, allowing one to increase the ETPA cross-section by an order of magnitude by increasing Te from a few fs to ~10 ps. This enhancement is due to the entanglement area, Ae, decreasing as the frequency bandwidth decreases, which is how Te is increased. This spatial-spectral coupling has previously been overlooked in the context of ETPA, causing previous theoretical models to underestimate the ETPA cross-section at ps Te by 3-4 orders of magnitude. Using ps Te Type-I SPDC, ps-scale excited state dynamics can be studied using ETPA, and photochemical reactions with intermediate steps, such as isomerizations or solvent reorganization, can be controlled. The experimental techniques and theoretical work here has provided a much needed improvement to make ETPA a more quantitatively robust analytical technique that is useful and accessible to the researchers who can benefit most from its applications: chemists, biologists, and anyone who studies the quantum principles underlying all aspects of chemistry.
Entangled photons offer quantum correlations with no classical analogue. In entangled pair two-photon absorption (ETPA), absorption rate is predicted to scale linearly rather than quadratically with photon flux, promising molecular excitation at fluxes far below the classical threshold. Reported ETPA cross sections nevertheless vary widely across experiments, largely since the observable, the differential of the transmitted flux or weak fluorescence, is difficult to separate from scattering and linear losses. We present a method which uses Franson interference to study entangled two-photon absorption through delay-dependent coincidence measurements on dye molecules. Applying the method to Rhodamine 6G, we observe a small asymmetry in the Franson interference envelope and obtain a model-derived effective cross section of approximately 2.1 E(-21) centimeter squared for Rhodamine 6G. However, the estimated systematic uncertainty does not allow a definitive ETPA assignment. Instead, the experiment establishes a quantitative bound on the ETPA response and provides a background-free benchmark for future measurements.
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.
Over the last 50 years entangled photon pairs have received attention for use in lowering the flux in two-photon absorption imaging and spectroscopy. Despite this, evidence for entangled two-photon absorption (ETPA) effects remain highly debated, especially at low-fluxes. Here, we structure the transverse spatial correlations of entangled photon pairs to evidence signs of ETPA at room-temperature in organic and inorganic chromophores, in the low-flux regime. We demonstrate our scheme to be robust to common artifacts that have previously hampered detection of ETPA such as linear absorption and background fluorescence, and show that ETPA scales with transverse correlation area and chromophore two-photon cross-sections. Our results present a step towards verifying ETPA and experimentally exploring entangled light-matter interactions.
Nonlinear spectroscopy and microscopy techniques are ubiquitous in a wide range of applications across physics and biology. However, these usually rely on high-powered pulsed laser systems. A promising alternative is to exploit entangled two-photon absorption (ETPA), which can lead to tens of orders of magnitude lower incident fluxes than in conventional two-photon absorption schemes. However, the role of different entangled degrees of freedom in ETPA was unclear following recent experimental studies, when compared to earlier theoretical works. Here, we first demonstrate a linear dependence of the ETPA rate with the photon-pair rate, a clear signature of ETPA, and estimate the values for the concentration-dependent ETPA cross section for Rhodamine 6G. We then investigate the signature of energy-time entanglement and polarization dependence in the ETPA fluorescence rate and demonstrate a strong dependence of the signal on the interphoton delay that reflects the coherence time of the entangled two-photon wave packet.