Multiphoton absorption of entangled photons offers ways for obtaining unique information about chemical and biological processes. Measurements with entangled photons may enable sensing biological signatures with high selectivity and at very low light levels to protect against photodamage. In this paper, we present a theoretical and experimental study of the excitation wavelength dependence of the entangled two-photon absorption (ETPA) process in a molecular system, which provides insights into how entanglement affects molecular spectra. We demonstrate that the ETPA excitation spectrum can be different from that of classical TPA as well as that for one-photon resonant absorption (OPA) with photons of doubled frequency. These results are modeled by assuming the ETPA cross-section is governed by a two-photon excited state radiative linewidth rather than by electron-phonon interactions, and this leads to excitation spectra that match the observed results. Further, we find that the two-photon-allowed states with highest TPA and ETPA intensities have high electronic entanglements, with ETPA especially favoring states with the longest radiative lifetimes. These results provide concepts for the development of quantum light-based spectroscopy and microscopy that will lead to much higher efficiency of ETPA sensors and low-intensity detection schemes.
Multiphoton absorption of entangled photons offers ways for obtaining unique information about chemical and biological processes. Measurements with entangled photons may enable sensing biological signatures with high selectivity and at very low light levels to protect against photodamage. In this paper, we present a theoretical and experimental study of the excitation wavelength dependence of the entangled two-photon absorption (ETPA) process in a molecular system, which provides insights into how entanglement affects molecular spectra. We demonstrate that the ETPA excitation spectrum can be different from that of classical TPA as well as that for one-photon resonant absorption (OPA) with photons of doubled frequency. These results are modeled by assuming the ETPA cross-section is governed by a two-photon excited state radiative linewidth rather than by electron-phonon interactions, and this leads to excitation spectra that match the observed results. Further, we find that the two-photon-allowed states with highest TPA and ETPA intensities have high electronic entanglements, with ETPA especially favoring states with the longest radiative lifetimes. These results provide concepts for the development of quantum light-based spectroscopy and microscopy that will lead to much higher efficiency of ETPA sensors and low-intensity detection schemes.
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.
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.
The Two-photon absorption processes (TPA) is of great interest due to its variety of technical applications. Two-photon transitions can be excited using light with different statistical properties, such as laser light (coherent light), pseudo-thermal light and entangled photons-pairs (ETPA). For a TPA process driven with entangled photons (ETPA), a linear dependence of the signal with the photon flux has been theoretically predicted and experimentally demonstrated by various groups. The main motivation for studying ETPA relies on the capability of inducing TPA with a low photon flux. This is advantageous to analyze, among other things, biological samples with a reduced probability of destroying them in the process. However, ETPA experiments developed to date raise many questions regarding the difficulty of isolating a signal in organic molecules. One of the main problems in these experiments is the difficulty to study independently all the systematic effects due to the complex structure of organic molecules. An alternative to clarify the origin of the signal is to study the ETPA process in atomic systems. In this case, the possibility to excite narrow band transitions with tunable single frequency lasers can provide a clear signature of the ETPA process. To achieve this objective, the first step consists of understanding TPA with coherent light, or classical TPA, and measuring the corresponding cross-section in atomic systems. Once the classical regime is characterized, the ETPA process can be studied considering decoherence effects and coherence between intermediate states. For this, the ETPA cross section must be calculated and measured. In this work we present a theoretical calculation of the ETPA transition probability and cross-section with the density operator formalism for the transition 62S1/2 →82S1/2. The theoretical results will be complemented with an experiment with atomic Cesium based in a coincidence detection scheme.
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.
Two-photon absorption (TPA) and other nonlinear interactions of molecules with time-frequency-entangled photon pairs have been predicted to display a variety of fascinating effects. Therefore, their potential use in practical quantum-enhanced molecular spectroscopy requires close examination. This Tutorial presents a detailed theoretical study of one- and two-photon absorption by molecules, focusing on how to treat the quantum nature of light. We review some basic quantum optics theory and then we review the density-matrix (Liouville) derivation of molecular optical response, emphasizing how to incorporate quantum states of light into the treatment. For illustration, we treat in detail the TPA of photon pairs created by spontaneous parametric down conversion, with an emphasis on how quantum light TPA differs from that with classical light. In particular, we treat the question of how much enhancement of the TPA rate can be achieved using entangled states. This Tutorial includes a review of known theoretical methods and results as well as some extensions, especially the comparison of TPA processes that occur via far-off-resonant intermediate states only and those that involve off-resonant intermediate states by virtue of dephasing processes. A brief discussion of the main challenges facing experimental studies of entangled two-photon absorption is also given.