Quantum Entanglement Molecular Absorption Spectrum Simulator (QE-MASS) is a computer program for simulating two photon molecular-absorption spectroscopy using quantum-entangled photons. More specifically, QE-MASS simulates the molecular absorption of two quantum-entangled photons generated by the spontaneous parametric down-conversion (SPDC) of a fixed-frequency photon from a laser. The two-photon absorption process is modeled via a combination of rovibrational and electronic single-photon transitions, using a wave-function formalism. A two-photon absorption cross section as a function of the entanglement delay time between the two photons is computed, then subjected to a fast Fourier transform to produce an energy spectrum. The program then detects peaks in the Fourier spectrum and displays the energy levels of very short-lived intermediate quantum states (or virtual states) of the molecule. Such virtual states were only previously accessible using ultra-fast (femtosecond) laser systems. However, with the use of a single-frequency continuous wave laser to produce SPDC photons, and QEMASS program, these short-lived molecular states can now be studied using much simpler laser systems. QE-MASS can also show the dependence of the Fourier spectrum on the tuning range of the entanglement time of any externally introduced optical-path delay time. QE-MASS can be extended to any molecule for which an appropriate spectroscopic database is available. It is a means of performing an a priori parametric analysis of entangled photon spectroscopy for development and implementation of emerging quantum-spectroscopic sensing techniques. QE-MASS is currently implemented using the Mathcad software package.
Quantum Entanglement Molecular Absorption Spectrum Simulator (QE-MASS) is a computer program for simulating two photon molecular-absorption spectroscopy using quantum-entangled photons. More specifically, QE-MASS simulates the molecular absorption of two quantum-entangled photons generated by the spontaneous parametric down-conversion (SPDC) of a fixed-frequency photon from a laser. The two-photon absorption process is modeled via a combination of rovibrational and electronic single-photon transitions, using a wave-function formalism. A two-photon absorption cross section as a function of the entanglement delay time between the two photons is computed, then subjected to a fast Fourier transform to produce an energy spectrum. The program then detects peaks in the Fourier spectrum and displays the energy levels of very short-lived intermediate quantum states (or virtual states) of the molecule. Such virtual states were only previously accessible using ultra-fast (femtosecond) laser systems. However, with the use of a single-frequency continuous wave laser to produce SPDC photons, and QEMASS program, these short-lived molecular states can now be studied using much simpler laser systems. QE-MASS can also show the dependence of the Fourier spectrum on the tuning range of the entanglement time of any externally introduced optical-path delay time. QE-MASS can be extended to any molecule for which an appropriate spectroscopic database is available. It is a means of performing an a priori parametric analysis of entangled photon spectroscopy for development and implementation of emerging quantum-spectroscopic sensing techniques. QE-MASS is currently implemented using the Mathcad software package.
A new form of absorption spectroscopy is introduced in which the energy levels and matrix elements of virtual-state transitions in a medium are determined by the use of continuous-wave degenerate entangled photons without varying their wavelengths. Extractable spectroscopic information is embedded in the entangled-photon absorption cross section when measured over a range of entanglement and interbeam-delay times. This technique exploits the fundamental quantum interference that arises during the interaction of an entangled quantum state with a quantum system.
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 two-photon absorption spectroscopy (TPA) has been widely recognized as a powerful tool for revealing relevant information about the structure of complex molecular systems. However, to date, the experimental implementation of this technique has remained elusive, mainly because of two major difficulties: first, the need to perform multiple experiments with two-photon states bearing different temporal correlations, which translates into the necessity to have at the experimenter's disposal tens, if not hundreds, of sources of entangled photons; second, the need to have a priori knowledge of the absorbing medium's lowest-lying intermediate energy level. In this work, we put forward a simple experimental scheme that successfully overcomes these two limitations. By making use of a temperature-controlled entangled-photon source, which allows the tuning of the central frequencies of the absorbed photons, we show that the TPA signal, measured as a function of the temperature of the nonlinear crystal that generates the paired photons, and a controllable delay between them, carries all information about the electronic level structure of the absorbing medium, which can be revealed by a simple Fourier transformation.
The application of quantum states of light such as entangled photons, for example, created by parametric down conversion, has experienced tremendous progress in the almost 40 years since their first experimental realization. Initially, they were employed in the investigation of the foundations of quantum physics, such as the violation of Bell's inequalities and studies of quantum entanglement. They later emerged as basic platforms for quantum communication protocols and, in the recent experiments on single-photon interactions, in photonic quantum computation. These applications aim at the controlled manipulation of the photonic degrees of freedom, and therefore rely on simple models of matter, where the analysis is simpler. Furthermore, quantum imaging with entangled light can achieve enhanced resolution, and quantum metrology can overcome the shot noise limit for classical light. This Account focuses on an entirely different emerging class of applications using quantum light as a powerful spectroscopic tool to reveal novel information about complex molecules. These applications utilize two appealing properties of quantum light: its distinct intensity fluctuations and its nonclassical bandwidth properties. These give rise to new and surprising behavior of nonlinear optical signals. Nonclassical intensity fluctuations can enhance nonlinear optical signals relative to linear absorption. For instance, the two-photon absorption of entangled photon pairs scales linearly (rather than quadratically) in the photon flux, just like a single photon absorption. This enables nonlinear quantum spectroscopy of photosensitive, for example, biological, samples at low light intensities. We will discuss how the two-photon absorption cross section becomes a function of the photonic quantum state, which can be manipulated by properties of the entangled photon pairs. In addition, the quantum correlations in entangled photon states further influence the nonlinear signals in a variety of ways. Apart from affecting the signal's scaling with intensity, they also constitute an entirely new approach to shaping and controlling excitation pathways in molecular aggregates in a way that cannot be achieved with shaped classical pulses. This is because between the two absorption events in entangled two-photon absorption, the light and material system are entangled. Classical constraints for the simultaneous time and frequency resolution can thus be circumvented, since the two are not Fourier conjugates. Here we review the simplest manifestation of quantum light spectroscopy, two-photon absorption spectroscopy with entangled photons. This will allow us to discuss exemplarily the impact of quantum properties of light on a nonlinear optical signal and explore the opportunities for future applications.
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.
In recent years, the process of absorbing two photons using a non-classical light sources has been studied thoroughly. In particular, it has been experimentally demonstrated that the absorption cross section of two entangled photons is much greater than the absorption cross section of two uncorrelated photons. In this dissertation a source of entangled photons produced through spontaneous parametric down conversion or SPDC was constructed and characterized. It was found that for a pump laser of 411.32 nm and a BBO crystal of 1 mm the produced photons followed the same optical path as the laser, it was also found that the joint spectrum is symmetric at the degenerate wavelength of 822 ± 0.5 nm. This work conclude by showing a first approximation to execute a measurement of the Hong-Ou-Mandel effect. The objective of the measurements of joint spectrum and the Hong-Ou-Mandel effect are to understand the correlation properties of the photon source, with this information a virtual state spectroscopy technique can be implemented on the experiment constructed