Abstract Tracking the progression of the bone resorption process is important for clinical research and development of pharmaceuticals for bone-related diseases. Although fluorescence imaging is found to be one of the powerful tools for studying bone, conventional fluorescence microscopy relies on extremely high excitation powers that induce photobleaching and phototoxic effects on biological samples, preventing long-term observations. To overcome these limitations, we propose a quantum light-based approach using entangled two-photon fluorescence microscopy (ETPFM). We demonstrate ETPFM for imaging fixed demineralized unstained and Hoechst- and DAPI-stained bone sections at photon fluxes nearly 6 orders of magnitude lower than classical two-photon excitation. This quantum-correlated light preserves tissue integrity, eliminates photobleaching, and reveals microstructures with high contrast. Endogenous fluorescence is detected without exogenous dyes, and osteoclast cells are clearly resolved in the stained bone sections. ETPFM thus provides a minimally invasive, quantum-enhanced approach for long-term imaging of bone microenvironments.

Photochemistry is a powerful tool for synthesizing important molecules that are challenging to create without light. We report compelling results that indicate that photochemical reaction rate (oxygenation) can be notably enhanced by utilizing a very small number of entangled photons. Measurements with the same small number of classical photons show that the rate of product formation is considerably lower. This suggests that the reaction rate with entangled photons is enhanced by many orders of magnitude. Theoretical calculations show that classical and entangled photons excite the photocatalyst to different final excited states. This chemical synthesis approach with entangled photons could have a large impact on our understanding of chemical reactivity and provide new insights into photochemical processes.

Quantum-to-quantum transduction is the coherent exchange of information between quantum systems, which is an essential element of quantum information science (QIS). Molecular architectures provide unmatched flexibility for tailoring the properties that are critical to quantum transduction, and molecular synthesis affords the opportunity to build novel molecular materials from the bottom-up. Singlet fission (SF) in organic chromophore solids provides a potential means of executing photon-to-spin quantum transduction because absorption of a single photon results in formation of four entangled spins that constitute a triplet pair state (TT). We have demonstrated that SF in single crystals of 5,12-bis(tricyclohexylsilylethynyl)-tetracene (TCHS-tetracene) produces a long-lived quintet state 5(TT) that provides a promising new route to multiqubit systems for QIS. Here, we investigate the interaction of classical and entangled photons with TCHS-tetracene as well as the related TIPS-tetracene and tetracene to assess the molecular properties required for quantum transduction. From the classical two-photon absorption (TPA) experiment, we observed that TCHS-tetracene and TIPS-tetracene have a higher TPA cross-section than tetracene. The entangled TPA cross-sections of TCHS-tetracene and TIPS-tetracene (6.914 x 10-19 and 5.057 x 10-19 cm2/molecule, respectively) are about an order of magnitude larger than that of tetracene (5.146 x 10-20 cm2/molecule), making TCHS-tetracene and TIPS-tetracene excellent candidates for demonstrating photon-to-spin transduction using SF. We also observed a longer coherence time for the TCHS-tetracene in comparison to the tetracene molecular systems investigated, which suggests its suitability for possible QIS applications.

Investigations of entangled and classical two-photon absorption have been carried out for six donor (D)-acceptor (A)-donor (D) compounds containing the dithieno pyrrole (DTP) unit as donor and acceptors with systematically varied electronic properties. Comparing ETPA (quantum) and TPA (classical) results reveals that the ETPA cross section decreases with increasing TPA cross section for molecules with highly off-resonant excited states for single-photon excitation. Theory (TDDFT) results are in semiquantitative agreement with this anticorrelated behavior due to the dependence of the ETPA cross section but not TPA on the two-photon excited state lifetime. The largest cross section is found for a DTP derivative that has a single photon excitation energy closest to resonance with half the two-photon excitation energy. These results are important for the possible use of quantum light for low-intensity energy-conversion applications.

The application of light in studying and influencing cellular behavior with improved temporal and spatial resolution remains a key objective in fields such as chemistry, physics, medicine, and engineering. In the brain, nonexcitable cells called astrocytes play essential roles in regulating homeostasis and cognitive function through complex calcium signaling pathways. Understanding these pathways is vital for deciphering brain physiology and neurological disorders like Parkinson's and Alzheimer's. Despite challenges in selectively targeting astrocyte signaling pathways due to shared molecular equipment with neurons, recent advancements in laser technology offer promising avenues. However, the effort to use laser light properties to study astroglial cell function is still limited. This work aims to exploit an in-depth pharmacological analysis of astrocyte calcium channels to determine the physiological mechanism induced by exposure to classical nanosecond-pulsed light. We herein report molecular clues supporting the use of visible-nanosecond laser pulses as a promising approach to excite primary rat neocortical astrocytes and unprecedentedly report on the implementation of entangled two-photon microscopy to image them.

Light-induced tissue damage is a crucial limitation for traditional microscopy of the living brain, underscoring the need for new techniques that minimize exposure of samples to light. Here, we tested the hypothesis that quantum light, i.e., entangled photons, could detect brain structures at a lower excitation energy. In a proof of principle, we show microscopic images of fixed brain tissue in the hippocampus area created by fluorescence selective excitation in the process of entangled two-photon absorption in a scanning microscope. Quantum-enhanced entangled two-photon microscopy (TPM) had brain imaging capabilities at an unprecedented low excitation intensity of ∼3.6 × 107 photons/s, orders of magnitude lower than the excitation level for the classical two-photon fluorescence image obtained in the same microscope. The extremely low light probe intensity demonstrated in entangled TPM is of critical importance in the investigation of neural activity to minimize heating and photobleaching during repetitive imaging. It may have important functional implications in optogenetic technology, removing unintended heating and accumulated photodamage effects. This technology also opens avenues in spatially resolved brain tissue investigations with quantum light, providing new capabilities in local spectroscopy.

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.

Correct biological interpretation from cell imaging can be achieved only if the observed phenomena proceed with negligible perturbation from the imaging system. Herein, we demonstrate microscopic images of breast cancer cells created by the fluorescence selectively excited in the process of entangled two-photon absorption in a scanning microscope at an excitation intensity orders of magnitude lower than that used for classical two-photon microscopy. Quantum enhanced entangled two-photon microscopy has shown cell imaging capabilities at an unprecedented low excitation intensity of ∼3.6 × 107 photons/s, which is a million times lower than the excitation level for the classical two-photon fluorescence image obtained in the same microscope. The extremely low light probe intensity demonstrated in entangled two-photon microscopy is of critical importance to minimize photobleaching during repetitive imaging and damage to cells in live-cell applications. This technology opens new avenues in cell investigations with light microscopy, such as enhanced selectivity and time-frequency resolution.

CONSPECTUS: The enhanced interest in quantum-related phenomena has provided new opportunities for chemists to push the limits of detection and analysis of chemical processes. As some have called this the second quantum revolution, a time has come to apply the rules learned from previous research in quantum phenomena toward new methods and technologies important to chemists. While there has been great interest recently in quantum information science (QIS), the quest to understand how nonclassical states of light interact with matter has been ongoing for more than two decades. Our entry into this field started around this time with the use of materials to produce nonclassical states of light. Here, the process of multiphoton absorption led to photonnumber squeezed states of light, where the photon statistics are sub-Poissonian. In addition to the great interest in generating squeezed states of light, there was also interest in the formation of entangled states of light. While much of the effort is still in foundational physics, there are numerous new avenues as to how quantum entanglement can be applied to spectroscopy, imaging, and sensing. These opportunities could have a large impact on the chemical community for a broad spectrum of applications. In this Account, we discuss the use of entangled (or quantum) light for spectroscopy as well as applications in microscopy and interferometry. The potential benefits of the use of quantum light are discussed in detail. From the first experiments in porphyrin dendrimer systems by Dr. Dong-Ik Lee in our group to the measurements of the entangled two photon absorption cross sections of biological systems such as flavoproteins, the usefulness of entangled light for spectroscopy has been illustrated. These early measurements led the way to more advanced measurements of the unique characteristics of both entangled light and the entangled photon absorption cross-section, which provides new control knobs for manipulating excited states in molecules. The first reports of fluorescence-induced entangled processes were in organic chromophores where the entangled photon cross-section was measured. These results would later have widespread impact in applications such as entangled two-photon microscopy. From our design, construction and implementation of a quantum entangled photon excited microscope, important imaging capabilities were achieved at an unprecedented low excitation intensity of 10(7) photons/s, which is 6 orders of magnitude lower than the excitation level for the classical two-photon image. New reports have also illustrated an advantage of nonclassical light in Raman imaging as well. From a standpoint of more precise measurements, the use of entangled photons in quantum interferometry may offer new opportunities for chemistry research. Experiments that combine molecular spectroscopy and quantum interferometry, by utilizing the correlations of entangled photons in a Hong-Ou-Mandel (HOM) interferometer, have been carried out. The initial experiment showed that the HOM signal is sensitive to the presence of a resonant organic sample placed in one arm of the interferometer. In addition, parameters such as the dephasing time have been obtained with the opportunity for even more advanced phenomenology in the future.

Cesium-halide perovskite quantum dots (QDs) have gained tremendous interest as quantum emitters in quantum information processing applications due to their optical and photophysical properties. However, engineering excitonic states in quantum dots requires a deep knowledge of the coherent dynamics of their excitons at a single-particle level. Here, we use femtosecond time-resolved two-photon near-field scanning optical microscopy (NSOM) to reveal coherences involving a single cesium lead bromide perovskite QD (CsPbBr3) at room temperature. We show that, compared to other nonperovskite nanoparticles, the electronic coherence on a single perovskite QD has a relatively long lifetime of ca. 150 fs, whereas CdSe QDs have exciton coherence times shorter than 75 fs at room temperature. One possible explanation for the longer coherence time observed for the CsPbBr3 perovskite system is related to the exciton fine structure of these perovskite QDs compared to other nanoparticles. These perovskite QDs exhibit interesting optical properties that differ from those of the traditional QDs including bright triplet exciton states. In fact, due to the small amplitude of the energy gap fluctuations of dipole-allowed triplet states in perovskite QDs, the coherent superposition could be preserved for longer times. Furthermore, single-particle excitation approach implemented in this work allows us to remove effects of heterogeneity that are usually present in ensemble averaging experiments at room temperature. The realization of quantum-mechanical phase-coherence of a charge carrier that can operate at room temperature is an issue of great importance for the potential application of coherent electronic phenomena in electronic and optoelectronic devices. These interesting findings provide further evidence of the great potential of these perovskite QDs as candidates for quantum computing and information processing applications.

Entangled photon pairs have been used for molecular spectroscopy in the form of entangled two-photon absorption and in quantum interferometry for precise measurements of light source properties and time delays. We present an experiment that combines molecular spectroscopy and quantum interferometry by utilizing the correlations of entangled photons in a Hong-Ou-Mandel (HOM) interferometer to study molecular properties. We find that the HOM signal is sensitive to the presence of a resonant organic sample placed in one arm of the interferometer, and the resulting signal contains information pertaining to the light-matter interaction. We can extract the dephasing time of the coherent response induced by the excitation on a femtosecond time scale. A dephasing time of 102 fs is obtained, which is relatively short compared to times found with similar methods and considering line width broadening and the instrument entanglement time As the measurement is done with coincidence counts as opposed to simply intensity, it is unaffected by even-order dispersion effects, and because interactions with the molecular state affect the photon correlation, the observed measurement contains only these effects and no other classical losses. The experiments are accompanied by theory that predicts the observed temporal shift and captures the entangled photon joint spectral amplitude and the molecule's transmission in the coincidence counting rate. Thus, we present a proof-of-concept experimental method based of entangled photon interferometry that can be used to characterize optical properties in organic molecules and can in the future be expanded on for more complex spectroscopic studies of nonlinear optical properties.

Quantum entanglement has been shown to imply correlations stronger than those allowed by classical models. The possibility of performing tasks that are classically impossible has made quantum entanglement a powerful resource for the development of novel methods and applications in various fields of research such as quantum computing, quantum cryptography, and quantum metrology. There is a great need for the development of next generation instrumentation and technologies utilizing entangled quantum light. Among the many applications of nonclassical states of light, nonlinear microscopy has the potential to make an impact in broad areas of science from physics to biology. Here, the microscopic image created by the fluorescence selectively excited by the process of the entangled two-photon absorption is reported. Entangled two-photon microscopy offers nonlinear imaging capabilities at an unprecedented low excitation intensity 107, which is 6 orders of magnitude lower than the excitation level for the classical two-photon image. The nonmonotonic dependence of the image on the femtosecond delay between the components of the entangled photon pair is demonstrated. This delay dependence is a result of specific quantum interference effects associated with the entanglement and this is not observable with classical excitation light. In combination with novel spectroscopic capabilities provided by a nonclassical light excitation, this is of critical importance for sensing and biological applications.

Conventional spectroscopy uses classical light to detect matter properties through the variation of its response with frequencies or time delays. Quantum light opens up new avenues for spectroscopy by utilizing parameters of the quantum state of light as novel control knobs and through the variation of photon statistics by coupling to matter. This Roadmap article focuses on using quantum light as a powerful sensing and spectroscopic tool to reveal novel information about complex molecules that is not accessible by classical light. It aims at bridging the quantum optics and spectroscopy communities which normally have opposite goals: manipulating complex light states with simple matter e.g. qubits versus studying complex molecules with simple classical light, respectively. Articles cover advances in the generation and manipulation of state-of-the-art quantum light sources along with applications to sensing, spectroscopy, imaging and interferometry.

The use of nonclassical states of light to probe organic molecules has received great attention due to the possibility of providing new and detailed information regarding molecular excitations. Experimental and theoretical results have been reported which show large enhancements of the nonlinear optical responses in organic materials due to possible virtual-electronic-state interactions with entangled photons. In order to predict molecular excitations with nonclassical light, more detailed investigations of the parameters involved must be carried out. In this report we investigate the details of the state-to-state parameters important in calculating the contribution of particular transitions involved in the entangled two-photon absorption process for diatomic molecules. The theoretical discussion of the entangled two-photon process is described for a set of diatomic molecules. Specifically, we provide detailed quantum chemical calculations which give accurate energies and transition moments for selection-rule allowed intermediate states important in the entangled nonlinear effect for the diatomic molecules. These results are used to estimate in a more accurate manner the nonmonotonic behavior of the entangled two-photon absorption cross-section. We also derive accurate approximations that can be used to predict the period between entanglement-induced transparencies without needing exact values of the transition dipole moments. These results suggest that with the additional parameters allotted by the entangled two-photon absorption (in comparison to the classical case), it may be possible to predict and later control the nonlinear absorption and transparency of a molecule at a constant incident photon frequency.

In this contribution, the entangled two-photon absorption (ETPA) process on naturally occurring flavoproteins was studied. Low temperature responsive protein (LOT6P) and b-type dihydroorotate dehydrogenase (DHOD B), which possess flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) chromophores embedded in the protein environment, were investigated. The ETPA cross-section was measured, and we found that it increases when going from an aqueous solution of the free flavin chromophore to the chromophore embedded in the protein. This enhancement is particularly evident when entangled photons are used as excitation light compared to classical light. Our results prove the potential of ETPA as a sensing technique for fluorescent proteins even for those whose classical TPA cross-section is small compared to well-known fluorescent proteins.

Solid-state organic light conversion devices are known to intrinsically possess structural and electronic inhomogeneity, which limits their performance. To better understand the local transport dynamics in these systems, there is a great need to probe transport processes in the condensed phase and to follow the optical excitation dynamics with fast time resolution and high spatial resolution. Here, femtosecond pulse sequence near-field optical microscopy, which allows monitoring of fast coherent processes in the excited state at the nanoscale has been demonstrated. Utilizing phase-locked femtosecond light pulses in a two-photon regime, we have investigated the fast excited state dynamics in bulk heterojunction film consisting of poly[[4,8-bis [(2-ethylhexyl)oxy]benzo [1,2-b :4,5-b']dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]-thiophenediyl]] as a donor and dimeric perylene diimide (DPDI) as a nonfullerene acceptor as well as in the neat film of DPDI. Specifically, we have observed and analyzed the inhomogeneity of the ultrafast fluorescence oscillations as a function of the interpulse delay at the nanoscale. The oscillating fluorescence amplitude profile indicated dynamics with a characteristic time of similar to 450 fs that can be associated with the intermolecular energy transport in DPDI system.

The recent discovery of stable Ag nanoclusters presents new opportunities to understand the detailed electronic and optical properties of the metal core and the ligands using ultrafast spectroscopy. This paper focuses on Ag-32 and Ag-15 (with thiolate ligands), which are stable in solution. The steady state absorption spectra of Ag nanoclusters show interesting quantum size effects, expected for this size regime. Using a simple structural model for Ag-32, TDDFT calculations show absorption at 480 nm and 680 nm that are in reasonable correspondence with experiments. Ag-32(SG)(19) and Ag-15(SG)(11) have quantum yields up to 2 orders of magnitude higher than Au nanoclusters of similar sizes, with an emission maximum at 650 nm, identified as the metalligand state. The emission from both Ag nanoclusters has a common lifetime of about 130 ps and a common energy transfer rate of KEET (3) 9.7 X 10(9) s(1). A dark state competing with the emission process was also observed and was found to be directly related to the difference in quantum yield (QY) for the two Ag clusters. Two-photon excited emission was observed for Ag-15(SG)(11), with a cross-section of 34 GM under 800 nm excitation. Femtosecond transient absorption measurements for Ag-32 recorded a possible metal core state at 530 nm, a metalligand state at 651 nm, and ground state bleaches at 485 and 600 nm. The ground state bleach signals in the transient spectrum for Ag-32 are 100 nm blue-shifted in comparison to Au-25. The transient spectrum for Ag-15 shows a weak ground state bleach at similar to 480 nm and a broad excited state centered at 610 nm. TDDFT calculations indicate that the electronic and optical properties of Ag nanoclusters can be divided into core states and metalligand states, and photoexcitation generally involves a ligand to metal core transition. Subsequent relaxation leaves the electron in a core state, but the hole can be either ligand or core-localized. This leads to emission/relaxation that is consistent with the observed photophysics.

The rapid rise of organolead trihalide perovskites as solar photovoltaic materials has been followed by promising developments in light emitting devices and lasers due to their unique and promising optical properties. Evolution of the photophysical properties in as-deposited or annealed CH3NH3PbBr3 and CH3NH3PbI3 perovskite films processed through the interdiffusion method has been investigated. Absorption spectra showed broad band edge saturation in the as-deposited films in contrast to sharp excitonic absorption in the annealed films. Fluorescence emission of the perovskite films showed strong dependence on the halogen type with a very high quantum yield of ',90% for the annealed CH3NH3PbBr3 film. An explanation for this was provided based on its crystallinity and quantum confinement of the excitons. The emission showed weakly Stokes shifted bands. Time-resolved spectroscopic measurements were carried out to probe the ultrafast dynamics for the perovskites for the as deposited or annealed films. We classified the evolution in the absorption features in the excited state of CH3NH3PbBr3 perovskite films for the first time and compared them to CH3NH3PbI3. We suggest a bleach feature below 400 nm as the charge transfer band, which results in the photoinduced absorption in the CH3NH3PbBr3 perovskite film, a charge-separated band gap state, and the existence of intermediate excited-state species that regenerate the ground state.