By variation of the photon statistics and the entanglement between time and frequency components, quantum states of light feature spectroscopy signals which are not subject to classical Fourier limitations on temporal and spectral resolution. This review surveys the properties of entangled photon pairs relevant to spectroscopic applications and it presents an intuitive diagrammatic approach which allows calculation of a variety of ultrafast, nonlinear spectroscopy signals from multilevel model systems exposed to quantum light.
By variation of the photon statistics and the entanglement between time and frequency components, quantum states of light feature spectroscopy signals which are not subject to classical Fourier limitations on temporal and spectral resolution. This review surveys the properties of entangled photon pairs relevant to spectroscopic applications and it presents an intuitive diagrammatic approach which allows calculation of a variety of ultrafast, nonlinear spectroscopy signals from multilevel model systems exposed to quantum light.
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.
Classical light such as lasers and thermal radiation has long been utilized for spectroscopy to explore material structures and dynamics. However, quantum states of light open new avenues for ultrafast optics and spectroscopy, by the variation of their unique properties, e.g., entanglement and photon statistics, after interacting with materials. Quantum light possesses unique characteristics that distinguish it from classical light, particularly the phase-space distribution which reflects its fundamental quantum statistics and the quantum control protocol that may take advantage of noise reduction. This Review will focus on spectroscopic applications using entangled photons and squeezed light, in conjunction with quantum coherent control. We provide a brief introduction to the generation of the entangled and squeezed light, discussing their fundamental properties and connections to the control protocol that may enhance spectroscopic techniques. A microscopic theory for the spectroscopy and corresponding signals induced by certain transition processes is developed. Nonlinear optical signals induced by entangled photons for the coherent anti-Stokes Raman spectra (CARS) and the stimulated Raman spectra (SRS) are investigated. We further extend our efforts to the transient absorption process that enables real-time monitoring of the exciton transfer in the transition metal dichalcogenides (TMDs), namely, using the WS 2 monolayer. This series of works on nonlinear optical spectroscopy using quantum light explicitly demonstrates the quantum supremacy in ultrafast spectroscopy through jointly enhancing the time and energy scales beyond the classical bound.
Classical time-resolved optical spectroscopy experiments are performed using sequences of ultrashort light pulses, with photon fluxes incident on the sample which are many orders of magnitude higher than real-world conditions corresponding to sunlight illumination. Here we overcome this paradigm by exploiting quantum correlations to perform time-resolved spectroscopy with entangled photons. Starting from spontaneous parametric down-conversion driven by a continuous-wave laser, we exploit the temporal entanglement between randomly generated signal/idler pairs to obtain temporal resolution, and their spectral entanglement to select the excitation frequency. We also add spectral resolution in detection, using a Fourier transform approach which employs a common-path interferometer. We demonstrate the potential of our entangled-photon streak camera by resolving, at the single-photon level, excitation energy transfer cascades from LH2 to LH1 in the photosynthetic membrane and disentangling the lifetimes of two dyes in a mixture. We show that time-resolved spectroscopy with quantum light can be performed without compromising measurement time, recording a fluorescence time trace in less than a minute even for samples with low quantum yield, which can be reduced to sub-second times with acceptable signal-to-noise ratio. Our results provide a new approach to ultrafast optical spectroscopy, where experiments are performed under conditions comparable to real-world sunlight illumination.
Spectroscopy based on individual-photon-level light sources brings new variables to both spectroscopy and imaging. We refer to this approach as quantum-light spectroscopy (QLS). The photon statistics of different types of photon sources and photon entanglements are briefly described. We then illustrate the potential of QLS by means of some current experimental examples including time-resolved spectroscopy performed with single photons and quantum-enhanced optical imaging, showing how all the methods rely on correlations between entangled photons in the pair. We then ask: how might physical chemists exploit QLSs to expose new physical insights? For example, can we address fundamental questions in interactions between light and molecules or materials that are very difficult to address with classical light sources? Does QLS open up techniques to study highly congested spectra with higher combined spectral and temporal resolution than is possible with classical light? The concept of QLS also opens the potential study of fundamental questions such as what initial excited states are produced by individual photon absorption? We also point out the experimental challenges in extending QLS to more complex spectroscopies, such as multidimensional spectroscopies. We conclude with a brief selection of recent theoretical proposals to extend the capabilities of the QLS.
Recent advances in techniques for generating quantum light have stimulated research on novel spectroscopic measurements using quantum entangled photons. One such spectroscopy technique utilizes non-classical correlations among entangled photons to enable measurements with enhanced sensitivity and selectivity. Here, we investigate the spectroscopic measurement utilizing entangled three photons. In this measurement, time-resolved entangled photon spectroscopy with monochromatic pumping [A. Ishizaki, J. Chem. Phys. 153, 051102 (2020)] is integrated with the frequency-dispersed two-photon counting technique, which suppresses undesired accidental photon counts in the detector and thus allows one to separate the weak desired signal. This time-resolved frequency-dispersed two-photon counting signal, which is a function of two frequencies, is shown to provide the same information as that of coherent two-dimensional optical spectra. The spectral distribution of the phase-matching function works as a frequency filter to selectively resolve a specific region of the two-dimensional spectra, whereas the excited-state dynamics under investigation are temporally resolved in the time region longer than the entanglement time. The signal is not subject to Fourier limitations on the joint temporal and spectral resolution, and therefore, it is expected to be useful for investigating complex molecular systems in which multiple electronic states are present within a narrow energy range.
Quantum light is a key resource for promoting quantum technology. One such class of technology aims to improve the precision of optical measurements using engineered quantum states of light. In this study, we investigate transmission measurement of frequency-entangled broadband photon pairs generated via parametric down-conversion with a monochromatic laser. It is observed that state-to-state dynamics in the system under study are temporally resolved by adjusting the path difference between the entangled twin beams when the entanglement time is sufficiently short. The non-classical photon correlation enables time-resolved spectroscopy with monochromatic pumping. It is further demonstrated that the signal corresponds to the spectral information along anti-diagonal lines of, for example, two-dimensional Fourier-transformed photon-echo spectra. This correspondence inspires us to anticipate that more elaborately engineered photon states would broaden the availability of quantum light spectroscopy.