The most important problem of spectroscopic chiral analysis is the enantioselective effects of the light-molecule interactions are inherently weak and severely reduced by the environment noises. Enormous efforts had been spent to overcome this problem by enhancing the symmetry break in the light-molecule interactions or reducing the environment noises. Here, we propose an alternative way to solve this problem by using frequency-entangled two-photon pairs as probe signals and detecting them in coincidence, i.e., using quantum chiral spectroscopy. For this purpose, we develop the theory of entanglement-assisted quantum chiral spectroscopy. Our results show that the quantum spectra of the left- and right-handed molecules are always distinguishable by suitably configuring the frequency-entangled two-photon pairs. In contrast, the classical spectra of the two enantiomers, where the broadband signal photon is frequency-uncorrelated with the idle one, become indistinguishable in the strong dissipation region. This offers our quantum chiral spectroscopy a great advantage over the classical chiral spectroscopy. Our work opens up an exciting area that exploring profound advantages of the quantum spectroscopy in chiral analysis.
The most important problem of spectroscopic chiral analysis is the enantioselective effects of the light-molecule interactions are inherently weak and severely reduced by the environment noises. Enormous efforts had been spent to overcome this problem by enhancing the symmetry break in the light-molecule interactions or reducing the environment noises. Here, we propose an alternative way to solve this problem by using frequency-entangled two-photon pairs as probe signals and detecting them in coincidence, i.e., using quantum chiral spectroscopy. For this purpose, we develop the theory of entanglement-assisted quantum chiral spectroscopy. Our results show that the quantum spectra of the left- and right-handed molecules are always distinguishable by suitably configuring the frequency-entangled two-photon pairs. In contrast, the classical spectra of the two enantiomers, where the broadband signal photon is frequency-uncorrelated with the idle one, become indistinguishable in the strong dissipation region. This offers our quantum chiral spectroscopy a great advantage over the classical chiral spectroscopy. Our work opens up an exciting area that exploring profound advantages of the quantum spectroscopy in chiral analysis.
Entangled photons carry nontrivial quantum correlations that defy classical physics and provide new tools for monitoring quantum dynamics in molecules. The use of low-flux entangled photons in molecular spectroscopy has been proposed in the past to probe excited-state dynamics with enhanced temporal and spectral resolutions. However, these techniques have been challenging to implement experimentally since the resulting signal has a low signal-to-noise ratio. The signal strength can be boosted by using intense entangled-photon beams, where multiple pairs of entangled photons are present in the environment. This allows the molecule under study to interact with both entangled and unentangled photons, where the classical component of the spectroscopic signal stemming from the latter can mask the spectral features associated with the former. In this Letter, we show how the signal arising from unentangled photons can be eliminated when intense entangled-photon beams are used in two-dimensional electronic spectroscopy (2DES), and demonstrate the advantage of photonic entanglement.
Spectroscopy is an important tool for probing the properties of materials, chemicals, and biological samples. We design a practical transmitter-receiver system that exploits entanglement to achieve a provable quantum advantage over all spectroscopic schemes based on classical sources. To probe the absorption spectra, modeled as a pattern of transmissivities among different frequency modes, we employ broadband signal-idler pairs in two-mode squeezed vacuum states. At the receiver side, we apply photodetection after optical parametric amplification. Finally, we perform a maximum likelihood decision test on the measurement results, achieving an error probability orders of magnitude lower than the optimum classical systems in various examples, including "wine tasting" and "drug testing" where real molecules are considered. In detecting the presence of an absorption line, our quantum scheme achieves the optimum performance allowed by quantum mechanics. The quantum advantage in our system is robust against noise and loss, which makes near-term experimental demonstration possible.
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.
Chiral discrimination of enantiomeric biomolecules is vital in chemistry, biology, and medicine. Conventional methods, relying on circularly polarized light, face weak chiroptical signals and potential photodamage. Despite extensive efforts to improve sensitivity under low-photon exposure, classical chiral probes remain fundamentally bounded by the shot-noise limit due to quantum fluctuations. To beat these limitations, we demonstrate quantum-elevated chiral discrimination using continuous-variable polarization-entangled states as moderate-photon-flux, high-sensitivity, quantum-noise-squeezed chiral probes. We achieve a 5 dB improvement beyond the SNL in distinguishing L- and D-amino acids in liquid phase. This non-destructive, biocompatible protocol enables high-sensitivity chiral analysis, with broad implications for drug development, biochemical research, environmental monitoring, and asymmetric synthesis.
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.
Quantum light, such as entangled photon pairs, is a special state of light that cannot be described by classical electrodynamics. Owing to its exotic nonclassical properties, the use of quantum light can open up a new possibility for spectroscopy that cannot be realized by a classical light source. Recently, using entangled photon pairs as the light source, we demonstrated ultrasensitive absorption spectroscopy with quantum-enhanced sensitivity [1]. This novel approach allows us to obtain absorption spectra with a noise level suppressed below the shot-noise limit, which is a fundamental and unavoidable limit in conventional absorption spectroscopy. In this presentation, we will discuss two applications of this sub-shot-noise absorption spectroscopy.