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ACS Photonics

Sub-shot-Noise Circular Dichroism Spectroscopy for the Accelerated Characterization of Molecular Chirality

Korenobu Matsuzaki, Tahei Tahara

Published Apr 2, 2024• 4 citations
Publisher

Abstract

Circular dichroism (CD), which detects differential absorption with left- and right-circularly polarized light, is widely used for characterizing molecular chirality. Despite its similarity to absorption spectroscopy, CD spectroscopy typically requires a significantly longer time to acquire a spectrum due to the low intensity of the CD signal. The CD measurements can be accelerated if the noise level in the spectra can be reduced, which facilitates the detection of the weak CD signal. Here we show that such noise suppression is indeed possible using entangled photon pairs as the light source. By taking advantage of the photon number correlation of entangled photon pairs, we suppressed the noise contained in the CD spectra by 30% below the shot-noise limit which is a fundamental limit in conventional CD measurements. As a consequence, sub-shot-noise CD spectroscopy developed in this study is capable of characterizing the molecular chirality twice as fast as conventional CD spectroscopy.

DOI
10.1021/acsphotonics.4c00182

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Circular dichroism (CD), which detects differential absorption with left- and right-circularly polarized light, is widely used for characterizing molecular chirality. Despite its similarity to absorption spectroscopy, CD spectroscopy typically requires a significantly longer time to acquire a spectrum due to the low intensity of the CD signal. The CD measurements can be accelerated if the noise level in the spectra can be reduced, which facilitates the detection of the weak CD signal. Here we show that such noise suppression is indeed possible using entangled photon pairs as the light source. By taking advantage of the photon number correlation of entangled photon pairs, we suppressed the noise contained in the CD spectra by 30% below the shot-noise limit which is a fundamental limit in conventional CD measurements. As a consequence, sub-shot-noise CD spectroscopy developed in this study is capable of characterizing the molecular chirality twice as fast as conventional CD spectroscopy.
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.
The measurement of circular dichroism (CD) has widely been exploited to distinguish the different enantiomers of chiral structures. It has been applied to natural materials (e.g., molecules) as well as to artificial materials (e.g., nanophotonic structures). However, especially for chiral molecules the signal level is very low and increasing the signal-to-noise ratio is of paramount importance to either shorten the necessary measurement time or to lower the minimum detectable molecule concentration. As one solution to this problem, we propose here to use quantum states of light in CD sensing to reduce the noise below the shot noise level encountered when using coherent states of light. Through a multiparameter estimation approach, we identify the ultimate quantum limit of the precision in CD sensing, allowing for general schemes including additional ancillary modes. We show that the ultimate quantum limit can be achieved by various optimal schemes. These include using a pair of Fock-state probes in a direct sensing configuration and pairs of twin beams in an ancilla-assisted sensing configuration, for both of which photon number-resolved detection is shown to be the optimal measurement. These optimal schemes offer a significant quantum enhancement even in the presence of some additional system loss. The near optimality of a scheme using a single twin beam in a direct sensing configuration is also shown for cases where the actual CD signal is very small. Alternative optimal schemes involving single-photon sources and detectors are also proposed. This work paves the way for further investigations of quantum metrological techniques in chirality sensing.
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.
Absorption spectroscopy is one of the most widely used spectroscopic methods. The signal-to-noise ratio in conventional absorption spectroscopy is ultimately limited by the shot noise, which arises from the statistical property of the light used for the measurement. Here we show that the noise in absorption spectra can be suppressed below the shot-noise limit when entangled photon pairs are used for the light source. By combining broadband entangled photon pairs and multichannel detection, we realize the acquisition of sub-shot-noise absorption spectra in the entire visible wavelength. Furthermore, we demonstrate the strength of sub-shot-noise absorption spectroscopy for the identification and quantification of chemical species, which are two primary aims of absorption spectroscopy. For highly diluted binary mixture solutions, sub-shot-noise absorption spectroscopy enables us to determine the concentration of each chemical species with precision beyond the limit of conventional absorption spectroscopy. That is, sub-shot-noise absorption spectroscopy achieves superresolution in concentration measurements.
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.
In recent times there has been growing interest in Raman optical activity (ROA) for its label free detection of absolute configuration, conformation, and stereochemical structure in chiral biosamples and drug molecules. Since ROA signals are generally small, techniques such as stimulation by a probe beam can be used to enhance the signal strength. However, with a classical probe, the measurement precision is still fundamentally limited by its shot noise. To solve this problem we propose the use of two-mode squeezed vacuum and show that it can achieve sub-shot noise limited measurement sensitivity. Using quantum estimation theory, we derived the quantum Fisher information and the quantum Cramér-Rao bound (QCRB) for stimulated ROA measurement to quantify the precision enhancement. This improvement comes from photon-number correlations which suppress the intensity fluctuation common to both modes. We further show that balanced detection of the output intensity difference is a practical measurement scheme that approaches the QCRB and becomes optimal in the small-chirality limit. This opens a promising path toward more sensitive Raman chiroptical spectroscopy of weak and photosensitive samples.