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Quantum Technology: Driving Commercialisation of an Enabling Science

Bell inequality in chiral liquids

Sara Restuccia, Graham M. Gibson, Leroy Cronin, Miles J. Padgett

Published Oct 6, 2021• 0 citations
Publisher

Abstract

Optical activity is a macroscopic property of chiral molecules which manifests as a rotation of the plane of linear polarization when light passes through a sample. We have developed a compact Bell inequality experiment for quantum probing of chiral liquids, based on polarization measurements. In particular, we show that we can use a Bell-type inequality configuration to measure the optical activity of D-Limonene, a chiral molecule which is a major component in the oil of citrus fruit peels.

DOI
10.1117/12.2601516

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Optical activity is a macroscopic property of chiral molecules which manifests as a rotation of the plane of linear polarization when light passes through a sample. We have developed a compact Bell inequality experiment for quantum probing of chiral liquids, based on polarization measurements. In particular, we show that we can use a Bell-type inequality configuration to measure the optical activity of D-Limonene, a chiral molecule which is a major component in the oil of citrus fruit peels.
The phenomenon of molecular optical activity manifests itself as the rotation of the plane of linear polarization when light passes through chiral media. Measurements of optical activity and its wavelength dependence, that is, optical rotatory dispersion, can reveal information about intricate properties of molecules, such as the three-dimensional arrangement of atoms comprising a molecule. Given a limited probe power, quantum metrology offers the possibility of outperforming classical measurements. This has particular appeal when samples may be damaged by high power, which is a potential concern for chiroptical studies. We present the first experiment in which multiwavelength polarization-entangled photon pairs are used to measure the optical activity and optical rotatory dispersion exhibited by a solution of chiral molecules. Our work paves the way for quantum-enhanced measurements of chirality, with potential applications in chemistry, biology, materials science, and the pharmaceutical industry. The scheme that we use for probing wavelength dependence not only allows one to surpass the information extracted per photon in a classical measurement but also can be used for more general differential measurements.
We perform experimental quantum polarimetry using a heralded single photon to analyze the optical activity of linearly polarized light traversing a chiral medium. Three kinds of estimators are considered to estimate the concentrations of sucrose solutions from measuring the rotation angle of the linear polarization of the output photons. Through repetition of independent and identical measurements performed for each individual scheme and different concentration sucrose solutions, we compare the estimation uncertainty among the three schemes. The results are also compared to classical benchmarks for which a coherent state of light is taken into account. The quantum enhancement in the estimation uncertainty is evaluated and the impact of experimental and technical imperfections is discussed. In this work, we lay out a route for future applications relying on quantum polarimetry.
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.
Probing the polarization response of an object with the well-acknowledged technique of optical polarimetry enables determination of the specimen's inner properties. Prominent examples include the measurement of the chirality of proteins and molecules [1] or the induced anisotropy of disease-affected cells and tissues [2]. Aiming for better signal-to-noise ratio, lower photon flux of the probing beam and non-local diagnostics, polarimetric approaches employing the quantum nature of light and entanglement are currently rapidly developing [3]–[6]. In the present work we study the experimental feasibility of the non-local identification of different polarizing samples by performing only few coincidence measurements [6] without necessity for full density matrix reconstruction of the quantum state.
Entanglement, one of the most representative phenomena in quantum mechanics, has been widely used for fundamental studies and modern quantum technologies. In this paper, we report the observation of nonlocal cancellation and addition of optical rotations with polarization-entangled photons in fructose solutions. The entanglement also enables probing optical activities at a distance by joint measurements on the entangled photons. The good agreement between the experimental results and theoretical predictions demonstrates the potential for extending these measurements to other chiral molecules, with a sensitivity that improves as the number of entangled photons increases.
Abstract We perform experimental quantum polarimetry using a heralded single photon to analyze the optical activity of linearly polarized light traversing a chiral medium. Three kinds of estimators are considered to estimate the concentrations of sucrose solutions from measuring the rotation angle of the linear polarization of the output photons. Through repetition of independent and identical measurements performed for each individual scheme and different concentration sucrose solutions, we compare the estimation uncertainty among the three schemes. The results are also compared to classical benchmarks for which a coherent state of light is taken into account. The quantum enhancement in the estimation uncertainty is evaluated and the impact of experimental and technical imperfections is discussed. In this work, we lay out a route for future applications relying on quantum polarimetry.