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Quantum-Enhanced Spectroscopy and Imaging: Coherent anti-Stokes Raman Scattering with Squeezed Light

Santiago Pinto, Frank Hagelberg, Ali Passian, Xu Yi

Published Jan 1, 2026• 0 citations
Publisher

Abstract

We study theoretically quantum-enhanced CARS that exploits intrinsic photon-number correlations in two-mode squeezed light to boost vibrational transitions at low photon flux, enabling sensitivity gains without interferometric detection or high pump powers, promising nondestructive bioimaging.
DOI
10.1364/cleo_fs.2026.ftu2c.6

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Quantum correlations offer the possibility of surpassing classical limits in optical measurements, but their integration into nonlinear vibrational spectroscopy has remained largely unexplored. Here, we introduce a quantum-enhanced implementation of coherent Raman spectroscopy that employs broadband squeezed-light detection to interrogate relative intensity correlations between Stokes and anti-Stokes fields generated in an ultrafast four-wave-mixing process. By exploiting nonclassical correlations between these fields, we suppress the nonresonant background and enhance the signal-to-noise performance in both temporal and spectral domains, enabling detection sensitivities beyond the classical shot-noise limit. Importantly, the quantum advantage demonstrated here does not arise solely from improved detection statistics. Rather, it originates from microscopic intermolecular correlations mediated by cascaded exchange of a virtual photon between molecular pairs. This mechanism establishes a fundamentally distinct and significant pathway for information extraction in vibrational spectroscopy, providing access to structural and dynamical observables that are intrinsically inaccessible by conventional classical Raman techniques. Numerical simulations illustrate the power of the technique by resolving intradimer [Formula: see text] bond splitting in cyclobutane thymine dimers following ultraviolet excitation, highlighting its potential for probing ultrafast photochemical processes with quantum-limited sensitivity and for label-free biomedical optical sensing and imaging.
The non-classical effects of squeezing of light in coherent anti-Stokes Raman scattering (CARS) and coherent anti-Stokes hyper-Raman scattering (CAHRS) are investigated under the short-time approximation based on a fully quantum mechanical approach. In an idealized model CARS, the interaction is looked upon as a process involving absorption of a pump photon and emission of a Stokes photon, which is followed by absorption of a pump photon and subsequently emission of an anti-Stokes photon at a different frequency, whereas CAHRS is looked upon as a process involving absorption of two pump photons and emission of a Stokes photon, which is followed by absorption of two more pump photons and subsequent emission of an anti-Stokes photon at a different frequency. The coupled Heisenberg equations of motion involving real and imaginary parts of the quadrature operators are established. It is shown that squeezing in the Stokes and anti-Stokes mode in CARS is found to be dependent on amplitude-squared squeezing of the initial pump field while, in CAHRS, it is dependent on the fourth-order amplitude squeezing of the initial pump field. Higher-order squeezing can be converted to linear squeezing through these processes. It is also shown that squeezing is greater in CAHRS than the corresponding squeezing in CARS.
Abstract The sensitivity of classical Raman spectroscopy methods, such as coherent anti-stokes Raman spectroscopy (CARS) or stimulated Raman spectroscopy (SRS), is ultimately limited by shot-noise from the stimulating fields. We present the complete theoretical analysis of a squeezing-enhanced version of Raman spectroscopy that overcomes the shot-noise limit of sensitivity with enhancement of the Raman signal and inherent background suppression, while remaining fully compatible with standard Raman spectroscopy methods. By incorporating the Raman sample between two phase-sensitive parametric amplifiers that squeeze the light along orthogonal quadrature axes, the typical intensity measurement of the Raman response is converted into a quantum-limited, super-sensitive estimation of phase. The resonant Raman response in the sample induces a phase shift to signal-idler frequency-pairs within the fingerprint spectrum of the molecule, resulting in amplification of the resonant Raman signal by the squeezing factor of the parametric amplifiers, whereas the non-resonant background is annihilated by destructive interference. Seeding the interferometer with classical coherent light stimulates the Raman signal further without increasing the background, effectively forming squeezing-enhanced versions of CARS and SRS, where the quantum enhancement is achieved on top of the classical stimulation.
Stimulated Raman scattering (SRS) microscopy has emerged as a powerful technique for probing the spatiotemporal dynamics of molecular bonds with exceptional sensitivity, resolution, and speed. However, classically, its performance remains fundamentally constrained by optical shot noise, which imposes a strict limit on detection sensitivity and speed. Here, we demonstrate a quantum-enhanced SRS microscopy platform that circumvents this barrier by harnessing amplitude-squeezed light. Specifically, we generate a Stokes beam with 5.2 dB5.2~\mathrm{dB}5.2 dB of amplitude squeezing using traveling-wave optical parametric amplification in second-order nonlinear waveguides, and combine it with a tunable coherent pump to access vibrational modes spanning from 100010001000 to 3100 cm−13100~\mathrm{cm}^{-1}3100 cm−1. Applied to quantum imaging of metabolites in biological tissue (pork muscle), our quantum-enhanced Raman microscope achieves an average noise suppression of 3.6 dB3.6~\mathrm{dB}3.6 dB and a 51%51\%51% enhancement in signal-to-noise ratio (SNR) -- to the best of our knowledge, the largest improvement reported to date in quantum-enhanced SRS microscopy of biological samples.
The integration of quantum-enabled technology into remote sensing revolutionizes sensitivity and accuracy, enabling the detection of subtle environmental changes, biochemical markers, hazardous materials, and other critical targets beyond the reach of traditional sensors. We have developed advanced remote sensing methodologies by integrating linearly chirped pulses into Coherent Anti-Stokes Raman Scattering (C-CARS) [1-4] and Fractional Stimulated Raman Adiabatic Passage (CFSTIRAP) [5], thereby enhancing their performance. This integration facilitates the generation of highly directional, backward-propagating signals, significantly amplifies signal intensity, and reduces noise by optimizing coherence within the target species, which ensures superior detection accuracy. The proposed methodologies open new possibilities for precise and reliable sensing in complex and challenging environments, paving the way for groundbreaking applications in high-resolution climate monitoring, secure long-range communication, and precise material analysis over vast distances [6]. The work is partially supported the Office of Naval Research through award N00014-22-1-2374. [1] J. Chathanathil, S. A. Malinovskaya, Chirped pulse control of Raman coherence in atoms and molecules. Adv. Quant. Chem. 89, 225-289 (2024). [2] J. Chathanathil, D. Budker, S. A. Malinovskaya, Quantum control via chirped coherent anti-Stokes Raman spectroscopy, Quantum Sci. Tech. 8, 045005 (2023). [3] J. Chathanathil, G. Lui, S. A. Malinovskaya, A semi-classical control theory of Coherent Anti-Stokes Raman Scattering (CARS) maximizing vibrational coherence for remote detection, Phys. Rev. A 104, 043701 (2021). [4] N. Pandya, G. Lui, F. A. Narducci, J. Chathanathil, S. A. Malinovskaya, Creation of the maximum coherence via adiabatic passage in the four-wave mixing process of coherent anti-Stokes Raman scattering, Chem. Phys. Lett., 738, 136763 (2020). [5] J. Chathanathil, A. Ramaswamy, V. S. Malinovsky, D. Budker, S. A. Malinovskaya, Chirped fractional stimulated Raman adiabatic passage, Phys. Rev. A 108, 043710 (2023). [6] G. Liu, F. A. Narducci, S. A. Malinovskaya, Limits to remote molecular detection via coherent anti-Stokes Raman spectroscopy using a maximal coherence control technique, J. Mod. Opt. 67, 21-25 (2020).