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Proceedings of the National Academy of Sciences

Quantum-enhanced coherent Raman spectroscopy with broadband squeezed-light detection

Konstantin E. Dorfman, Vladislav V. Yakovlev, Shaul Mukamel

Published Sep 8, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1073/pnas.2611941123

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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.
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.
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 is a powerful label-free imaging technique that probes the vibrational response of chemicals with high specificity and sensitivity. High-power, quantum-enhanced SRS microscopes have been recently demonstrated and applied to polymers and biological samples. Quantum correlations, in the form of squeezed light, enable the microscopes to operate below the shot noise limit, enhancing their performance without increasing the illumination intensity. This addresses the signal-to-noise ratio (SNR) and speed constraints introduced by photodamage in shot noise-limited microscopes. Previous microscopes have either used single-beam squeezing, but with insufficient brightness to reach the optimal ratio of pump-to-Stokes intensity for maximum SNR, or have used twin-beam squeezing and suffered a 3 dB noise penalty. Here we report a quantum-enhanced Raman microscope that uses a bright squeezed single-beam, enabling operation at the optimal efficiency of the SRS process. The increase in brightness leads to multimode effects that degrade the squeezing level, which we partially overcome using spatial filtering. We apply our quantum-enhanced SRS microscope to biological samples and demonstrate quantum-enhanced multispectral imaging of living cells. The imaging speed of 100×100 pixels in 18 seconds allows the dynamics of cell organelles to be resolved. The SNR achieved is compatible with video-rate imaging, with the quantum correlations yielding a 20% improvement in imaging speed compared to shot noise-limited operation.
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.
Stimulated Raman spectroscopy has become a powerful tool to study the spatiodynamics of molecular bonds with high sensitivity, resolution, and speed. However, the sensitivity and speed of state-of-the-art stimulated Raman scattering spectroscopy are currently limited by the shot-noise of the light beam probing the Raman process. Here, we demonstrate in a proof-of-principle experiment an enhancement of the sensitivity of continuous-wave stimulated Raman spectroscopy by reducing the quantum noise of the probing light below the shot-noise limit by means of amplitude squeezed states of light. Probing polymer samples with Raman shifts around 2950 c m − 1 with squeezed states, we demonstrate a quantum enhancement of the stimulated Raman signal-to-noise ratio (SNR) of 3.60 dB relative to the shot-noise limited SNR. Our proof-of-concept demonstration of quantum-enhanced continuous-wave Raman spectroscopy paves the way for more elaborate demonstrations using state-of-the-art stimulated Raman scattering microscopes, and thus constitutes the very first step towards a new generation of Raman microscopes, where weak Raman transitions can be imaged without the use of markers or an increase in the total optical power.