Coherent anti-Stokes Raman scattering with squeezed light: CARS for quantum-enhanced spectroscopy and imaging*

We present a theoretical framework showing how squeezed light can enhance coherent anti-Stokes Raman scattering (CARS) without interferometry at low photon flux, enabling nondestructive spectroscopy and imaging. Using Gaussian input states, we derive fourth-order transition amplitudes and cross-sections,and identify two practical implementations: 1. single-mode squeezing of the pump, and 2. two-mode squeezing that entangles pump and Stokes fields. A key result is that the ordering of displacement and squeezing operations significantly affects the outcome. Applying squeezing before displacement yields, at equal photon number, larger CARS transition amplitudes than the reverse order, due to distinct photon-number moments and quantum-enhancement coefficients. These gains are achieved without increased pump power. We also propose a hybrid quantum-plasmonic approach, where nanoparticle-induced local field enhancements selectively amplify the relevant CARS modes. This scheme combines quantum noise reduction with nanoscale field concentration. Finally, we discuss feasible experimental regimes and the compatibility of our approach with existing background-suppression techniques. This is the first report of CARS enhancement mechanisms involving the combination of photonic effects and localized plasmonic fields.