The sensitivity of coherent Raman spectroscopy methods such as Stimulated Raman Spectroscopy (SRS) or Coherent Anti-Stokes Raman Spectroscopy (CARS), is ultimately limited by shot noise from the stimulating fields. We present sub-shot-noise and background-free squeezed-light Raman spectroscopy, where the resonant Raman gain of the sample is enhanced by the quantum squeezing of two parametric amplifiers, while the nonresonant background of the Raman response in the sample is eliminated by destructive interference. Our configuration incorporates the Raman sample between two parametric amplifiers that squeeze the light in orthogonal quadrature axes (forming a nonlinear SU(1,1) interferometer), where the presence of a resonant Raman response induces a nonlinear phase shift, which can be measured below the shot-noise limit due to the squeezed illumination. Seeding the interferometer with coherent input further increases the Raman signal, similar to classical coherent methods. Thus, this method gains the benefits of both the coherent (classical) amplification of the seed and the squeezing-enhanced (quantum) sub-shot-noise sensitivity.
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