SQUEEZING-ENHANCED QUANTUM SENSING: RAMAN SPECTROSCOPY

Raman spectroscopy is a major tool for chemical sensing due to its ability to identify molecules based on their vibrational and rotational fingerprint in the emission spectrum. Raman spectroscopy is an ideal contrasting method for chemically resolved microscopy that does not require prior preparation or fluorescent tagging of the sample. The sensitivity of classical Raman 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 developed squeezing-enhanced Raman spectroscopy [9] that overcomes the shot-noise limit in a simple manner, fully compatible with standard Raman spectroscopy methods. By incorporating the Raman sample between two crossed parametric amplifiers (that squeeze the light along orthogonal quadrature axes), a nonclassical (SU1,1) interferometer is formed, where the Raman response of the sample induces a nonlinear phase shift, which can be detected with sub-shot-noise sensitivity. The resonant Raman response of the sample is amplified by the squeezing factor of the parametric amplifiers, while the non-resonant background is fully 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.