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.
Highlights
Quantum-enhanced measurements utilize the unique correlation properties of non-classical light for highly sensitive detection
The Raman sample, which can be considered as a weak parametric amplifier via Raman-based four-wave mixing (FWM), is placed inside a nonlinear interferometer that is composed of two external optical parametric amplifiers (OPAs), where each OPA amplifies one quadrature of the two-mode signal-idler field
This relative phase could be achieved by changing the optical path of the beams, but if OPA1 and OPA2 generate non-resonant light, the resonant Raman amplification of the sample will occur at φr = π/2 with no additional phase adjustments required
Summary
Quantum-enhanced measurements utilize the unique correlation properties of non-classical light for highly sensitive detection. Common examples include NOON1 and squeezing-based[2,3] interferometers that employ entangled quantum states to achieve subshot-noise phase sensitivity. This enhancement can be useful for measurements of extremely weak signals, with a crowning example being the detection of gravitational waves.[4,5] A major field that can greatly benefit from sub-shot-noise detection is Raman spectroscopy, which is widely used for chemical sensing,[6,7,8] due to its ability to identify the molecular contents of a sample based on its Raman fingerprint spectrum.
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