Classical Model for Broadband Squeezed Vacuum Driving Two-Photon Absorption or Sum Frequency Generation

TL;DR

This paper demonstrates that classical stochastic models can accurately replicate quantum predictions for nonlinear optical processes driven by broadband squeezed vacuum, bridging classical and quantum descriptions in nonlinear spectroscopy.

Contribution

It shows that classical stochastic fields, with appropriate renormalization, can mimic quantum states in nonlinear optics, including TPA and SFG, across different regimes.

Findings

Classical models match quantum predictions for TPA and SFG rates.

Linear flux scaling observed at low photon flux.

Dependence of rates on linewidths aligns with quantum theory.

Abstract

We address theoretically the question of classical stochastic fields mimicking quantum states of light in the context of nonlinear spectroscopy and nonlinear optics, in particular two-photon absorption (TPA) and sum-frequency generation (SFG) driven by weak or bright broadband squeezed vacuum with time-frequency entanglement between photons. Upon using a well-defined but ad hoc subtraction of vacuum-energy terms (renormalization), we find that the classical stochastic model yields exactly the same predictions as the full quantum-field theory for all of the phenomena considered here, in both the low-gain and high-gain regimes of squeezed vacuum. Such predictions include the linear-flux scaling of TPA and SFG rates at low incident photon flux, as well as the dependence of TPA and SHG rates on the relative linewidths of the squeezed light and the ground-to-final-state transition in the material system, and the spectrum of SFG generated by bright squeezed vacuum.