Fluctuation properties of laser light after interaction with an atomic system: comparison between two-level and multilevel atomic transitions

TL;DR

This paper compares the fluctuation properties of laser light after interaction with two-level and multilevel atomic systems, revealing distinct spectral features and noise behaviors influenced by atomic structure and polarization.

Contribution

It provides a detailed analysis of how atomic internal structure and laser polarization affect the spectral noise properties of transmitted laser light, highlighting differences between two-level and multilevel systems.

Findings

Mollow triplet observed in two-level systems with circular polarization

Additional spectral features appear in multilevel systems with linear polarization

Multilevel systems exhibit extra noise from spontaneous Raman transitions in weak drive regimes

Abstract

The complex internal atomic structure involved in radiative transitions has an effect on the spectrum of fluctuations (noise) of the transmitted light. A degenerate transition has different properties in this respect than a pure two-level transition. We investigate these variations by studying a certain transition between two degenerate atomic levels for different choices of the polarization state of the driving laser. For circular polarization, corresponding to the textbook two-level atom case, the optical spectrum shows the characteristic Mollow triplet for strong laser drive, while the corresponding noise spectrum exhibits squeezing in some frequency ranges. For a linearly polarized drive, corresponding to the case of a multilevel system, additional features appear in both optical and noise spectra. These differences are more pronounced in the regime of a weakly driven transition: whereas the two-level case essentially exhibits elastic scattering, the multilevel case has extra noise terms related to spontaneous Raman transitions. We also discuss the possibility to experimentally observe these predicted differences for the commonly encountered case where the laser drive has excess noise in its phase quadrature.