# Two-photon absorption in a two-level system enabled by noise

## TL;DR

This paper investigates how environmental noise enables two-photon absorption in a two-level spin system, revealing non-Lorentzian line shapes and resonance phenomena influenced by drive strength.

## Contribution

It demonstrates that environmental noise can induce two-photon absorption in a two-level system, with detailed analysis of line shape dependence on drive amplitude and noise characteristics.

## Key findings

- Noise enables forbidden two-photon absorption.
- Line shape varies from monotonic to two-peak with drive strength.
- Environmental interaction significantly alters absorption spectra.

## Abstract

We address the textbook problem of dynamics of a spin placed in a dc magnetic field and subjected to an ac drive. If the drive is polarized in the plane perpendicular to the dc field, the drive photons are resonantly absorbed when the spacing between the Zeeman levels is close to the photon energy. This is the only resonance when the drive is circularly polarized. For linearly polarized drive, additional resonances corresponding to absorption of three, five, and multiple odd numbers of photons is possible. Interaction with the environment causes the broadening of the absorption lines. We demonstrate that the interaction with environment enables the forbidden two-photon absorption. We adopt a model of the environment in the form of random telegraph noise produced by a single fluctuator. As a result of the synchronous time fluctuations of different components of the random field, the shape of the two-photon absorption line is non-Lorentzian and depends dramatically on the drive amplitude. This shape is a monotonic curve at strong drive, while, at weak drive, it develops a two-peak structure reminiscent of an induced transparency on resonance.