Quantum nonlinear spectroscopy via correlations of weak Faraday-rotation measurements

TL;DR

This paper introduces a quantum nonlinear spectroscopy method using polarized light as a quantum sensor to measure higher-order correlations in magnetic materials, broadening the applicability of quantum sensing techniques.

Contribution

It proposes a novel QNS protocol employing light polarization as a quantum sensor, overcoming limitations of central spin sensors for studying quantum many-body systems.

Findings

Correlated photon counts relate to higher-order magnetic fluctuations.

The method enables non-invasive, weak measurements of quantum correlations.

Applicable to complex quantum many-body systems.

Abstract

The correlations of fluctuations are key to studying fundamental quantum physics and quantum many-body dynamics. They are also useful information for understanding and combating decoherence in quantum technology. Nonlinear spectroscopy and noise spectroscopy are powerful tools to characterize fluctuations, but they can access only very few among the many types of higher-order correlations. A systematic quantum sensing approach, called quantum nonlinear spectroscopy (QNS), is recently proposed for extracting arbitrary types and orders of time-ordered correlations, using sequential weak measurement via a spin quantum sensor. However, the requirement of a central spin as the quantum sensor limits the versatility of the QNS since usually a central spin interacts only with a small number of particles in proximity and the measurement of single spins needs stringent conditions. Here we propose to employ the polarization (a pseudo-spin) of a coherent light beam as a quantum sensor for QNS. After interacting with a target system (such as a transparent magnetic material), the small Faraday rotation of the linearly polarized light can be measured, which constitutes a weak measurement of the magnetization in the target system. The correlated difference photon counts of a certain numbers of measurement shots can be made proportional to a certain type and order of correlations of the magnetic fluctuations in the material. This protocol of QNS is advantageous for studying quantum many-body systems.