# Temperature-Controlled Entangled-Photon Absorption Spectroscopy

## TL;DR

This paper introduces a temperature-controlled entangled-photon spectroscopy method that overcomes previous experimental challenges, enabling detailed analysis of molecular electronic structures through a novel, tunable photon source and simple data processing.

## Contribution

The work presents a new experimental scheme using temperature-tuned entangled photons to efficiently probe molecular structures without extensive source requirements or prior knowledge.

## Key findings

- Successful demonstration of temperature-tuned entangled-photon spectroscopy
- Extraction of electronic energy levels via Fourier analysis of TPA signals
- Simplification of experimental setup for molecular structure analysis

## Abstract

Entangled two-photon absorption spectroscopy (TPA) has been widely recognized as a powerful tool for revealing relevant information about the structure of complex molecular systems. However, to date, the experimental implementation of this technique has remained elusive, mainly because of two major difficulties. First, the need to perform multiple experiments with two-photon states bearing different temporal correlations, which translates in the necessity to have at the experimenter's disposal tens, if not hundreds, of sources of entangled photons. Second, the need to have \emph{a priori} knowledge of the absorbing medium's lowest-lying intermediate energy level. In this work, we put forward a simple experimental scheme that successfully overcomes these two limitations. By making use of a temperature-controlled entangled-photon source, which allows the tuning of the central frequencies of the absorbed photons, we show that the TPA signal, measured as a function of the temperature of the nonlinear crystal that generates the paired photons, and a controllable delay between them, carries all information about the electronic level structure of the absorbing medium, which can be revealed by a simple Fourier transformation.