Entangled Two-Photon Absorption by Atoms and Molecules: A Quantum Optics Tutorial

TL;DR

This tutorial explores the quantum optical theory of two-photon absorption (TPA) in molecules using entangled photon pairs, highlighting how quantum light can enhance TPA rates and discussing theoretical methods and experimental challenges.

Contribution

It provides a comprehensive theoretical framework for entangled two-photon absorption, including new insights into quantum light effects and extensions of existing models.

Findings

Quantum entangled photons can significantly enhance TPA rates.

Differences between TPA with quantum and classical light are characterized.

Theoretical methods for modeling entangled TPA are reviewed and extended.

Abstract

Two-photon absorption (TPA) and other nonlinear interactions of molecules with time-frequency-entangled photon pairs (EPP) has been predicted to display a variety of fascinating effects. Therefore, their potential use in practical quantum-enhanced molecular spectroscopy requires close examination. This paper presents in tutorial style a detailed theoretical study of one- and two-photon absorption by molecules, focusing on how to treat the quantum nature of light. We review some basic quantum optics theory, then we review the density-matrix (Liouville) derivation of molecular optical response, emphasizing how to incorporate quantum states of light into the treatment. For illustration we treat in detail the TPA of photon pairs created by spontaneous parametric down conversion, with an emphasis on how quantum light TPA differs from that with classical light. In particular, we treat the question of how much enhancement of the TPA rate can be achieved using entangled states. The paper includes review of known theoretical methods and results, as well as some extensions, especially the comparison of TPA processes that occur via far-off-resonant intermediate states only and those that involve off-resonant intermediate state by virtue of dephasing processes. A brief discussion of the main challenges facing experimental studies of entangled TPA is also given.