Fundamental limits of pulsed quantum light spectroscopy: Dipole moment estimation

TL;DR

This paper investigates the fundamental quantum limits of estimating atomic interaction parameters using pulsed quantum light, revealing that entanglement offers no advantage over separable states in certain regimes.

Contribution

It introduces a quantum information theoretic framework for quantum light spectroscopy and analyzes the role of entanglement in parameter estimation accuracy.

Findings

Single-photon absorption limits estimation precision.

Entangled biphoton states do not outperform separable states.

Approximate models for short pulses neglecting spontaneous emission.

Abstract

We study the fundamental limits of the precision of estimating parameters of a quantum matter system when it is probed by a travelling pulse of quantum light. In particular, we focus on the estimation of the interaction strength between the pulse and a two-level atom, equivalent to the estimation of the dipole moment. Our analysis of single-photon pulses highlights the interplay between the information gained from the absorption of the photon by the atom as measured in absorption spectroscopy, and the perturbation to the temporal mode of the photon due to spontaneous emission. Beyond the single-photon regime, we introduce an approximate model to study more general states of light in the limit of short pulses, where spontaneous emission can be neglected. We also show that for a vast class of entangled biphoton states, quantum entanglement between the signal mode interacting with the atom and the idler mode provides no fundamental advantage and the same precision can be obtained with a separable state. We conclude by studying the estimation of the electric dipole moment of a sodium atom using quantum light. Our work initiates a quantum information theoretic methodology for developing the theory and practice of quantum light spectroscopy.