Comment on "Nonlinear fluctuations and dissipation in matter revealed by quantum light"

TL;DR

This paper clarifies that nonlinear quantum spectroscopy signals can be derived from coherent-state experiments, challenging previous claims that classical approximations cannot reproduce quantum effects in nonlinear regimes.

Contribution

It demonstrates that the connection between coherent states and quantum states allows quantum spectroscopy to be obtained from coherent-state measurements, even in nonlinear systems.

Findings

Quantum spectroscopy can be projected from coherent-state experiments.

Classical approximations are insufficient for nonlinear quantum signals.

Coherent states bridge the gap between classical and quantum spectroscopy.

Abstract

In a recent paper [Phys.~Rev.~A {\bf 91}, 053844 (2015)], Mukamel and Dorfman compare spectroscopies performed with classical vs.~quantum light, and conclude that \textit{nonlinear} quantum-spectroscopy signals cannot be obtained from averaging their classical-spectroscopy counterparts over the Glauber--Sudarshan quasiprobability distribution of the quantum field. In this Comment, we show that this interpretation is correct only if one assumes that a classical signal is given by a classical approximation for the field. While such an assumption can be useful for comparing theoretical results, it is never realized in laser spectroscopy experiments that typically use coherent states. Thus, instead of using classical signals, the connection between coherent states and quantum states of light must be considered. We rigorously show that quantum spectroscopy can always be projected from the experimentally realized coherent-state spectroscopy regardless how nonlinear the system response is.