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

Abstract

In a recent paper [Phys. Rev. A 91, 053844 (2015)], Mukamel and Dorfman compare spectroscopies performed with classical vs quantum light and conclude that 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 when classical spectroscopy is perceived as a theoretical description which neglects quantum fluctuations of light altogether. While such an assumption can be a good approximation and useful for comparing theoretical results, it is never realized exactly in laser-spectroscopy experiments that typically use coherent states. Even though coherent states represent the most classical form of light, their quantumness must be considered to fully understand laser-spectroscopy experiments and their connection to quantum spectroscopy, performed with true quantum sources, such as Schr\odinger's cat states. Thus, instead of using a classical approximation, the connection between coherent states and true 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.