Genuinely quantum effects in nonlinear spectroscopy: vacuum fluctuations and their induced superradiance

TL;DR

This paper reveals that classical laser spectroscopy signals can contain quantum vacuum effects, which can induce superradiance, challenging the traditional classical-quantum classification of spectroscopic techniques.

Contribution

It demonstrates that vacuum fluctuations contribute to pump-probe signals and can induce superradiance, highlighting a quantum aspect in classical laser spectroscopy.

Findings

Vacuum contributions act as correction terms to classical signals.

Vacuum effects can lead to observable superradiance.

Quantum vacuum effects are significant in collinear pump-probe setups.

Abstract

The classical or quantum nature of optical spectroscopy signals is a topic that has attracted great attention recently. Spectroscopic techniques have been classified as quantum or classical depending on the light-source used in their implementations. In this way, experiments performed with quantum light---such as entangled photon pairs---have been labeled as quantum spectroscopies, whereas those performed with coherent laser pulses are generally referred to as classical ones. In this work, we highlight the fact that typical laser-spectroscopy signals should sometimes be deemed quantum too, as they contain information about the quantum vacua of the modes that interact with the sample. Using a minimalistic model, namely frequency-integrated pump-probe spectroscopy, we demonstrate that vacuum contributions can be expressed as a correction term to the \emph{classical} pump-probe signals, which scales linearly with the intensity of the pump field. Remarkably, we show that these vacuum contributions may not be negligible and lead to the observation of superradiance in pump-probe experiments, provided that fields interacting with the medium are arranged in a collinear configuration.