Experimentally separating vacuum fluctuations from source radiation

TL;DR

This paper demonstrates an experimental method to distinguish vacuum fluctuations from source radiation in quantum fields using laser pulses, verifying fundamental quantum theorems and enabling new studies in relativistic quantum information.

Contribution

It introduces a novel phase-sensitive detection technique to separately probe vacuum and source radiation correlations in a laboratory setting.

Findings

Vacuum fluctuations and source radiation correlate different quadratures of laser pulses.

Experimental verification of the quantum fluctuation-dissipation theorem.

Potential for studying quantum field phenomena in relativistic contexts.

Abstract

The unique distinction between vacuum-field and source-radiation induced effects in processes such as the Lamb shift, Casimir forces or spontaneous emission, remains unresolved even at the theoretical level, and an experimental approach was never considered feasible [1-4]. In 1932, Fermi introduced the two-atom problem, which is a Gedanken-experiment that explores how two atoms interact with the surrounding electromagnetic field via vacuum and source-radiation induced processes, providing fundamental insights into the behavior of quantum fields [5-9]. Recent advancements in ultrafast optics have enabled experimental analogues of this system using two laser pulses inside a nonlinear crystal [10-12]. Here, we demonstrate the detection of vacuum and source radiation induced correlations, separated by their causal properties, between two laser pulses. In particular, we show that vacuum fluctuations and source radiation correlate different quadratures of near-infrared laser pulses, allowing them to be individually probed through phase-sensitive detection. This result provides an experimental verification of the time-domain fluctuation-dissipation theorem at the quantum level and offers a novel path to studying quantum radiation effects in time-dependent media. Beyond resolving a longstanding theoretical ambiguity, our findings open new possibilities for investigating quantum field phenomena in the context of relativistic quantum information such as entanglement harvesting from the quantum vacuum or quantum field detection in analogues of curved space-times.