The ground state of the electromagnetic field—the vacuum state—is not as ‘inert’ as one might think: in fact, electric field fluctuations permeate empty space at all frequencies. However, as these fluctuations cannot be measured directly with intensity detectors it has so far been necessary to rely largely on indirect evidence for their existence. Here, Ileana-Cristina Benea-Chelmus and collaborators report a direct investigation of the spatial and temporal coherence properties of vacuum fluctuations in the terahertz frequency range with a method based on electro-optic detection in a nonlinear crystal embedded in a cryogenic environment. The data give access to the spectral composition of the ground state of the electromagnetic radiation lying within the bandwidth of electro-optic detection, and the adopted approach—which enables shot-noise-limited measurements—could facilitate further studies of light emission in the ultra-strong coupling regime, and of a variety of coupled light-matter systems.
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