Recent progress in electro-optic sampling has allowed direct access to the fluctuations of the electromagnetic ground state. Here, we present a theoretical formalism that allows for an in-depth characterisation and interpretation of such quantum-vacuum detection experiments by relating their output statistics to the quantum statistics of the electromagnetic vacuum probed. In particular, we include the effects of absorption, dispersion and reflections from general environments. Our results agree with available experimental data while leading to significant corrections to previous theoretical predictions and generalises them to new parameter regimes. Our formalism opens the door for a detailed experimental analysis of the different characteristics of the polaritonic ground state, e.g. we show that transverse (free-field) as well as longitudinal (matter or near-field) fluctuations can be accessed individually by tuning the experimental parameters.
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