A time-domain approach that gives access to the quantum properties of light and possibly other fundamental excitations in condensed matter is presented, covering the entire mid-infrared and terahertz frequency ranges. Ultrabroadband electro-optic sampling with few-femtosecond and highly stable laser pulses allows direct detection of the vacuum fluctuations of the electric field in free space [1,2]. Besides the Planck and electric field fundamental constants, the variance (ΔE)2 of the ground state is determined solely by the inverse of the four-dimensional space-time volume over which a measurement process or physical system integrates. Therefore, we can vary the contribution of multi-terahertz vacuum noise in the statistical readout of our nonlinear technique and discriminate against the trivial shot noise due to the constant flux of near-infrared probe photons. A subcycle temporal resolution and an off-resonant character provide signals even from purely virtual photons, enabling access to the ground-state wave function without amplification to finite intensity. Recently, we have succeeded in generation and analysis of mid-infrared squeezed transients with quantum noise patterns that are time-locked to the probe pulses. We find temporal positions with a noise level distinctly below the bare vacuum input which serves as a reference. Delay times with increased differential noise indicate generation of highly correlated photons by spontaneous parametric fluorescence. Our time-domain approach offers a generalized understanding of spontaneous emission processes as a consequence of local anomalies in the co-propagating reference frame modulating the quantum vacuum, in combination with the boundary conditions set by the uncertainty principle.