Fig. 2: Experimentally observed correlations.

a Within the optical setup to measure electro-optic correlations, a prism compressor is used to compensate for the dispersion of the transmission optics in the setup to achieve 110 fs long pulses inside the nonlinear crystal. Two individual probe signals are prepared whose time-delay δt is controlled using a delay stage. Inside a cryostat, the probe signals are focused tightly into the zinc-telluride crystal (ZnTe) at 4K. Afterwards the polarization change on both probe beams is individually evaluated on a balanced detector. Depending on the experiment, a quarter-wave plate (QWP) is used to access the change in ellipticity or a half-wave plate (HWP) to observe a rotation in the polarization. b Correlations between the signals from the two balanced detectors, which is sensitive to the correlations caused by terahertz vacuum fluctuations (blue, two quarter wave plates in the detection setup) and source radiation (green, one quarter and one half-wave plate in the detection setup), is plotted depending on the time-delay between the two pulses δt. c Experimentally observed correlation are shown obtained from the Fourier-transformation of the time-traces shown in (b). The vacuum-induced correlation is plotted in blue and green is the imaginary part of the correlation caused by source radiation. The simulated spectral correlations (see “Methods” and ref. ⁷) are plotted with dashed lines and the vacuum-induced correlation multiplied by a factor of \(-\frac{1}{2}\) is plotted in gray. Details about the acquisition and analysis of the experimental data is reported in the Supplementary Note 2.A–C.