Spatial and temporal correlation measurent on the ground state of light

In quantum field theory probing of aquantum state of light is often illustraded by the picture of atoms described by a two-level system interacting with the quantum state. Thereby, the ground state of light – well known as vacuum fluctuations – can create non-causal connections between two atoms, since the vacuum fluctuation being a pure quantum state is shows entanglement. For the discussion of causality, a precisely defined starting time for the interaction of the atoms with each other as well as with the vacuum starts is crucial. Consequently, an experimental realization failed at the permanent presence of the vacuum fluctuations. At the same time, optical signals do not interact with the vacuum field in general, but interaction can be achieved by overlapping the contributing fields inside a non-linear crystal. Within the spectral range of THz up to mid-infrared, electro-optic sampling allows mapping the electric field of a signal onto the polarization state of near-infrared laser pulses by the use of second-order nonlinear interaction. In this manner, the statistics of vacuum fluctuations and their first-order temporal field correlation within a single spatial point have been analysed in the past. Now we expand the latter investigation by separating the near-infrared probing pulses also in the spatial dimension. For a distance of 50 μm – corresponding to a time-of-flight of 470 fs – we demonstrate a non-vanishing vacuum-induced correlation between two 195 fs pulses. In conclusion, we build an experimental analogy to the theoretical two-atom picture paving the way to novel insights into quantum field theory.