Ultrabroadband electro-optic sampling is presented as an extremely sensitive technique to detect
electric field amplitudes in free space. The temporal resolution provided by few-femtosecond laser
pulses results in a bandwidth exceeding 100 THz, potentially covering the entire infrared spectral
range. A pedagogic introduction to the operational principle of the method is given along the lines
of a classical coherent input field and a zincblende-type electro-optic sensor. We then show that
even the bare vacuum fluctuations of the electric field in the quantum ground state may be detected.
This time-domain approach to quantum physics operates directly on sub-cycle scales where no local
energy conservation holds. Therefore, signals may be obtained from purely virtual photons without
amplification to finite intensity.