Direct sampling of electric-field vacuum fluctuations

Probing the fluctuating vacuum According to quantum mechanics, a vacuum is not empty space. A consequence of the uncertainly principle is that particles or energy can come into existence for a fleeting moment. Such vacuum or quantum fluctuations are known to exist, but evidence for them has been indirect. Riek et al. present an ultrafast optical based technique that probes the vacuum fluctuation of electromagnetic radiation directly. Science, this issue p. 420 Ultrafast optics can directly probe the electric-field vacuum fluctuations. The ground state of quantum systems is characterized by zero-point motion. This motion, in the form of vacuum fluctuations, is generally considered to be an elusive phenomenon that manifests itself only indirectly. Here, we report direct detection of the vacuum fluctuations of electromagnetic radiation in free space. The ground-state electric-field variance is inversely proportional to the four-dimensional space-time volume, which we sampled electro-optically with tightly focused laser pulses lasting a few femtoseconds. Subcycle temporal readout and nonlinear coupling far from resonance provide signals from purely virtual photons without amplification. Our findings enable an extreme time-domain approach to quantum physics, with nondestructive access to the quantum state of light. Operating at multiterahertz frequencies, such techniques might also allow time-resolved studies of intrinsic fluctuations of elementary excitations in condensed matter.

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