title: Direct sampling of electric-field vacuum fluctuations.
abstract: 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.
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Riek
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany.
authors.forename: D V
authors.initials: DV
authors.lastname: Seletskiy
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany.
authors.forename: A S
authors.initials: AS
authors.lastname: Moskalenko
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany.
authors.forename: J F
authors.initials: JF
authors.lastname: Schmidt
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Krauspe
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Eckart
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Eggert
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany.
authors.forename: G
authors.initials: G
authors.lastname: Burkard
authors.affiliation: Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany. alfred.leitenstorfer@uni-konstanz.de.
authors.forename: A
authors.initials: A
authors.lastname: Leitenstorfer