  Quantum light is considered to be one of the key resources of the coming
second quantum revolution expected to give rise to groundbreaking technologies
and applications. If the spatio-temporal and polarization structure of modes is
known, the properties of quantum light are well understood. This information
provides the basis for contemporary quantum optics and its applications in
quantum communication and metrology. However, thinking about quantum light at
the most fundamental timescale, namely the oscillation cycle of a mode or the
inverse frequency of an involved photon, we realize that the corresponding
picture has been missing until now. For instance, how to comprehend and
characterize a single photon at this timescale? To fill this gap, we
demonstrate theoretically how local quantum measurements allow to reconstruct
and visualize a quantum field under study at subcycle scales, even when its
temporal mode structure is a priori unknown. In particular, generation and
tomography of ultrabroadband squeezed states as well as photon-subtracted
states derived from them are described, incorporating also single-photon
states. Our results set a cornerstone in the emerging chapter of quantum
physics termed time-domain quantum optics. We expect this development to elicit
new spectroscopic concepts for approaching e.g. fundamental correlations and
entanglement in the dynamics of quantum matter, overcoming the temporal
limitation set by the oscillation cycles of both light and elementary
excitations.
