  This review covers recent theoretical and experimental efforts to extend the
application of the continuous-variable quantum technology of light beyond
"Gaussian" quantum states, such as coherent and squeezed states, into the
domain of "non-Gaussian" states with negative Wigner functions. Starting with
basic Gaussian nonclassicality associated with single- and two-mode vacuum
states produced by means of parametric down-conversion and applying a set of
standard tools, such as linear interferometry, coherent state injection, and
conditional homodyne and photon number measurements, one can implement a large
variety of optical states and processes that are relevant in fundamental
quantum physics as well as quantum optical information processing. We present a
systematic review of these methods, paying attention to both fundamental and
practical aspects of their implementation, as well as a comprehensive overview
of the results achieved therewith.
