  The ability to engineer the quantum state of traveling optical fields is a
central requirement for quantum information science and technology, including
quantum communication, computing and metrology. In this video article, we
describe the reliable generation of non-Gaussian states, including
single-photon states and coherent state superpositions, using a conditional
preparation method operated on the non-classical light emitted by optical
parametric oscillators. Type-I and type-II phase-matched OPOs operated below
threshold, i.e. single-mode or two-mode squeezed vacuum sources, are considered
and common procedures, such as the required frequency filtering or the
high-efficiency quantum state characterization by homodyning, are detailed. The
reported method enables a high fidelity with the targeted state and the
generation of the state in a well-controlled spatiotemporal mode, a crucial
feature for their use in subsequent protocols.
