  Continuous-wave (CW) squeezed light is used in generation of various optical
quantum states thus is a fundamental resource of fault-tolerant universal
quantum computation using optical continuous variables. To realize a practical
quantum computer, a waveguide optical parametric amplifier (OPA) is an
attractive CW squeezed light source in terms of its THz-order bandwidth and
suitability for modularization. The usages of a waveguide OPA in quantum
applications thus far, however, are limited due to the difficulty of the
generation of the squeezed light with a high purity. In this paper, we report
the first observation of Wigner negativity of the states generated by a
heralding method using a waveguide OPA. We generate Schr\"{o}dinger cat states
at the wavelength of 1545 nm with Wigner negativity using a quasi-single-mode
ZnO-doped periodically poled ${\rm LiNbO_3}$ waveguide module we developed.
Wigner negativity is regarded as an important indicator of the usefulness of
the quantum states as it is essential in the fault-tolerant universal quantum
computation. Our result shows that our waveguide OPA can be used in wide range
of quantum applications leading to a THz-clock optical quantum computer.
