  Integrated optics provides a platform for the experimental implementation of
highly complex and compact circuits for practical applications as well as for
advances in the fundamental science of quantum optics. The lithium niobate (LN)
waveguide is an important candidate for the construction of integrated optical
circuits. Based on the bound state in the continuum (BIC) in a LN waveguide, we
propose an efficient way to produce polarization-entangled photon pairs. The
implementation of this method is simple and does not require the polarization
process needed for periodically poled LN. The generation rate of the entangled
photon pairs increases linearly with the length of the waveguide. For visible
light, the generation efficiency can be improved by more than five orders of
magnitude with waveguides having the length of only a few millimeters, compared
with the corresponding case without BICs. The phenomena can appear in a very
wide spectrum range from the visible to THz regions. This study is of great
significance for the development of active integrated quantum chips in various
wavelength ranges.
