  Photonic entanglement is a crucial resource for quantum information
technologies, including quantum information processing and long-distance
communication. These applications call for narrow-band entanglement sources,
being compatible with the photon-electron interfaces and relax the
dispersion-induced entanglement-degradation in the network mediums. However,
such Giga-hertz-level bandwidth is a challenge for the photonic entanglement
based on conventional spontaneous parametric down-conversion. Here we introduce
a state-of-art domain-engineering technology for the counterpropagating phase
matching in the polarization-entanglement generation, which results in an
inherent bandwidth of 7.1 GHz at telecom wavelength. This geometry also
resolves the fundamental challenge for the deterministic separation of the
entangled photon pairs at frequency degeneracy, so that efficient collinear
interaction can be achieved in a periodically-poled KTP waveguide and results
in a spectral brightness of 3.4*10^3(GHz*mW*s)^-1. The 155 ps base-to-base
Hong-Ou-Mandel dip also confirms the source bandwidth, with and a high
interference visibility of 97.1%. The entanglement is measured to violate the
Bell inequality by up to 18.5 standard deviations, with
Clauser-Horne-Shimony-Holt S-parameter of 2.720. The quantum state tomography
further characterizes the entanglement, with fidelity F=95.71%. These unique
features establish a cornerstone for the photonic entanglement sources
development and warrant broad practical quantum information applications.
