  Entanglement enables many promising applications in quantum technology.
Devising new generation methods and harnessing entanglement are prerequisites
for practical applications. Here we realize a distinct polarization-entangled
source by simultaneously achieving type-0 and type-I backward quasi-phase
matching (BQPM) through spontaneous parametric down-conversion in a single bulk
crystal, which is different from all previous entangled-source configurations.
Pumping the crystal with a single polarized beam generates a non-maximally
polarization-entangled state, which can be further projected to a maximal Bell
state with a pair of Brewster windows. Hong-Ou-Mandel interference experiments
are done on polarization-degenerate photon pairs for both type-0 and type-I
BQPM processes for the first time. The emitted photons in both processes have a
bandwidth as narrow as 15.7 GHz. The high quality of this source is
characterized by various methods. The rather simple configuration, narrow
bandwidth, and high entanglement quality make the source very promising for
many quantum information tasks.
