  We discuss the generation of entangled photons using a nonlinear photonic
crystal and beam splitter. In our method, the photonic crystal is assumed to be
composed of a material with a large second-order nonlinear optical
susceptibility $\chi^{(2)}$. Our proposal relies on two facts: (1) A nonlinear
photonic crystal changes coherent incident light into squeezed light. (2) A
beam splitter transforms the squeezed light into entangled light beams flying
in two different directions. We estimate the yield efficiency of pairs of
entangled photons per pulse of the very weak coherent light for our method at
$0.0783$ for a specific concrete example. Our method is more effective because
the conversion efficiency (entangled biphotons per incident pump photons) in
the spontaneous parametric downconversion is of the order of $4\times 10^{-6}$.
The only drawback is that it requires very fine tuning of the frequency of
signal photons fed into the photonic crystal; for example, an adjustment is
given by $\Delta\nu=3.11\times 10^{8}$ Hz for the signal light of
$\nu=3.23\times 10^{14}$ Hz. We investigate the application of entangled
photons produced by our method to the BB84 quantum key distribution protocol,
and we explore how to detect an eavesdropper. We suggest that our method is
promising for decoy-state quantum key distribution using weak coherent light.
