  Nonclassical light sources, such as correlated photon-pairs, play an
important role in quantum optics and quantum information processing systems.
This study proposes a process to generate antibunched photon-pairs in a
nondegenerate optical parametric oscillator. It is found that when the
parameters of the system satisfy certain conditions, the generated photons in
subharmonic modes exhibit a strong antibunching behavior and are strongly
correlated with one another. In particular, the average photon-pair number is
resonantly enhanced. It is also observed that the conventional photon blockade
contributes to this phenomenon. In addition, it is interesting to note that
fundamental mode photons can blockade the subharmonic mode photons. We refer to
this phenomenon as a heterogeneous photon blockade.
