  We review the unconventional photon blockade mechanism. This quantum effect
remarkably enables a strongly sub-Poissonian light statistics, even from a
system characterized by a weak single photon nonlinearity. We revisit the past
results, which can be interpreted in terms of quantum interferences or optimal
squeezing, and show how recent developments on input-output field mixing can
overcome the limitations of the original schemes towards passive and integrable
single photon sources. We finally present some valuable alternative schemes for
which the unconventional blockade can be directly adapted.
