  We study how one-particle correlations transfer to manifest as two-particle
correlations in the context of parametric down-conversion (PDC), a process in
which a pump photon is annihilated to produce two entangled photons. We work in
the polarization degree of freedom and show that for any two-qubit generation
process that is both trace-preserving and entropy-nondecreasing, the
concurrence $C(\rho)$ of the generated two-qubit state $\rho$ follows an
intrinsic upper bound with $C(\rho)\leq (1+P)/2$, where $P$ is the degree of
polarization of the pump photon. We also find that for the class of two qubit
states that is restricted to have only two non-zero diagonal elements such that
the effective dimensionality of the two-qubit state is same as the
dimensionality of the pump polarization state, the upper bound on concurrence
is the degree of polarization itself, that is, $C(\rho)\leq P$. Our work shows
that the maximum manifestation of two-particle correlations as entanglement is
dictated by one-particle correlations. The formalism developed in this work can
be extended to include multi-particle systems and can thus have important
implications towards deducing the upper bounds on multi-particle entanglement,
for which no universally accepted measure exists.
