  Duality in the entanglement of identical particles manifests that
entanglement in only one variable can be revealed at a time. We demonstrate
this using polarization and orbital angular momentum (OAM) variables of
indistinguishable photons generated from parametric down conversion. We show
polarization entanglement by sorting photons in even and odd OAM basis, while
sorting them in two orthogonal polarization modes reveals the OAM entanglement.
The duality assisted observation of entanglement can be used as a verification
for the preservation of quantum indistinguishability over communication
channels. Indistinguishable photons entangled in complementary variables could
also evoke interest in distributed quantum sensing protocols and remote
entanglement generation.
