Polarization–orbital angular momentum duality assisted entanglement observation for indistinguishable photons

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. Indistinguishable photons entangled in two variables could find applications in robust quantum communication, remote entanglement generation and distributed quantum sensing.