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

TL;DR

The paper investigates duality in entanglement for indistinguishable photons by demonstrating polarization entanglement through sorting in an even-odd OAM basis and, conversely, OAM entanglement through polarization sorting. Using a ppKTP-based type-II SPDC source and a robust folded Mach-Zehnder OAM sorter, the authors show high-visibility entanglement across complementary variables, quantified by visibilities and Bell parameters. The approach preserves indistinguishability and avoids post-selection losses, enabling potential applications in quantum networking, distributed sensing, and verification of channel-induced decoherence. The results highlight the practical utility of even-odd OAM bases for entanglement verification and suggest improvements to reach unity visibility with a complete even-odd basis.

Abstract

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.

Figures (11)

• Figure 1: Interferometric sorter for even and odd OAM states of light using Dove prism in (a) Mach-Zehnder arrangement and (b) Folded Mach-Zehnder arrangement. In both figures, the orientation of the two Dove prisms are perpendicular to each other.
• Figure 2: Experimental setup to observe the entanglement in a non-collinear type-II SPDC from a ppKTP crystal ($\chi^{(2)}$). Polarization measurements are carried out using a combination of half-wave plate (HWP) and a polarizer (P) in each arm. $\mathrm{L_{FC}}$ - Aspheric lens associated with the fiber coupler (FC), SMF - Single mode fiber, SPCM - Single photon counting module, CC - Coincidence counter.
• Figure 3: The collimated SPDC output images obtained using an EMCCD kept at different distances from the crystal plane. A 10 cm lens placed after the SPDC output collimates the diverging cone of photon pairs.
• Figure 4: The crystal position and tilt are adjusted such that the SPDC output corresponding to both the polarizations are spatially overlapping. Diametrically opposite regions (in red circles) corresponds to the entangled photons.
• Figure 5: Polarization correlations corresponding to the spatially separated photons in the non-collinear down conversion pair. Error bars indicate statistical uncertainty of one standard deviation.