Towards generation of hybrid entangled photon pairs by spectrally overlapping intermodal-vectorial four-wave mixing bands in optical fibers

Optical fiber-based sources of photon pairs with hybrid entanglement across multiple degrees of freedom are essential for a wide range of quantum technologies, while remaining compatible with existing communication networks. In this paper, we use a birefringent few-mode fiber to demonstrate an intermodal-vectorial four-wave mixing process that generates two pairs of spectrally overlapping signal-idler bands. Using phase-matching conditions, we show that the pairs of bands become spectrally indistinguishable when the group refractive indices of the signal and idler modes intersect at the pump wavelength. Our theoretical predictions are confirmed through experimental observations and corroborated by numerical simulations. Furthermore, we introduce the two-photon state associated with the overlapping bands, which simultaneously exhibits entanglement in a single degree of freedom and hybrid entanglement across multiple degrees of freedom. We also explain how the type and degree of entanglement can be controlled by tailoring the spectral overlap between the signal-idler bands. This study presents a promising scheme for a fiber-based source of photon pairs with entanglement spanning the spatial, polarization, and frequency degrees of freedom.