Entangled photon-pair sources are foundational to advancing quantum technologies, including secure communication, quantum sensing, and imaging. For deployment in space-constrained environments such as satellite-based quantum networks or portable devices, compact, reconfigurable, and efficient entanglement sources are essential. Here, we present an electrically tunable entangled photon-pair source utilizing a nanostructured NbOCl
2 crystal, engineered for operation in the telecommunication C-band. The inherent non-centrosymmetric lattice symmetry of NbOCl
2 enables direct generation of polarization-entangled Bell states without the need for post-selection, leveraging its exceptional second-order nonlinear susceptibility, which surpasses conventional nonlinear materials. By nanopatterning NbOCl
2 into a high-quality-factor metasurface, we achieve three orders of magnitude enhancement in photon-pair generation efficiency via resonant excitation of bound states in the continuum resonance, which intensify light-matter interactions. Furthermore, we demonstrate in situ electrical tunability of the photon-pair emission wavelength over a 250 nm range from 1450 nm to 1700 nm by dynamically modulating surrounding liquid crystal layer. Remarkably, the decoupling of photon-pair generation rate and spectral tunability ensures high brightness, above 10,000 coincidences, under active tuning. The air stability and mechanical robustness of NbOCl
2 further enhance its practicality for real-world deployment. This work establishes NbOCl
2 as a superior material for scalable, on-chip quantum light sources, paving the way for integrated quantum communication systems, adaptive sensors, and portable quantum devices.
Tunable photonic quantum entanglement promises to advance information processing beyond classical approaches, facilitating applications in quantum communications, imaging and computing. A practical approach to preparing quantum entanglement at room temperature is to engineer photon-pair generation through spontaneous parametric down-conversion (SPDC). Nonlinear flat-optics devices, incorporating subwavelength-thick nonlinear materials, are emerging as new platforms for generating photon pairs at the nanoscale. In particular, nanostructured metasurfaces supporting optical resonances can enhance and tailor SPDC processes, allowing for the generation of spectral, polarization, and spatial entanglement [1]. However, the tunability of polarization entanglement is subject to the symmetry of nonlinear tensors of materials, where the pump photon polarization serves as the only tuning parameter [2].
Entangled photon-pairs generated through spontaneous parametric down-conversion (SPDC) have evolved into an indispensable resource for various quantum communication, imaging, sensing and computing schemes. While photon-pair sources are so far mostly based on bulk-optical nonlinear components, resonant nonlinear metasurfaces with sub-wavelength thickness have recently been demonstrated as a new platform for photon-pair generation [1]–[3]. Contrary to bulk and integrated nonlinear structures where phase matching steers the pair generation efficiency in different directions and wavelengths, nanostructures can control these properties via their optical resonances. However, the tunability of spatial photon emission in multiple directions remained an open challenge.