Symmetry Engineering Enables Deterministic Ensemble Polarization Control in Hexagonal Boron Nitride
Optical polarization is a key degree of freedom in quantum photonic and sensing technologies, enabling efficient light--matter coupling and directional emission. However, in defect ensemble-based quantum systems, orientational averaging across many defects suppresses optical anisotropy and eliminates a deterministic polarization axis. Here, symmetry engineering is introduced as a strategy to restore collective optical anisotropy in negatively charged boron vacancy (V_B^-) ensembles hosted in hexagonal boron nitride (h-BN). By imposing anisotropic in-plane tensile strain through lithographically defined nano-ridge arrays, in-plane symmetry is broken and the ensemble emission dipole is aligned with the ridge-defined strain symmetry axis. Polarization-resolved photoluminescence reveals that the polarization visibility scales with the magnitude of strain anisotropy, while the emission orientation rigidly follows the engineered symmetry axis across devices with varying strain direction. This deterministic alignment is independent of crystal orientation, establishing the optical foundation for polarization-defined ensemble spin readout and programmable photonic integration in two-dimensional quantum materials