Rotation of the Transition Dipole in Single hBN Quantum Emitters via Vibronic Coupling

The design of polarization-encoded quantum interfaces relies on the assumption that solid-state emitters possess static transition dipoles defined by the host lattice symmetry. Here, we demonstrate the vibronic breakdown of this static dipole approximation in hexagonal boron nitride quantum emitters. Through high-resolution energy-resolved spectroscopy, we reveal a continuous, spectral rotation of the emission dipole orientation reaching up to 40^circ, driven by coupling to the phonon bath. This spectral gradient is significantly suppressed at cryogenic temperatures (6 K), identifying thermally activated lattice vibrations as the primary driver of the dipole reorientation. First-principles calculations on two representative defect types indicate the microscopic origin of this phenomenon as a coordinate-dependent transition dipole, where phonon-induced atomic displacements fundamentally perturb the electronic wavefunctions. By comparing the distinct defect environments, we demonstrate that the magnitude of the polarization rotation scales with the strength of the vibronic coupling. Our results not only identify a fundamental limit for polarization fidelity in solid-state quantum networks but also suggest a new class of strain-tunable quantum photonic devices based on vibronic dipole reorientation.