Transition Dipole Rotation Beyond the Condon Approximation in Single hBN Quantum Emitters

TL;DR

This study reveals that the transition dipole moment of single hBN quantum emitters rotates with photon energy due to phonon interactions, challenging the static dipole assumption in solid-state quantum interfaces.

Contribution

It demonstrates that transition dipoles are dynamic and energy-dependent, driven by phonon coupling, thus extending the understanding beyond the Condon approximation in quantum emitters.

Findings

Dipole rotation reaches up to 40 degrees across vibronic states at room temperature.

Cryogenic temperatures suppress spectral dipole rotation, indicating phonon involvement.

First-principles calculations confirm phonon-induced deviations in dipole orientation.

Abstract

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 that the transition dipole moment of single hexagonal boron nitride quantum emitters is not a static property but rotates as a function of photon energy. Through high-resolution energy-resolved spectroscopy, we reveal a continuous rotation of the emission dipole orientation reaching up to $40^{\circ}$ across the vibronic manifold at room temperature, driven by coupling to the phonon bath. This spectral rotation is effectively suppressed at cryogenic temperatures (6 K), where the acoustic phonon population is negligible, identifying thermally activated lattice vibrations as the primary driver of the reorientation. First-principles calculations on two representative defects spanning weak and strong electron-phonon coupling regimes confirm that phonon-displaced geometries produce a systematic deviation of the transition dipole orientation from the zero-phonon line, with the magnitude scaling with vibronic coupling strength. The experimental observations and calculations demonstrate that single quantum emitters can operate beyond the Condon approximation, with the transition dipole acquiring a dependence on the instantaneous nuclear configuration. Our results identify a fundamental limit for polarization fidelity in solid-state quantum networks and connect solid-state single-emitter physics to a class of effects previously accessible only in ensemble measurements in molecular and biological spectroscopy.