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arXiv:2604.07314

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

Serkan Paçal, Chanaprom Cholsuk, Mouli Hazra, Çağlar Samaner, Özgür Çakır, Tobias Vogl, Serkan Ateş

quant-phcond-mat.mes-hallSubmitted Apr 8, 2026• Announced Apr 9, 2026
v1 · Apr 8, 2026v2 · Apr 21, 2026
arXivHTMLPDF

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 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.

Comments
8 pages, 4 figures

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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.
Collective light emission from many-body quantum systems is a cornerstone of quantum optics, yet its implementation in solid-state platforms operating under ambient conditions remains highly challenging. Large-bandgap van der Waals materials such as hexagonal boron nitride (hBN) host stable room-temperature single-photon emitters with narrow linewidths across a broad spectral range. However, cooperative radiative effects in this system have not been previously explored. Here we demonstrate collective emission from quantum-emitter ensembles in hBN layers when the emitters are nearly indistinguishable and positioned within a sub-wavelength proximity. Using confocal microscopy and a Hanbury Brown-Twiss (HBT) configuration, we identify both isolated emitters and ensembles activated by localized electron-beam irradiation. Time-resolved photoluminescence measurements reveal a superlinear intensity enhancement and a pronounced acceleration of the radiative decay in tightly confined ensembles, with lifetimes approaching the temporal resolution of our experimental system (about 500 ps), compared to approximately 1.85 ns for single emitters or large, spatially extended ensembles. Complementary second-order photon-correlation measurements exhibit sub-Poissonian antidip consistent with emission from a few indistinguishable emitters. The simultaneous observation of lifetime shortening and enhanced emission provides direct evidence of cooperative emission at room temperature, achieved without optical cavities or cryogenic cooling. These results establish optically active defect ensembles in hBN as a scalable solid-state platform for engineered collective quantum optics in two-dimensional materials, opening avenues toward ultrabright superradiant light sources and nonclassical photonic states for quantum technologies.
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Quantum emitters in solid-state crystals have recently attracted a great deal of attention due to their simple applicability in optical quantum technologies. The polarization of single photons generated by quantum emitters is one of the key parameters that plays a crucial role in various applications, such as quantum computation, which uses the indistinguishability of photons. However, the degree of single-photon polarization is typically quantified using the time-averaged photoluminescence intensity of single emitters, which provides limited information about the dipole properties in solids. In this work, we use single defects in hexagonal boron nitride and nanodiamond as efficient room-temperature single-photon sources to reveal the origin and temporal evolution of the dipole orientation in solid-state quantum emitters. The angles of the excitation and emission dipoles relative to the crystal axes were determined experimentally and then calculated using density functional theory, which resulted in characteristic angles for every specific defect that can be used as an efficient tool for defect identification and understanding their atomic structure. Moreover, the temporal polarization dynamics revealed a strongly modified linear polarization visibility that depends on the excited-state decay time of the individual excitation. This effect can potentially be traced back to the excitation of excess charges in the local crystal environment. Understanding such hidden time-dependent mechanisms can further improve the performance of polarization-sensitive experiments, particularly that for quantum communication with single-photon emitters.
The negatively charged boron vacancy center in hexagonal boron nitride is a premier candidate for quantum sensing, yet its performance is critically limited by longitudinal spin-lattice relaxation time (T1T_1T1​). A microscopic understanding of spin relaxation in the high magnetic field regime remains elusive, as the relevant Zeeman transitions lie far below the optical phonon energies typically invoked to describe the relaxation process. Here, we apply an \textit{ab initio} acoustic mode spin-phonon relaxation theory to this problem and quantitatively reproduce the experimental magnetic field and temperature dependence of T1T_1T1​ without empirical fitting parameters. We demonstrate that the relaxation dynamics are driven by a direct one-phonon emission and absorption process resonant with the Zeeman splitting. Furthermore, we identify the out-of-plane flexural phonon branch which is unique to two-dimensional hosts, as the primary source of decoherence, creating a distinct low-energy spectral function that facilitates spin relaxation. Our results provide a microscopic interpretation of the experimentally observed non-monotonic field and temperature dependence in two-dimensional quantum defect centers.
Quantum emitters in hexagonal boron nitride (hBN) are promising building blocks for solid-state quantum photonics because of their bright room-temperature emission. These emitters strongly interact with lattice vibrations, which partially underpins their excitation efficiency. Here, using complementary cathodoluminescence (CL) and photoluminescence (PL) spectroscopy, we demonstrate cavity-mediated interactions between hBN quantum emitters coupled to the same plasmonic Au void cavity at the room temperature. Cathodoluminescence measurements reveal that a vibronic transition of the 443 nm emitter hybridizes with the cavity mode, giving rise to a spectrally resolved splitting, while the neighboring 537 nm emitter remains in the weak-coupling regime despite spectral overlap. The energy split of the CL peak is at the order of 36.2 meV, whereas the PL-resolved peak is approximately 24.1 meV, which is due to the collective excitations of multiple emitters per electron excitation and their mutual interactions. Maxwell-Bloch simulations reproduce the experimental spectra and identify the observed splitting as the result of cavity-induced hybridization. Excitation-dependent PL further shows that the hybridized state can be accessed through phonon-assisted excitations below the zero-phonon transition. These results demonstrate that vibronic transitions provide an effective pathway for controlling interactions between multiple quantum emitters in hybrid plasmonic nanophotonic systems.