Mechanical decoupling of quantum emitters in hexagonal boron nitride from low-energy phonon modes.

Quantum emitters in hexagonal boron nitride were recently reported to hold unusual narrow homogeneous linewidths of tens of megahertz within the Fourier transform limit at room temperature. This unique observation was traced back to decoupling from in-plane phonon modes. Here, we investigate the origins for the mechanical decoupling. New sample preparation improved spectral diffusion, which allowed us to reveal a gap in the electron-phonon spectral density for low phonon frequencies. This sign for mechanical decoupling persists up to room temperature and explains the observed narrow lines at 300 kelvin. We investigate the dipole emission directionality and reveal preferred photon emission through channels between the layers supporting the claim for out-of-plane distorted defect centers. Our work provides insights into the underlying physics for the persistence of Fourier transform limit lines up to room temperature and gives a guide to the community on how to identify the exotic emitters.

Efficient Coupling of an Ensemble of Nitrogen Vacancy Center to the Mode of a High-Q, Si3N4 Photonic Crystal Cavity.

Integrated nanophotonics is an emerging field with high potential for quantum technology applications such as quantum sensing or quantum networks. A desired photonics platform is Si3N4 due to low-photon loss and well-established fabrication techniques. However, quantum optics applications are not yet established. Here, we investigate an approach toward Si3N4-based quantum photonics utilizing a crossed waveguide, pump-probe design. The platform enables efficient, on-chip excitation, strong background suppression, and at the same time, efficient coupling to the mode of a high- Q photonic crystal cavity. The freestanding photonic crystal cavities reach high Q-factors up to 47 × 103. To test our platform, we positioned an ensemble of negatively charged nitrogen vacancy centers located in a nanodiamond within the interaction zone of the photonic crystal cavity. We quantify the efficiency of the coupling with the ßλ-factor reaching values as large as 0.71. We further demonstrate on-chip excitation of the quantum emitter with strong suppression (∼20 dB) of the background fluorescence. Our results unfold the potential to utilize negatively charged nitrogen vacancy centers in nanodiamonds and Si3N4 platforms as an efficient, on-chip spin-photon interface in quantum photonics experiments.