Quantum emitters in solid-state crystals have recently attracted a lot of attention due to their simple applicability in optical quantum technologies. Color centers such as fluorescent defects hosted by diamond and hexagonal boron nitride (hBN) emit single photons at room temperature and can be used for nanoscale sensing. The atomic structure of the hBN defects, however, is not yet well understood. In this work, we fabricate an array of identical hBN emitters by localized electron irradiation. This allows us to correlate the dipole orientations with the host crystal axes. The angle of excitation and emission dipoles relative to the crystal axes are also calculated using density functional theory, which reveals characteristic angles for every specific defect. Moreover, we also investigate the temporal polarization dynamics and discover a mechanism of time-dependent polarization visibility and dipole orientation of color centers in hBN and diamond. This can be traced back to the excitation of excess charges in the local crystal environment. We therefore provide a promising pathway for the identification of color centers as well as important insight into the dynamics of solid-state quantum emitters.

Efficient single photon generation is an important requirement for several practical applications in quantum technologies, including quantum cryptography. A proof-of-concept demonstration of free-space quantum key distribution (QKD) is presented with single photons generated from an isolated defect in hexagonal boron nitride (hBN). The bright source operating at room temperature is integrated into a QKD system based on B92 protocol and a sifted key rate of 238 bps with a quantum bit error rate of 8.95% are achieved at 1 MHz clock rate. The effect of temporal filtering of detected photons on the performance of QKD parameters is also studied. It is believed that these results will stimulate the research on optically active defects in hBN as well as other 2D-based quantum emitters and their applications within quantum information technologies including practical QKD systems.

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