Next-generation single-photon sources using two-dimensional hexagonal boron nitride
Creators: name: Vogl, Tobias
Publisher: name: The Australian National University
Year: 2019

description: With the second quantum revolution unfolding, the realization of optical quantum technologies will transform future information processing, communication, and sensing. One of the crucial building blocks of quantum information architectures is a single-photon source. Promising candidates for such quantum light sources are quantum dots, trapped ions, color centers in solid-state crystals, and sources based on heralded spontaneous parametric down-conversion. The recent discovery of optically active defects hosted by 2D materials has added yet another class to the solid-state quantum emitters. Stable quantum emitters have been reported in semiconducting transition metal dichalcogenides (TMDs) and in hexagonal boron nitride (hBN). Owing to the large band gap, the energy levels of defects in hBN are well isolated from the band edges. In contrast to TMDs, this allows for operation at room temperature and prevents non-radiative decay, resulting in a high quantum yield. Unlike NV centers in diamond and other solid-state quantum emitters in 3D systems, the 2D crystal lattice of hBN allows for an intrinsically ideal extraction efficiency. In this thesis, advances in developing this new type of emitter are described. In the first experiment, quantum emitters hosted by hBN are attached by van der Waals force to the core of multimode fibers. The system features a free space and fiber-coupled single-photon generation mode. The results can be generalized to waveguides and other on-chip photonic quantum information processing devices, thus providing a path toward integration with photonic networks. Next, the fabrication process, based on a microwave plasma etching technique, is substantially improved, achieving a narrow emission linewidth, high single-photon purity, and a significant reduction of the excited state lifetime. The defect formation probability is influenced by the plasma conditions, while the emitter brightness correlates with the annealing temperature. Due to their low size, weight and power requirements, the quantum emitters in hBN are promising candidates as light sources for long-distance satellite-based quantum communication. The next part of this thesis focuses on the feasibility of using these emitters as a light source for quantum key distribution. The necessary improvement in the photon quality is achieved by coupling an emitter with a microcavity in the Purcell regime. The device is characterized by a strong increase in spectral and single-photon purity and can be used for realistic quantum key distribution, thereby outperforming efficient state-of-the-art decoy state protocols. Moreover, the complete source is integrated on a 1U CubeSat, a picoclass satellite platform encapsulated within a cube of length 10cm. This makes the source among the smallest, fully self-contained, ready-to-operate single-photon sources in the world. The emitters are also space-qualified by exposure to ionizing radiation. After irradiation with gamma-rays, protons and electrons, the quantum emitters show negligible change in photophysics. The space certification study is also extended to other 2D materials, suggesting robust suitability for use of these nanomaterials for space instrumentation. Finally, since the nature of the single-photon emission is still debated and highly controversial, efforts are made to locate the defects with atomic precision. The positions at which the defects form correlate with the fabrication method. This allows one to engineer the emitters to be close to the surface, where high-resolution electron microscopy can be utilized to identify the chemical defect. The results so far prove that quantum emitters in hBN are well suited for quantum information applications and can also be integrated on satellite platforms. A device based around this technology would thus provide an excellent building block for a worldwide quantum internet, where metropolitan fiber networks are connected through satellite relay stations.