Ultrafast Dynamics of Quantum Emitters in hBN

Key challenges in the development of quantum networks and communications include the availability of efficient and robust quantum light sources and their optical coherent control. Recently, single-photon sources in atomically thin transition metal dichalcogenides and other 2D van der Waals materials joined the family of solid-state quantum light emitters [1]. In the 2D insulator hexagonal boron nitride (hBN), optically active states in the band gap have been discovered, efficiently emitting single photons even at room temperature. The wide variability of the emission wavelength, narrow emission lines and tunability makes this emitter particularly compelling for quantum sensing and wavelength division multiplexed quantum communications.<br/>The quantum emitters in hBN can be found in commercially available nanocrystals, making them ideal for creating large arrays of single-photon emitters using the capillary assembly technique with high positioning yields up to 95% [2]. This method opens the way for systematic optical characterization of easily addressable single-photon emitters and offers the possibility of deterministically fabricating photonic nanostructures around individual emitters.<br/>Notably, photonic microstructures and components can be created using 3D direct-laser writing. This technique allows the printing of polymer microlenses in various shapes to effectively collect and direct light from embedded emitters. However, the auto-fluorescence of commercially available photoresins limits the application in quantum optics. To address this issue, we have developed an ultra-low-fluorescence photoresin for 3D direct laser writing. We demonstrate the 3D-printing of microlenses that effectively collect quantum light emission from the emitters and collimate the single photons into a low-divergent beam [3].<br/>Furthermore, we demonstrate the coherent state manipulation of a single hBN quantum emitter with ultrafast laser pulses using a double-pulse experiment [4]. The coherence properties of the two-level system are detected by measuring the emitted photons as a function of the pulse delay. Our joint experiment-theory study reveals the effects of different sources of spectral jitter on the ultrafast coherence dynamics. We also demonstrate that coherent control can not only be exerted resonantly on the optical transition but also phonon-assisted, provides profound insight into the internal phonon quantum dynamics. We find that increased decoherence rates in optical phonons are due to dephasing processes of the phonon states, partly due to their anharmonic decay. Similarly, dephasing induced by acoustic phonon generation results in a rapid decrease in coherence when propagating phonon wave packets are emitted. Our experiments on phonon-assisted coherent control of individual hBN color centers are a significant step towards hybrid quantum technologies that combine electronic and phononic excitations.<br/> <br/>References<br/>[1] S. Michaelis de Vasconcellos et al., physica status solidi (b) 259, 2100566 (2022)<br/>[2] J. A. Preuß et al., 2D Materials 8 035005 (2021)<br/>[3] J. A. Preuß et al., Nano Letters 23, 407 (2023).<br/>[4] J. A. Preuß et al., Optica 9, 522 (2022)