QUANTUM-Seminar

This talk explores the rapidly evolving field of quantum technologies, with a particular focus on semiconductor quantum dots (QDs) and their potential in quantum communication and distributed quantum computing. Quantum dots are excellent sources of single and entangled photons and offer significant tunability in their optical properties through variations in material composition, shape, and confinement. Our work focuses on epitaxially grown GaAs/AlGaAs QDs that emit near the optical transitions of rubidium or the zero-phonon line of silicon-vacancy centres [1]. There is strong potential for such QDs in hybrid systems, which are vital for future quantum repeaters and extended quantum networks [2]. Several milestone experiments have been realized, incorporating single quantum dots. These include entanglement swapping between photon pairs [3] and quantum key distribution (QKD) experiments between Hannover and Braunschweig [4]. In order for QDs to realise their full potential in quantum technology applications, it is imperative to realize a seamless integration of QDs into photonic devices, with the objective of enhancing the efficiency of photon extraction and optimising the coupling to fibre networks. This necessitates the development of innovative strategies in the fields of optical positioning, photonic design and fabrication. A calibration model is introduced with the objective of enhancing the accuracy of wide-field optical positioning for the alignment of solid-state single photon emitters within photonic nanostructures. This is expected to result in a significant increase in the yield of high performance quantum photonic devices. Furthermore, the development of hybrid circular photonic crystal gratings for the generation of entangled photon pairs at telecom wavelengths represents a promising advancement for direct coupling efficiency into single-mode fibres [5]. [1] X. Cao, J. Yang, T. Fandrich, Y. Zhang, E. P. Rugeramigabo, B. Brechtken, R. J. Haug, M. Zopf, and F. Ding, Nano Letters 23, 6109 (2023). [2] P. van Loock, W. Alt, C. Becher, O. Benson, H. Boche, C. Deppe, J. Eschner, S. Höfling, D. Meschede, P. Michler, et al., Advanced Quantum Technologies 3, 1900141 (2020), https://advanced.onlinelibrary.wiley.com/doi/pdf/10.1002/qute.201900141. [3] M. Zopf, R. Keil, Y. Chen, J. Yang, D. Chen, F. Ding, and O. G. Schmidt, Phys. Rev. Lett. 123, 160502 (2019). [4] J. Yang, Z. Jiang, F. Benthin, J. Hanel, T. Fandrich, R. Joos, S. Bauer, S. Kolatschek, A. Hreibi, E. P. Rugeramigabo, et al., Light: Science & Applications 13, 150 (2024), ISSN 2047-7538. [5] C. Ma, J. Yang, P. Li, E. P. Rugeramigabo, M. Zopf, and F. Ding, Opt. Express 32, 14789 (2024).

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Diamond quantum sensors for magnetic fields have transformed several areas of science, most prominently magnetic resonance and magnetic field imaging at the micro- and nanoscale. However, these breakthroughs have largely remained limited to specialized laboratories. I will present two lines of research of our laboratory to change this state of affairs and significantly simplify the use of diamond quantum sensors. One direction concerns scanning probe imaging, where we have developed a simplified approach to scanning probe positioning. While conventional setups image magnetic fields by scanning a nanofabricated diamond tip hosting a single NV center across a sample, we developed a setup where we can scan an extended (10 µm to mm) bulk diamond in 10 nm-scale proximity of a sample, using interferometric alignment to maintain the sensor perfectly parallel to the sample. Beyond a technical simplification, this approach opens the door to massively parallel scanning probe microscopy using multiple NV centers, as well as to novel plasmonic near-field microscopes. Another direction concerns the electric readout of large ensembles of NV center spins, as they might find application in large-scale commercial devices like gyroscopes or magnetic field sensors. Here, we have shown in recent research that readout in a microwave cavity is remarkably competitive with more established optical readout for large ensembles, and provides a straightforward all-electric way to integrate diamond spin sensors into microfabricated circuits.

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