SS 2020

Department colloquia

General Physical Colloquium
  • Online-Kolloquium

    07.05.2020

    Development and physics of quantum dot devices for applications in photonic quantum technology

    Prof. Dr. Stefan Reitzenstein
    TU Berlin / Institut für Festköperphysik


    Zeit: Donnerstag, 16 Uhr c.t.

    Zugangsdaten:
    https://wwu.zoom.us/j/93012126842
    Meeting-ID: 930 1212 6842
    Passwort: WWU_2020


    Photonic quantum devices are key building blocks for the implementation of quantum communication protocols and for the realization of photonic quantum processors. Moreover, they allow one to explore exciting physics in the quantum regime of single emitters and single photons. Of particular interest are devices generating, routing, processing and detecting single photons which act as information carriers in the field of photonic quantum technologies. In this talk I present recent progress in the development and deterministic fabrication of high-performance single-photon sources and on-chip quantum circuits based on semiconductor quantum dots. Here, quantum dots act as close-to-ideal photon emitters with high quantum efficiency and excellent quantum nature in terms of single-photon purity and photon indistinguishability [1, 2]. Using an advanced nanoprocessing technology platform, deemed in-situ electron beam lithography, we pre-select suitable quantum dots and integrate them with nm accuracy into photonic nanostructures such as microlenses to enhance the brightness of the sources [3], to realize fiber-coupled stand-alone single-photon sources [4, 5], and to enable the development of highly functional quantum circuits [6-8]. The talk gives insight into the physics of such devices and discusses technological challenges, present limitations as well as future prospects of semiconductor quantum dot based quantum devices.

    [1] Thoma A. et al., Physical Review. Letters 116, 033601 (2016)
    [2] Bounouar S. et al., Physical Review. Letters 118, 233601 (2017)
    [3] Gschrey M. et al., Nature Communications 6, 7662 (2015)
    [4] Schlehahn A. et al., Scientific Reports 8, 1340 (2018)
    [5] Musial A. et al., arxiv:1912.10351 (2019), Advanced Quantum Technologies, in press (2020)
    [6] Schnauber P. et al., Nano Letters 18, 2336-2342 (2018)
    [7] Mrowiński, P. et al., ACS Photonics, 6, 2231 (2019)
    [8] Schnauber P. et al., Nano Letters 19, 7164 (2019)

    Einladender: Bratschitsch

    PDF

  • Online-Kolloquium

    14.05.2020

    The electron spin and chiral systems-Merging that results in novel properties

    Prof. Dr. Ron Naaman
    Department of Chemical and Biological Physics / Weizmann Institute of Science / Israel


    Zeit: Donnerstag, 16 Uhr c.t.

    Zugangsdaten:
    https://wwu.zoom.us/j/93012126842
    Meeting-ID: 930 1212 6842
    Passwort: WWU_2020


    Spin based properties, applications, and devices are commonly related to magnetic effects and to magnetic materials. However, we found that chiral molecules act as spin filters for photoelectrons transmission, in electron transfer, and in electron transport. The spin polarization can exceed 85% at room in some systems.

    The new effect, termed Chiral Induced Spin Selectivity (CISS),1,2 was found, among others, in bio-molecules and in bio-systems. It has interesting implications for the production of new types of spintronics devices3,4 in controlling magnetization,5and on electron transfer and conduction. It was also found that charge polarization in chiral molecules is accompanied by spin polarization. This finding shed new light on spin dependent interaction between chiral molecules and between them and magnetic surfaces6.

    [1] The Chiral Induced Spin Selectivity Effect, Naaman, R.; Waldeck, D.H. J. Phys. Chem. Lett. (feature), 3, 2178 (2012)

    [2] Spintronics and Chirality: Spin Selectivity in Electron Transport Through Chiral Molecules, R. Naaman, D. H. Waldeck Ann. Rev. Phys. Chem., 66, 263 (2015).

    [3] Magnetization switching in ferromagnets by adsorbed chiral molecules without current or external magnetic field, O. Ben Dor, S. Yochelis, A. Radko, K. Vankayala, E. Capua, A. Capua, S.-H. Yang, L. T. Baczewski, S. S. P. Parkin, R. Naaman, and Y. Paltiel, Nat. Comm., 8:14567 (2017).

    [4] A new approach towards spintronics- spintronics with no magnets, K. Michaeli, V. Varade, R. Naaman, D. Waldeck, Journal of Physics: Condensed Matter, 29, 103002 (2017).

    [5] E. Z. B. Smolinsky, A. Neubauer, A. Kumar, S. Yochelis, E.l Capua, R. Carmieli, Y. Paltiel, R. Naaman, K. Michaeli, Electric field controlled magnetization in GaAs/AlGaAs heterostructures-chiral organic molecules hybrids, J. Phys. Chem. Lett. 10, 1139–1145 (2019).

    [6] Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates, K. Banerjee-Ghosh, O. Ben Dor, F. Tassinari, E. Capua, S. Yochelis, A. Capua, S.-H. Yang, S. S. P. Parkin, S. Sarkar, L. Kronik, L. T. Baczewski, R. Naaman, Y. Paltiel, Science360, 1331 (2018).

    Einladender: Zacharias

    PDF

  • Online-Kolloquium

    28.05.2020

    2D materials: from atomistic quantum emitters to many-body physics

    Prof. Dr. Alexander Holleitner
    TU München / Walter Schottky Institut


    Zeit: Donnerstag, 16 Uhr c.t.

    Zugangsdaten:
    https://wwu.zoom.us/j/93012126842
    Meeting-ID: 930 1212 6842
    Passwort: WWU_2020


    We introduce the manifold class of semiconducting 2D materials and their heterostacks, and demonstrate the deterministic generation of single photon emitters in monolayer MoS2 van der Waals heterostacks. The MoS2 is site-selectively bombarded with helium ions to generate optically active defects in arrays and lines on demand. We investigate single defects by performing low temperature photoluminescence and scanning tunneling spectroscopy. Moreover, we show first results on a Bose-Einstein condensate of photogenerated interlayer excitons in MoSe2–WSe2 van der Waals heterostacks.