Physikalisches Kolloquium

4.5.2020 17:15, Raum: T-1004
Kai Müller (TU München)
Semiconductor quantum dots as non-classical light sources and optically-active spin qubits

Semiconductor quantum dots are promising for applications in photonic quantum technologies. This can be traced to their excellent optical properties such as fast emission rates, nearly transform-limited linewidth, ease of integration into nanophotonic resonators to enhance the light-matter interaction and ability to host single electron or hole spins as optically active qubits.

For the generation of non-classical light resonant excitation is essential since it avoids the creation of free carriers which cause fluctuations in the environment. For on demand single-photon generation, a two-level system given by an excitonic transition in a quantum dot is typically excited with a resonant laser pulse of area π. This prepares the two-level system in its excited state from where it spontaneously emits a single photon. However, emission that occurs already during the presence of the laser pulse allows for re-excitation and, thus, multi-photon emission which limits the single-photon purity [1]. In contrast, when exciting the system with a pulse of area 2π, the system is expected to be returned to the ground state. However, in this case emission during the presence of the pulse is most likely to occur when the system is in its excited state – exactly after an area of π has been absorbed. This restarts the Rabi oscillation with a pulse area of π remaining in the pulse which leads to re-excitation with near-unity probability and the emission of a second photon within the excited state lifetime [2,3]. In addition to the dynamics of this process I will discuss the potential of multi-level systems for single-photon generation with ultra-low multi-photon probability [4]. For single spins confined to quantum dots, decoherence occurs due to interactions with the nuclear spin bath. For electron spins, we observe fast ensemble dephasing (~2ns), slow spin relaxation due to nuclear spin co-flips with the central spin (>1µs) and at intermediate timescales (~750ns) an additional stage of incomplete non-monotonic dephasing which results from quadrupolar coupling of the nuclear spins to strain induced electric field [5]. In contrast, for hole spins we observe a more than two-orders of magnitude slower ensemble dephasing due to reduced hyperfine interaction [6].

[1] K. A Fischer, K. Müller, K. G. Lagoudakis and J. Vučković, “Dynamical modeling of pulsed two-photon interference”, New J. Phys. 18, 113053 (2016). [2] K. A. Fischer, L. Hanschke, J. Wierzbowski, T. Simmet, C. Dory, J. J. Finley, J. Vučković and K. Müller, “Signatures of two-photon pulses from a quantum two-level system”, Nature Physics 13, 649-654 (2017). [3] K. A. Fischer, L. Hanschke, M. Kremser, J. J. Finley, K. Müller and J. Vuckovic, “Pulsed Rabi oscillations in quantum two-level systems: beyond the Area Theorem”, Quantum Science and Technology, 3, 1 (2017) [4] L. Hanschke, K. A. Fischer, S. Appel, D. Lukin, J. Wierzbowski, S. Sun, R. Trivedi, J. Vuckovic, J. J. Finley, Kai Müller, "Quantum dot single photon sources with ultra-low multi-photon probability" npj Quantum Information 4, 43 (2018) [5] A. Bechtold, D. Rauch, F. Li, T. Simmet, P.-L. Ardelt, A. Regler, K. Müller, N. A. Sinitsyn, J. J. Finley, "Three stage decoherence dynamics of electron spin qubits in an optically active quantum dot", Nature Physics 11, 1005-1008 (2015) [6] T. Simmet et al. in preparation

18.5.2020 17:15, Raum: T-1004
Prof. Philip J. W. Moll (EPFL Lausanne)
Microstructured Topological Materials: A novel route towards topological electronics (tentative)
22.6.2020 17:15, Raum: T-1004
Prof. Dr. Michael Thorwart (Universität Hamburg)
Quantum biology revisited
6.7.2020 17:15, Raum: T-1004
Prof. Dr. Ing. habil. Oliver Gutfleisch (Institut für Materialwissenschaft, Technische Universität Darmstadt)
Hysteresis Design of Magnetic Materials for Efficient Energy Conversion

Die Vorträge finden im Hörsaalzentrum Physik (Gebäude T) statt.

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