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Disputation Jaan Freudenfeld

12.02.2021 | 16:00
Thema der Dissertation:
Coupling Quantum Point Contacts via Ballistic Electron Optics
Abstract: In future electronics, quantum effects in integrated circuits containing coupled nanostructures will play a key role. In this framework, the coupling between distant on-chip components could be realized by the exchange of ballistic electrons. For this purpose, we aim at optimizing the exchange of ballistic electrons between quantum point contacts (QPCs), fundamental building blocks of quantum circuits. To couple distant QPCs in quantum transport experiments at low temperature, we use the concept of ballistic electron optics. We first study the actual electrostatic potential shape of a gate-defined single QPC by measuring its one-dimensional subband spacings. The potential shape is central for the emission process of ballistic electrons as well as interaction effects. Comparing the measured subband spacings of the QPC to models of lateral parabolic versus hard-wall confinement, we find that it is compatible with the parabolic saddle-point scenario near pinch-off. However, as we increase the number of populated subbands, Coulomb screening flattens the potential bottom and a description in terms of a hard-wall potential becomes more realistic. In a second experiment, we consider the ballistic and laterally coherent electron transport through two distant QPCs in series in a three-terminal configuration. We study the emission and detection properties of the QPCs in detail via magnetic detection of ballistic electrons in a perpendicular magnetic field. Additionally, we enhance the serial transmission by using a field effect electron lens. Comparing our measurements with quantum mechanical calculations we discuss generic features of the quantum circuit and demonstrate how the coherent and ballistic dynamics depend on the details of the QPC confinement potentials. In a third experiment, we consider an open ballistic electronic cavity formed by four QPCs. Performing bias voltage spectroscopy measurements, we characterize coherent Fabry-Pérot-like resonances and find good agreement with predictions for the dephasing of the electron ensemble as well as single particle decoherence due to electron-electron scattering.

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12.02.2021 | 16:00