Quantum interference is a cornerstone of quantum mechanics, providing deep insights into wave-particle duality and quantum coherence. In mesoscopic systems, interference has been predominantly studied in the DC regime, using Aharonov-Bohm rings and the electronic Mach-Zehnder interferometers (MZI). In this adiabatic limit, where the driving field is constant, phase coherence and dephasing mechanisms have been extensively studied. However, with the advent of electron quantum optics and the development of on-demand electron sources, it is now possible to generate ultrashort voltage pulses that induce propagating plasmonic excitations in quantum conductors. This enables access to new dynamical regimes, where internal system dynamics can be probed, offering exciting possibilities for studying quantum coherence and interference at the single-electron level—an essential building block for the realization of a fully-fledged flying electron qubit. In this thesis, we investigate quantum coherence of ultrashort plasmonic excitations and quantum interference beyond the adiabatic limit. First, we demonstrate the optimization and fabrication of a 14 µm long electronic MZI, based on a GaAs/AlGaAs heterostructure, through realistic electrostatic simulations and the development of advanced nanofabrication techniques. Our controlled fabrication process lays the groundwork for scaling up such devices and enables the realization of more complex quantum devices in the future. Aiming to reach the single electron regime, we demonstrate the development of an on-demand electron source, based on the generation of ultrashort voltage pulses through Fourier synthesis using a frequency comb. We demonstrate its application by performing time-resolved measurements of plasmonic excitations with pulse widths as short as 25 ps, directly measured in situ at the sample level under cryogenic conditions. The versatility of our source, along with its stability and precise waveform generation, establishes it as an excellent tool for electron quantum optics experiments. Thorough characterization of our electronic MZI in DC is presented, demonstrating coherent oscillations across the 14 µm length of the device and establishing a substantial coherence length of 80 µm. We reveal the emergence of nonlinearity, driven by the energy dependence of the beam splitter within the MZI. This nonlinearity accounts for the observation of coherent oscillations in the nonlinear component of the current, a behavior that is well captured and understood through our quantum transport simulations. Finally, we exploit this nonlinearity to investigate the AC response of the MZI through quantum rectification. Using sinusoidal drives, we demonstrate the onset of the dynamical regime at 1 GHz, directly probing the characteristic timescale of the device. Most notably, we achieve quantum coherence with pulses as short as 30 ps and offer insights into the coherence as a function of pulse width. Our results show that we reach the few-electron regime, supported by quantum transport simulations and Floquet calculations, which corroborate our experimental observations. Our work lays a strong foundation for a myriad of novel experiments at the single-electron level, such as the implementation of quantum teleportation protocols for single-electron states and the on-demand generation of entanglement. Additionally, these advances mark an important milestone toward the realization of a fully functional flying qubit.

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