Fundamental quantum electrodynamical (QED) processes such as spontaneous emission and electron-photon scattering encompass a wealth of phenomena that form one of the cornerstones of modern science and technology. Conventionally, calculations in QED and in other field theories assume that incoming particles are single-momentum states. The possibility that coherent superposition states, i.e. shaped wavepackets, will alter the result of fundamental scattering processes is thereby neglected, and is instead assumed to sum to an incoherent (statistical) distribution in the incoming momentum. Here, we show that free-electron wave-shaping can be used to engineer quantum interferences that alter the results of scattering processes in QED. Specifically, the interference of two or more pathways in a QED process (such as photon emission) enables precise control over the rate of that process. As an example, we apply our concept to Bremsstrahlung, a ubiquitous phenomenon that occurs, for instance, in X-ray sources for state-of-the-art medical imaging, security scanning, materials analysis, and astrophysics. We show that free electron wave-shaping can be used to tailor both the spatial and the spectral distribution of emitted photons, enhancing their directionality and monochromaticity, and adding more degrees of freedom that make emission processes like Bremsstrahlung more versatile. The ability to tailor the spatiotemporal attributes of photon emission via quantum interference provides a new degree of freedom in shaping radiation across the entire electromagnetic spectrum. More broadly, the ability to tailor general QED processes through the shaping of free electrons opens up new avenues of control in processes ranging from optical excitation processes (e.g., plasmon and phonon emission) in electron microscopy to free electron lasing in the quantum regime.
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