Two-Electron Quantum Walks Probe Entanglement and Decoherence in an Ultrafast Electron Microscope

According to the laws of quantum mechanics, an ensemble of many interacting particles evolves coherently, and the corresponding unitary evolution delocalizes the wavefunction of each particle while also entangling it with many other ones. However, in practice, only a subset of the particles is observed, while the rest is considered as the environment [1]. In such a scenario, decoherence theory predicts the appearance of classical properties. We experimentally examine such phenomena using multiple free electrons, which are elementary particles with a well-defined classical limit: point particles following Newton's laws. Electrons strongly interact by Coulomb forces and exhibit measurable classical [2] or quantum-mechanical [3] behaviors under different types of measurement. For example, electron pairs have strong classical anti-correlations in energy and time [4]–[6]. However, the quantum properties of multiple electrons and their degree of entanglement remain unexplored, despite theoretical studies indicating such possibilities [7].