Controlling the quantum state of free electrons by inelastic optical near-field scattering

Summary form only given. Ultrafast transmission electron microscopy is a promising laser-pump electron-probe technique, which allows for studying ultrafast dynamics with both high spatial and temporal resolution [1]. Besides, the high spatial and temporal coherence of the pulsed electron beam enables a coherent manipulation of the free-electron quantum state [2] by inelastic scattering in optical near-fields [3-5]. Upon traversal of an intense optical near-field, the free-electron kinetic energy spectrum develops sidebands that are separated by the photon energy, which can be attributed to a sinusoidal phase modulation of the electron wavefunction. This coherent electron-light interaction opens up the new research field of free-electron quantum optics.In this contribution, we discuss the results and applications of two experiments [6,7] featuring the coherent interaction with dual near-fields that differ in position (a) or in frequency (b). Fig. 1a illustrates the first experimental scheme: The electron beam sequentially interacts with two phase-locked optical near-fields, separated by a few micrometres in space. The final electron energy distribution sensitively depends on the relative phase between the two fields, which can be precisely controlled. The cancellation or enhancement of the first phase modulation by the second, i.e. its reversibility demonstrates the quantum coherence of this process. This experimental geometry represents a free-electron analogue of a Ramsey interferometer scheme [8].