Ramsey-type phase control of free-electron beams

Quantum coherent evolution, interference between multiple distinct paths(1-4) and phase-controlled sequential interactions are the basis for powerful multi-dimensional optical(5) and nuclear magnetic resonance(3) spectroscopies, including Ramsey's method of separated fields(6). Recent developments in the quantum state preparation of free electrons(7) suggest a transfer of such concepts to ultrafast electron imaging and spectroscopy. Here, we demonstrate the sequential coherent manipulation of free-electron superposition states in an ultrashort electron pulse, using nanostructures featuring two spatially separated near-fields with polarization anisotropy. The incident light polarization controls the relative phase of these near-fields, yielding constructive and destructive quantum interference of the subsequent interactions. Future implementations of such electron-light interferometers may provide access to optically phase-resolved electronic dynamics and dephasing mechanisms with attosecond precision.