A new way to make quantum light skips external lasers

Researchers are now harnessing a previously overlooked aspect of electron behavior, propagation-induced quadratic phase, to generate and control quantum light without relying on external lasers. This new approach bypasses the complexity and instability risks of current seeded Free-Electron Lasers, which couple modulation depth to spectral bandwidth and require multistage phase-space manipulation. Unlike quantum Free-Electron Lasers demanding exceptionally narrow initial electron energy spreads, this method uses the Talbot effect to program quantum states through free propagation. The work establishes a unified testbed for coherent short-wavelength emission and nonclassical-light synthesis, rooted in the coherent evolution of a quantum wave packet itself. Free-space propagation distance directly controls both the harmonic content of attosecond electron emission and the number of components in synthesized Schrödinger cat states, a team has demonstrated. This control is achieved through a technique termed quadratic-phase programming, which uses the intrinsic quadratic dispersion of electron wave packets to sculpt both electron density and quantum states without relying on external modulators or material nonlinearities. The method begins by encoding a coherent momentum-state ladder onto an electron wave packet using photon-induced near-field electron microscopy, or PINEM, a process that produces a phase-coherent ladder of photon-spaced energy…