Observation of Rabi dynamics with a short-wavelength free-electron laser.

Rabi oscillations are periodic modulations of populations in two-level systems interacting with a time-varying field 1 . They are ubiquitous in physics with applications in different areas such as photonics 2 , nano-electronics 3 , electron microscopy 4 and quantum information 5 . While the theory developed by Rabi was intended for fermions in gyrating magnetic fields, Autler and Townes realized that it could also be used to describe coherent light-matter interactions within the rotating-wave approximation 6 . Although intense nanometre-wavelength light sources have been available for more than a decade 7-9 , Rabi dynamics at such short wavelengths has not been directly observed. Here we show that femtosecond extreme-ultraviolet pulses from a seeded free-electron laser 10 can drive Rabi dynamics between the ground state and an excited state in helium atoms. The measured photoelectron signal reveals an Autler-Townes doublet and an avoided crossing, phenomena that are both fundamental to coherent atom-field interactions 11 . Using an analytical model derived from perturbation theory on top of the Rabi model, we find that the ultrafast build-up of the doublet structure carries the signature of a quantum interference effect between resonant and non-resonant photoionization pathways. Given the recent availability of intense attosecond 12 and few-femtosecond 13 extreme-ultraviolet pulses, our results unfold opportunities to carry out ultrafast manipulation of coherent processes at short wavelengths using free-electron lasers.