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Rabi oscillations are periodic modulations of populations in two-levelsystems interacting with a time-varying field. They are ubiquitous in physicswith applications in different areas such as photonics, nano-electronics,electron microscopy, and quantum information. While the theory developed byRabi was intended for fermions in gyrating magnetic fields, Autler and Townesrealized that it could also be used to describe coherent light-matterinteraction within the rotating wave approximation\cite. Although intensenanometer-wavelength light-sources have been available for more than a decade,Rabi dynamics at such short wavelengths have not been observed directly. Herewe show that femtosecond extreme-ultraviolet pulses from a seeded free-electronlaser can drive Rabi oscillations between the ground state and an excited statein helium atoms. The measured photoemission signal revealed an Autler-Townesdoublet as well as an avoided crossing, phenomena that are both trademarks ofquantum optics. Using theoretical analyses that go beyond the strong-fieldapproximation, we found that the ultrafast build-up of the doublet structurefollows from a quantum interference effect between resonant and non-resonantphotoionization pathways. Given the recent availability of intense attosecondand few-femtosecond extreme-ultraviolet pulses, our results offer opportunitiesto carry out ultrafast manipulation of coherent processes at short wavelengthsusing free-electron lasers.

Studying ultrafast Rabi dynamics with a short-wavelength seeded free-electron laser