Rabi oscillations are periodic modulations of populations in two-level systems interacting with a time-varying field¹. They are ubiquitous in physics with applications in different areas such as photonics², nano-electronics³, electron microscopy⁴ and quantum information⁵. 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 approximation⁶. Although intense nanometre-wavelength light sources have been available for more than a decade^7,8,9, Rabi dynamics at such short wavelengths has not been directly observed. Here we show that femtosecond extreme-ultraviolet pulses from a seeded free-electron laser¹⁰ 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 interactions¹¹. 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 attosecond¹² and few-femtosecond¹³ extreme-ultraviolet pulses, our results unfold opportunities to carry out ultrafast manipulation of coherent processes at short wavelengths using free-electron lasers.

拉比振荡是与时变场相互作用的两能级系统中粒子群的周期性调制¹ 。它们在物理学中无处不在，并应用于不同领域，例如光子学² 、纳米电子学³ 、电子显微镜⁴和量子信息⁵ 。虽然拉比提出的理论是针对旋转磁场中的费米子，但奥特勒和汤斯意识到它也可以用来描述旋转波近似中的相干光与物质相互作用⁶ 。尽管强纳米波长光源已经存在十多年了^7,8,9 ，但如此短波长的拉比动力学尚未被直接观察到。在这里，我们展示了来自种子自由电子激光器¹⁰的飞秒极紫外脉冲可以驱动氦原子中基态和激发态之间的拉比动力学。测量的光电子信号揭示了奥特勒-汤斯双峰和避免的交叉，这两种现象都是相干原子场相互作用的基础¹¹ 。使用基于拉比模型的微扰理论推导的分析模型，我们发现双峰结构的超快构建带有共振和非共振光电离路径之间量子干涉效应的特征。鉴于最近出现了强阿秒¹²和几飞秒¹³极紫外脉冲，我们的研究结果为使用自由电子激光器在短波长下对相干过程进行超快操纵提供了机会。