2017 年 45 巻 8 号 p. 493-
Coherent light-matter interaction drives a variety of interesting processes in a quantum system. Rabi oscillation is one such process offering a periodic population transfer between quantum states in an ultrashort time scale. Here we demonstrate a robust and simple scheme to achieve femtosecond twophoton Rabi oscillations by intense NIR laser pulses, using excited He atoms prepared by EUV-FEL. The extension to multiphoton regime will expand the applicability to highly excited or dipole-forbidden states.