Fig. 3: Resolving electronic coherences in neon.
a, Illustration of the pulse sequence in SPRINT, focusing on large delays Δt. Transient absorption relies on resolving the interference between the excitation pulse (magenta) and the nonlinear generated field (purple). Their large temporal separation reduces their interference signal, hindering its detection. In contrast, SPRINT resolves the interference of the generated field with the external reference, both being synchronized to the IR pulse, enhancing the detection of coherences at large delays. b, Harmonic 13 of an APT resonantly excites a wavepacket, composed of several spin–orbit split excited states in neon in the range of 20.5–20.8 eV. A delayed IR pulse maps its coherent evolution onto harmonic 15 via four-wave mixing. c, Fourier analysis of the SPRINT result, focusing on this four-wave-mixing process, revealing the coherent contribution of multiple electronic resonances. The inset presents a broader view of the horizontal axis, also showing the linear interferometric signal. d, Reconstruction of the evolution of electronic coherences with delay across multiple timescales. The measurement reflects quantum beats over tens of femtoseconds, 2ωIR oscillations corresponding to the four-wave-mixing timescale and attosecond beating at the XUV frequency itself (inset).
