Attosecond pulse trains (APT) emerged from high harmonics (Anne L’Huillier et al, 1987) and their spectral phase determination by RABBITT (Pierre-Marie Paul et al, 2001). Although the record of the shortest pulse of 43 attoseconds belongs to the technique of isolated attosecond pulse (IAP) [1], APT and RABBITT have been extremely popular over the last 20 years. In this talk I will briefly review the physics involved in both as well as some applications at Ohio-State including photoionization delays, and the recent timing of non-sequential double ionization [2]. In conclusion I will evoke the advent of zeptoseconds in photoionization delays [3] and the future of attosecond science high-power IAP at free-electron laser [4].

• [1] T. Gaumnitz et al., Opt. Express 25: 305, (2017).
• [2] A. Piper et al, submitted (2024).
• [3] S. Grundmann et al Science, 370: 339 (2020).
• [4] J. Duris et al Nature Photonics 14 :30 (2020).