Comment on "Brilliant source of 19.2 attosecond soft X-ray pulses below the atomic unit of time"

Abstract

A recent preprint by Ardana-Lamas et al. (Ref. [1], arXiv:2510.04086) re-analyzes the data set of the attosecond streaking experiment on krypton atoms originally reported in Phys Rev X 7, 041030 (2017) [2], and claims the characterization of a 19.2 attosecond light pulse with an overall photon flux of 4.8*10^10 photons per second. In this comment, we highlight a series of physical and technical issues concerning both the original experiment [2] and the new characterization [1]. Specifically, without accounting for the contribution of Auger electrons or employing filters to remove low-energy harmonics and compensate for the intrinsic pulse chirp, the authors attribute the measured photoelectrons predominantly to the 3d inner-shell orbital of krypton and claim to have retrieved a nearly chirp-free pulse based on the single atomic orbital. In comparison with our recent attosecond streaking experiments on krypton atoms, these physical and technical issues raise significant doubts about the validity of the characterization results.