The article reports on real-time observation of valence electron motion in atomic krypton ions using attosecond pump-probe spectroscopy. The researchers generated krypton ions with a controlled few-cycle laser field and probed them with an attosecond extreme-ultraviolet (EUV) pulse, allowing them to observe subfemtosecond electron motion over a multifemtosecond time span. By analyzing the density matrix of the valence electrons, they characterized the quantum mechanical electron motion and determined its coherence. The study reveals that strong-field ionization can create a broadband valence electron wave packet with high coherence that persists for much longer than 10 fs. The technique is applicable to molecules and solid-state materials, providing insights into the elementary electron motions that control physical, chemical, and biological properties and processes.The article reports on real-time observation of valence electron motion in atomic krypton ions using attosecond pump-probe spectroscopy. The researchers generated krypton ions with a controlled few-cycle laser field and probed them with an attosecond extreme-ultraviolet (EUV) pulse, allowing them to observe subfemtosecond electron motion over a multifemtosecond time span. By analyzing the density matrix of the valence electrons, they characterized the quantum mechanical electron motion and determined its coherence. The study reveals that strong-field ionization can create a broadband valence electron wave packet with high coherence that persists for much longer than 10 fs. The technique is applicable to molecules and solid-state materials, providing insights into the elementary electron motions that control physical, chemical, and biological properties and processes.