Attosecond (10−18s) science has emerged from the study of the interaction of matter with intense laser fields, and it is the natural successor of femtochemistry [1]. Its main objective is to resolve and ultimately control electron dynamics in real time. Electrons play a key role in photosynthesis, destroy or modify molecules, and can be used for information processing in human-made devices. This wide range of applications and huge potential for attosecond imaging of matter has made it a hugely popular and expanding area. Unfortunately, this rapid growth has led to a vast, fragmented methodological landscape, with many conflicting views. In order to explore these issues in a controlled way and to provide a debating platform for early career researchers with a strong training component, we propose a three-day workshop focusing on quantum aspects of attoscience. areas of tension are:
1) Tunneling, quantum interference and non-classicality. Quantum interference has been vital in strong-field physics since its inception, and, with the development of imaging techniques, also in ultrafast spectroscopy. While extracting phase differences between specific electron pathways for use in target reconstruction is a motivation for many pump-probe schemes [2] and ultrafast photoelectron holography [3], the connection with quantum sensing is, as yet, unclear. The criteria for tunneling ionisation are hotly contested [4], and there is no agreement on definitions for classical, semi-classical or quantum approaches. Notions of nonclassicality are much fuzzier than in quantum optics or semi-classical theory and need to be developed. Finally, decoherence has also remained largely unexplored. This is important for macroscopic effects and extended targets.
2) Bound- and continuum dynamics and electron-electron correlation. For complex targets, electron-electron correlation plays a key role. There are however, no systematic studies of whether this correlation is classical, or quantum, and classical approaches are used to describe strongly correlated dynamics without much justification. However, correlated electron dynamics in strong fields may involve processes for which there may be no classical counterpart. Examples are an appropriate treatment of bound states, the continuum, excitation, relaxation and electron-hole migration.

3) Analytical vs ab-initio methods and Coulomb-distorted approaches. Moving from an unstructured to a structured continuum and incorporating bound-state dynamics has led to myriad approaches, whose efficacy, interpretational power and validity range vary. Key questions related to analytic orbit-based approaches are how to incorporate sub-barrier corrections and Coulomb distortions consistently, account for excited states and electron recapture. For methods with complex trajectories, the presence and regularization of branch cuts and singularities has also been a contentious issue [5].