  Attosecond physics enables the study of ultrafast coherent electron dynamics
in matter upon photoexcitation and photoionization, revealing spectacular
effects such as hole migration and coherent Auger dynamics in molecules. In the
photoionization scenario, there has been a strong focus on probing the physical
manifestations of the internal quantum coherence within the individual parent
ion and photoelectron systems. However, quantum correlations between these two
subsystems emerging from the attosecond photoionization event have thus far
remained much more elusive. In this work, we design theoretically and model
numerically a direct probe of quantum entanglement in attosecond
photoionization in the form of a Bell test. We simulate from first principles a
Bell test protocol for the case of noble gas atoms photoionized by ultrashort,
circularly polarized infrared laser pulses in the strong-field regime
predicting robust violation of the Bell inequality. This theoretical result
paves the way to the direct observation of entanglement in the context of
ultrafast photoionization of many-electron systems. Our work provides a
different perspective on attosecond physics directed towards the detection of
quantum correlations between systems born during attosecond photoionization and
unravelling the signatures of entanglement in the ultrafast coherent molecular
dynamics, including in the chemical decomposition pathways of molecular ions.
