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 internal quantum coherence within the individual parent ion
and photoelectron systems. However, quantum correlations between these two subsystems emerging
from attosecond photoionization events 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 for the direct observation of entanglement in the context of ultrafast photoionization
of many-electron systems. Our work provides a novel perspective on attosecond physics directed
toward the detection of quantum correlations between systems born during attosecond photoionization and unraveling the signatures of entanglement in ultrafast coherent molecular dynamics,
including in the chemical decomposition pathways of molecular ions.