  We demonstrate entanglement between the orbital angular momentum (OAM) of two
photoelectrons ionized via the strongly correlated process of non-sequential
double ionization (NSDI). Due to the quantization of OAM, this entanglement is
easily quantified and has a simple physical interpretation in terms of
conservation laws. We explore detection by an entanglement witness,
decomposable into local measurements, which strongly reduces the difficulty of
experimental implementation. We compute the logarithmic negativity measure,
which is directly applicable to mixed states, to demonstrate that the
entanglement is robust to incoherent effects such as focal averaging. Using the
strong-field approximation, we quantify the entanglement for a large range of
targets and field parameters, isolating the best targets for experimentalists.
The methodology presented here provides a general way to use OAM to quantify
and, in principle, measure entanglement, that is well-suited to attosecond
processes, can enhance our understanding and may be exploited in imaging
processes or the generation of OAM-entangled electrons.
