Comment on "Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometers"

In a recent article Hensen et al. [Nature 526, 682 (2015)] report on a sophisticated Bell experiment, simultaneously closing, for the first time, loopholes for local hidden-variable theories (HVTs). The authors claim that 'local realism' has been refuted, under certain natural assumptions. The aim of the present Comment is twofold. First, it is urged that the class of local HVTs that is eliminated by the reported experiment should be specified in greater detail than is done by the authors. Second, it is argued that the class of local HVTs that still survives is wide and natural. For instance, hidden variables describing a 'background' field can exploit the freedom-of-choice loophole, which cannot be closed for this type of hidden variables. The dynamics of such background-based theories can be illustrated by existing systems, e.g. from fluid mechanics.