Robust preparation noncontextuality inequalities in the simplest scenario

Contextuality is the leading notion of nonclassicality for a single system. However, an experimental demonstration requires finding procedures that are operationally equivalent, which might seem impossible to achieve exactly. Here I focus on the simplest nontrivial case, four preparations and two tomographically complete binary measurements. Exploiting a subtle connection to the Clauser-Horne-Shimony-Holt scenario gives eight nonlinear inequalities which are together necessary and sufficient for the experimental statistics to admit a preparation noncontextual model in such a scenario. No fixed operational equivalences are required, removing a key difficulty with experimental tests of older preparation noncontextuality inequalities.