Not yet reviewed by Pith; the record is open.

Here we introduce the concept of the twinning field -- a driving electromagnetic pulse that induces an identical optical response from two distinct materials. We show that for pairs of generic many-body systems, a twinning field which renders the systems \emph{optically indistinguishable} always exists. The conditions under which this field is unique are derived, and this analysis is supplemented by numerical calculations of twinning fields for the Fermi-Hubbard model. The universal existence of twinning fields may lead to new research directions in non-linear optics, materials science, and quantum technologies.