  We study the holographic map between long open strings, which stretch between
D-branes separated in the bulk space-time, and operators in the dual boundary
theory. We focus on a generalization of the Sakai-Sugimoto holographic model of
QCD, where the simplest chiral condensate involves an operator of this type.
Its expectation value is dominated by a semi-classical string worldsheet, as
for Wilson loops. We also discuss the deformation of the model by this
operator, and in particular its effect on the meson spectrum. This deformation
can be thought of as a generalization of a quark mass term to strong coupling.
It leads to the first top-down holographic model of QCD with a non-Abelian
chiral symmetry which is both spontaneously and explicitly broken, as in QCD.
Other examples we study include half-supersymmetric open Wilson lines, and
systems of D-branes ending on NS5-branes, which can be analyzed using
worldsheet methods.
