  The employment of path entangled multiphoton states enables measurement of
phase with enhanced precision. It is common practice to demonstrate the unique
properties of such quantum states by measuring super-resolving oscillations in
the coincidence rate of a Mach-Zehnder interferometer. Similar oscillations,
however, have also been demonstrated in various configurations using classical
light only; making it unclear what, if any, are the classical limits of this
phenomenon. Here we derive a classical bound for the visibility of
super-resolving oscillations in a Mach-Zehnder. This provides an easy to apply,
fundamental test of non-classicality. We apply this test to experimental
multiphoton coincidence measurements obtained using photon number resolving
detectors. Mach-Zehnder super-resolution is found to be a highly distinctive
quantum effect.
