  The quest to have both which-path knowledge and interference fringes in a
double-slit experiment dates back to the inception of quantum mechanics (QM)
and to the famous Einstein-Bohr debates. In this paper we propose and discuss
an experiment able to spy on one photon's path with another photon. We modify
the quantum state inside the interferometer as opposed to the traditional
physical modification of the "wave-like" or "particle-like" experimental setup.
We are able to show that it is the ability to harvest or not which-path
information that finally limits the visibility of the interference pattern and
not the "wave-like" or "particle-like" experimental setups. Remarkably, a full
"particle-like" experimental setup is able to show interference fringes with
100 % visibility if the quantum state is carefully engineered.
