  It has been suggested by Sorkin that a three-slit Young experiment could
reveal the validity a fundamental ingredient in the foundations of one of the
cornerstones in modern physics namely quantum mechanics. In terms of a certain
parameter $\kappa_S$, it was argued that a non-zero value could imply a
breakdown of the fundamental Born's rule as well as the superposition
principle. Here we argue that a physical realization of such arguments could
lead to an erroneous conclusion and contradict the basic rules of quantum
mechanics. In fact, we argue that a proper interpretation of the procedures
involved in a physical determination of $\kappa_S$ does not necessarily lead to
$\kappa_S=0$. In order to show this we consider a mono-chromatic source of
photons prepared in an {\it arbitrary} quantum state and a simple version of
the well-established photon detection theory of Glauber which, by construction,
obeys all the rules of quantum mechanics. It is, however, also argued that
after a proper identification of the relevant quantum-mechanical probability
amplitudes one can be reach $\kappa_S=0$. As long as one only consider a single
photon detector, it is verified that, in this context, there is no fundamental
difference between quantum-mechanical interference and interference as
expressed in terms of classical electro-magnetic waves.
