  Alternative theories to quantum mechanics motivate important fundamental
tests of our understanding and descriptions of the smallest physical systems.
Here, using spontaneous parametric downconversion as a heralded single-photon
source, we place experimental limits on a class of alternative theories,
consisting of classical field theories which result in power-dependent
normalized correlation functions. In addition, we compare our results with
standard quantum mechanical interpretations of our spontaneous parametric
downconversion source over an order of magnitude in intensity. Our data match
the quantum mechanical expectations, and do not show a statistically
significant dependence on power, limiting on quantum mechanics alternatives
which require power-dependent autocorrelation functions.
