  The Born rule provides a fundamental connection between theory and
observation in quantum mechanics, yet its origin remains a mystery. We consider
this problem within the context of quantum optics using only classical physics
and the assumption of a quantum electrodynamic vacuum that is real rather than
virtual. The connection to observation is made via classical intensity
threshold detectors that are used as a simple, deterministic model of photon
detection. By following standard experimental conventions of data analysis on
discrete detection events, we show that this model is capable of reproducing
several observed phenomena thought to be uniquely quantum in nature, thus
providing greater elucidation of the quantum-classical boundary.
