  The interference pattern in electron double-slit diffraction is a hallmark of
quantum mechanics. A long standing question for stochastic electrodynamics
(SED) is whether or not it is capable of reproducing such effects, as
interference is a manifestation of quantum coherence. In this study, we use
excited harmonic oscillators to directly test this quantum feature in SED. We
use two counter-propagating dichromatic laser pulses to promote a ground-state
harmonic oscillator to a squeezed Schr\"{o}dinger cat state. Upon recombination
of the two well-separated wavepackets, an interference pattern emerges in the
quantum probability distribution but is absent in the SED probability
distribution. We thus give a counterexample that rejects SED as a valid
alternative to quantum mechanics.
