  We propose a scheme for creating quantum superposition states involving of
order $10^{14}$ atoms via the interaction of a single photon with a tiny
mirror. This mirror, mounted on a high-quality mechanical oscillator, is part
of a high-finesse optical cavity which forms one arm of a Michelson
interferometer. By observing the interference of the photon only, one can study
the creation and decoherence of superpositions involving the mirror. All
experimental requirements appear to be within reach of current technology.
