Discussing the particle analog of Thomas Young's classic double‐slit experiment, Richard Feynman wrote in 1964 that it “has in it the heart of quantum mechanics. In reality, it contains the only mystery.” That mystery is the one‐particle superposition principle. But Feynman's discussion and statement have to be generalized. Superposition may be the only true quantum mystery, but in multiparticle systems the principle yields phenomena that are much richer and more interesting than anything that can be seen in one‐particle systems.
We're just beginning to understand the ramifications of the superposition principle at the heart of quantum mechanics. Multiparticle interference experiments can exhibit wonderful new phenomena.