  Parallel worlds are imaginative ideas in quantum mechanics and cosmology. The
superpositions of parallel worlds are novel states of quantum gravity and have
no classical correspondences generally. In this letter, we investigate the
superposition or the Schr\"odinger's cat state of optical parallel worlds,
which could be realized in laboratory and may shed some light on the detection
of parallel universes in a real world. We propose two realizable experimental
schemes, which enable to explore the mysterious `parallel universes' by a
Mach-Zehnder interferometer. The first one is based on an atomic ensemble in a
superposition state, which is a fat Schr\"odinger's cat state. The second one
is to prepare a photon in a superposition of different paths, where each path
lies in an optical parallel universe.
