The cover page shows snapshots of a decaying Schrödinger cat Wigner function. This is a quasi-probability distribution for the amplitude of a microwave field stored in an ultra-high quality superconducting cavity. A single circular Rydberg atom interacting with a coherent state produces a quantum superposition of two fields with different phases. The two positive peaks in the Wigner function (front) are centered on the classical amplitudes of these components. The interference pattern in-between indicates the quantum nature of the superposition. As time proceeds (from front to rear), the interference pattern vanishes, revealing the rapid decoherence of a mesoscopic quantum state superposition.