  Arrays of atoms coupled to waveguides can behave as mirrors. We consider an
array of $\Lambda$-type three-level atoms wherein preparing the atoms in one
ground state or another leads to reflection or transmission of the guided
electromagnetic field; a superposition of the two ground states thus
corresponds to a coherent superposition of mirror-like and transparent boundary
conditions. We analyze the spontaneous emission of an excited two-level atom in
the presence of such a quantum mirror, and inside a cavity formed by quantum
mirrors, demonstrating that the resulting dynamics of the excited atom can
exhibit exotic features, e.g., a superposition of Rabi cycle and exponential
decay. Our results pave the way for exploring quantum electrodynamics (QED)
phenomena in a paradigm wherein boundary conditions can exhibit quantum
superpositions and correlations.
