  Quantum superposition is the cornerstone of quantum mechanics, where
interference fringes originate in the self-interference of a single photon via
indistinguishable photon characteristics. Wheeler delayed-choice experiments
have been extensively studied for the wave-particle duality over the last
several decades to understand the complementarity theory of quantum mechanics.
The heart of the delayed-choice quantum eraser is in the mutually exclusive
quantum feature violating the cause-effect relation. Here, we experimentally
demonstrate the quantum eraser using coherent photon pairs by the delayed
choice of a polarizer placed out of the interferometer. Coherence solutions of
the observed quantum eraser are derived from a typical Mach-Zehnder
interferometer, where the violation of the cause-effect relation is due to
selective measurements of basis choice.
