Coherence analysis of the observed delayed-choice quantum eraser using coherent photons

Quantum eraser is known as one of the mysterious quantum features, where the action of the delayed choice retrospectively determines the photon's nature. Over the last decades, various photon characteristics have been used to demonstrate the quantum eraser, especially for violating the cause-effect relation. In this manuscript, recent observations of the quantum eraser based on a Mach-Zehnder interferometer (MZI) using an attenuated laser are speculated as the origin of the quantum mystery, where the delayed choice is conducted for the MZI output photons via polarization projection measurements. For this, the wave nature of a photon is taken for the present pure coherence optics-based analysis. By the definition of group velocity-based information, the violation of the cause-effect relation in the quantum eraser should be related to the ensemble coherence of measured photons, but not to individual photon's coherence, where the ensemble coherence is dramatically decreased as the spectral bandwidth of the photon ensemble increases.