  The delayed-choice quantum eraser has long been a subject of controversy, and
has been looked at as being incomprehensible to having retro-causal effect in
time. Here the delayed-choice quantum eraser is theoretically analyzed using
standard quantum mechanics. Employing Mach-Zehnder interferometer, instead of a
conventional two-slit interference, brings in surprising clarity. Some common
mistakes in interpreting the experiment are pointed out. It is demonstrated
that in the delayed mode there is no which-way information present after the
particle is registered on the screen or the final detectors, contrary to
popular belief. However, it is shown that another kind of path information is
present even after the particle is registered in the final detectors. The
registered particle can be used to predict the results of certain yet to be
made measurements on the which-way detector. This novel correlation can be
tested in a careful experiment. It is consequently argued that there is no big
mystery in the experiment, and no retro-causal effect whatsoever.
