  A realizable delayed-choice quantum eraser, using a modified Mach-Zehnder
(MZ) interferometer and polarization entangled photons, is theoretically
analyzed here. The signal photon goes through a modified MZ interferometer, and
the polarization of the idler photon provides path information for the signal
photon. The setup is very similar to the delayed-choice quantum eraser
experimentally studied by the Vienna group. In the class of quantum erasers
with discrete output states, it is easy to see that the delayed mode leaves no
choice for the experimenter. The which-way information is always erased, and
every detected signal photon fixes the polarization state of the idler, and
thus gives information on precisely how the signal photon traversed the two
paths. The analysis shows that the Vienna delayed-choice quantum eraser is the
first experimental demonstration of the fact that the delayed mode leaves no
choice for the experimenter, and the which-way information is always erased.
Additionally it is shown that this argument holds even in a conventional
two-slit quantum eraser. Every photon registered anywhere on the screen, fixes
the state of the two-state which-way detector in a unique mutually unbiased
basis. In the delayed-choice quantum eraser experiments, the role of mutually
unbiased basis sets for the which-way detector, has been overlooked till now.
