  This experiment was conceived of as a method of transmitting information from
inside a black hole to the outside. As it turns out, it doesn't work in the
form described (and possibly not in any form), but the way in which Nature
prevents quantum-mechanical effects from transmitting usable information using
quantum correlations is illuminating. In the process, one can learn some
quantum theory, as well as quantum optics.
  The proposed scheme uses a double-slit experiment, in the manner of the
Delayed Choice set up (see Kim et. al.), where the region where the
interference takes place (between "signal" photons) is spatially separated from
the region where the Delayed Choice (with "idler" photons) is made. Indeed,
this Double-Delayed Choice, which is this thought experiment, has one of the
idler photons slip inside the event horizon and serves as the method to attempt
to communicate from the inside to the outside.
