  Quantum mechanics is central to coherent photoproduction in ultra-peripheral
collisions (UPCs). This writeup will discuss some surprising aspects of UPCs
that stem from these quantum mechanical roots. The Good-Walker (GW) paradigm,
which connects coherent photoproduction with the target nucleus remaining in
its ground state. This contrasts with a semi-classical picture, where coherence
depends on the positions of the individual nucleons and the momentum transfer.
Unlike the GW approach, the semiclassical picture is consistent with the
observed data on coherent photoproduction with nuclear breakup, and with
coherent photoproduction in peripheral collisions. The semiclassical approach
allows for a wider variety of coherent UPC reactions, such as coherent
photoproduction of charged mesons, including some non $q\overline q$ exotica.
Quantum mechanics is also key to the coherent photoproduction of multiple
vector mesons by the interactions of a single ion pair. The vector mesons share
a common impact parameter, and so can exhibit richer interference patterns than
single mesons. At forward rapidities, the cross sections to produce multiple
identical vector mesons are enhanced due to superradiance. With enough
statistics, multi-meson events may provide an opportunity to observe stimulated
decays.}
