  It is known that the overlap of two energy eigenstates in a decaying quantum
system is bounded from above by a function of the energy detuning and the
individual decay rates. This is usually traced back to the positive
definiteness of an appropriately defined decay operator. Here, we show that the
weaker and more realistic condition of positive semi-definiteness is
sufficient. We prove also that the bound becomes an equality for the case of
single-channel decay. However, we show that the condition of positive
semi-definiteness can be spoiled by quantum backflow. Hence, the overlap of
quasibound quantum states subjected to outgoing-wave conditions can be larger
than expected from the bound. A modified and less stringent bound, however, can
be introduced. For electromagnetic systems, it turns out that a modification of
the bound is not necessary due to the linear free-space dispersion relation.
Finally, a geometric interpretation of the nonorthogonality bound is given
which reveals that in this context the complex energy space can seen as a
surface of constant negative curvature.
