I've seen this statement in many papers. However the function $e^{-iHt}$ satisfies the Cauchy-Riemann condition for all t. So why do we say $e^{-iHt}$ is holomorphic in the lower half plane specifically?
Does this have something to do with infinity or does "holomorphic" imply "bounded"?
For the statement, see for example:
arXiv:1803.04993https://arxiv.org/abs/1803.04993 below eq.(2.7)
arXiv:0710.5373https://arxiv.org/abs/0710.5373 below eq.(2.62)