  The possibility of long-baseline quantum experiments in space makes it
necessary to better understand the time evolution of relativistic quantum
particles in a weakly varying gravitational field. We explain why conventional
treatments by traditional quantum optics and atomic physics based on quantum
mechanics may become inadequate when faced with issues related to locality,
simultaneity, signaling, causality, etc. Quantum field theory is needed. Adding
the effects of gravitation, we are led to Quantum Field Theory in Curved
Spacetime (QFTCST). This well-established theory should serve as the canonical
reference theory to a large class of proposed space experiments testing the
foundations of gravitation and quantum theory, and the basic notions of quantum
information theory in relativistic settings.
  This is the first in a series of papers treating near-term quantum optics and
matter waves experiments in space from the perspective of QFTCST. We analyze
the quantum motion of photons and of scalar massive particles using QFTCST with
application to interferometer experiments. Our main result is that, for
photons, the weak gravitational field is to leading order completely equivalent
to an inhomogeneous dielectric, thus allowing for a description of quantum
optics experiments in curved space using familiar notions from the theory of
optical media. We also discuss interference experiments that probe first-order
quantum coherence, the importance of a covariant particle detection theory, and
the relevance of time of arrival measurements. For massive particles with
internal structure, we identify a novel gravity-induced phase shift that
originates from the different gravitational masses attributed to the excited
internal states. This phase shift can in principle be measured in space
experiments.
