  The traditional view from particle physics is that quantum gravity effects
should only become detectable at extremely high energies and small length
scales. Due to the significant technological challenges involved, there has
been limited progress in identifying experimentally detectable effects that can
be accessed in the foreseeable future. However, in recent decades, the size and
mass of quantum systems that can be controlled in the laboratory have reached
unprecedented scales, enabled by advances in ground-state cooling and
quantum-control techniques. Preparations of massive systems in quantum states
paves the way for the explorations of a low-energy regime in which gravity can
be both sourced and probed by quantum systems. Such approaches constitute an
increasingly viable alternative to accelerator-based, laser-interferometric,
torsion-balance, and cosmological tests of gravity. In this review, we provide
an overview of proposals where massive quantum systems act as interfaces
between quantum mechanics and gravity. We discuss conceptual difficulties in
the theoretical description of quantum systems in the presence of gravity,
review tools for modeling massive quantum systems in the laboratory, and
provide an overview of the current state-of-the-art experimental landscape.
Proposals covered in this review include, among others, precision tests of
gravity, tests of gravitationally-induced wavefunction collapse and
decoherence, as well as gravity-mediated entanglement. We conclude the review
with an outlook and discussion of future questions.
