  No experiment to date has provided evidence for quantum features of the
gravitational interaction. Recently proposed tests suggest looking for the
generation of quantum entanglement between massive objects as a possible route
towards the observation of such features. Motivated by advances in optical
cooling of mirrors, here we provide a systematic study of entanglement between
two masses that are coupled gravitationally. We first consider the masses
trapped at all times in harmonic potentials (optomechanics) and then the masses
released from the traps. This leads to the estimate of the experimental
parameters required for the observation of gravitationally induced
entanglement. The optomechanical setup demands LIGO-like mirrors and squeezing
or long coherence times, but the released masses can be light and accumulate
detectable entanglement in a timescale shorter than their coherence times. No
macroscopic quantum superposition develops during the evolution. We discuss the
implications from such thought experiments regarding the nature of the
gravitational coupling.
