I'm working my way through the paper Observable quantum entanglement due to gravity
Equation 2 is a Taylor expansion of Newtonian gravity potential. They say the second term is bi-local and so cannot lead to quantum entanglement, and the third term couples the two test masses/leads to entanglement between them. Here's a snapshot of the equation, where $L$ is the separation between test masses and XA and XB their displacements from equilibrium:
Can you please tell me what bi-local means here? Maybe more importantly why can it not cause entanglement? Because it does not include a product of the form XA * XB?
The reason the 3rd term entangles the two masses is because when written out, it includes a XA * XB term. Correct?
Any further insight into why product terms are associated with entanglement between the masses will be much appreciated.