People drilling Quantum Mechanics for meaning noticed fairly early that whereas Quantum Mechanics was strange enough for a single quantum particle, it was definitely way too strange for multiparticle systems. When it comes to things that are really profound progress tends to be slow. Recall that the Berry's paper about geometric phase shift was published in 1983 only.
Function is a complex-valued function on the configuration space of two particles.
Quantum potential Q does not necessarily fall with the distance between the two particles. The particles may therefore be strongly coupled even at long distances. There are no retardation terms in the quantum potential Q , so this non-local interaction is instananeous. But one can carry out similar reasoning in context of the Dirac equation and the non-locality will still be present. Yet, one can also demonstrate that the form of the Quantum Potential that arises in the relativistic theory will not allow signals to be transmitted faster than light.