One of the great mysteries of modern physics is the link between quantum mechanics and general relativity or gravity. But quantum phenomena generally occur on the very smallest scales while gravity generally crops on the largest scales. Never the twain shall meet.
At least, not without some clever thinking. One idea is to entangle a pair of photons, hang on to one and send the other across a distance so vast that gravity is significant, in other words, far enough for the gravitational curvature of space to come into play.
The issue in question is whether the entanglement--a purely quantum phenomenon--'feels' this curvature in the same way as purely classical things, like humans.
The required distance isn't that far--a few hundred kilometres should do the trick.