Quantum sensors, like the Quantum Gravity Gradiometer Pathfinder (QGGPf) instrument developed by JPL, private companies and academic institutions, use clouds of atoms dropped in freefall and cooled to near absolute zero.
As they fall, lasers act like mirrors and splitters, separating the atoms and then bringing them back together. The way the atoms interfere when they recombine reveals how much they were accelerated by gravity, allowing scientists to measure the tiniest changes in gravity with extreme precision.
Huh, neat stuff. Seems like putting one in orbit would nicely take care of the "freefall" aspect.