Step 1: Construct a tiny Cavendish pendulum with two solid gold BBs hanging from a silica fiber. A perturbation, perhaps a truck passing a kilometer off, or an earthquake near the Canary Islands, will set it oscillating, that is, swaying to and fro once every five minutes, so wait until the Christmas season when things are quiet. Set aside three weeks for observations. Step 0: Don't forget to do this in a hard vacuum: 0.0000000006 bar. PV = nRT n = (0.0000000006 bar)(1 liter) / (0.08314 L⋅bar/K⋅mol)(293 K) moles of gas n = 0.000000000024629 moles or just about a billion molecules of gas per liter flying around the hypersensitive pendulum in your vacuum chamber, hopefully not adding too much noise to the signal. Don't forget air springs and Viton™ rubber feet to dampen vibrations. Step 3, etc.: Jiggle a third tiny gold ball mounted on a thin wire near the pendulum very carefully to induce oscillations, and observe. Thanks, that was fascinating, but I didn't get far enough along to see how they imagine possibly scaling this down below the gram scale, to say nothing of the Planck mass.
In principle? It could go to the nanoscale, so tens of atoms, before tunneling and Casimir effect influence would be too much to ignore. In practice? I think there'll be a plateau around tens of thousands of atoms due to the electrical charge becoming a growing concern and quantum effects surfacing their ugly heads. I'm very happy you liked it.