Just put it on top of one of the LIGO mirrors? :P
If you magically got an couple of order of magnitudes improvement (~0.5um), would you have to start to worry about van der Waals forces, or is that still weak enough? Or does it not matter at all for this kind of apparatus?
Regarding van der Waals -- yes. At the moment, we block all electrostatic/Casimir interactions with a metal foil. When the distances involved are "sufficiently short", we won't get complete screening any more. That day will be an interesting day.
On the bright side, if we start to see Casimir leakage, that will be a boon to the people who try to understand the Casimir force at finite temperature (another open problem), so we'll learn something useful no matter what...
In the short term, though, I'd love to have that problem. It will take a lot of work for us to get there.
Speaking of, do you have a link to share of a recorded presentation about this or one of the previous experiments?
For some of our experiments, we need the hill we're built into, and we can handle the tilts.
This short-range measurement was greatly benefited by a fortuitous snowstorm that brought Seattle's buses to a halt right as John was taking data with the fully-commissioned instrument. If we had foreseen with 6-8 weeks of foresight how much of a societal disruption Covid-19 would cause, we would have crushed it to get this short-range instrument back in operation in time.
We're working on new seismic-isolation systems for our next upgrade. If that doesn't work out, we'll be looking for a remote hole in the ground to put a short-range instrument in.
The big steps forward on this cycle were improved alignment systems, improved flatness of multiple critical parts, improved cleanliness, a new gluing arrangement that reduced Newtonian-gravity systematic uncertainty, even greater care to ensure non-magnetic fabrication, and another ~5 years of experience with the instrument.
The next big jump for us is active seismic isolation. A small improvement there, with other tweaks, may buy us another ~5-10 microns.