@johncarlosbaez

From the paper:

“Coincidence events within the time window narrower than 1 µs identify whether the same muons are detected by both the reference detector and the receiver detector.”

I don’t see how else you could obtain any information! The source is random, so the only way you can get any timing information is by detecting the same muon once as it passes through the surface detector, and then again by the detector below ground. Presumably the detector can register a muon without destroying it, because it has so much energy it can just lose a tiny bit to the detector and then continue on its way.

@gregeganSF - oh, okay, not quantum trickiness like a superposition of position eigenstates.

They say that one way to detect muons is with "plastic scintillators and photomultiplier tubes". I guess a muon travelling fast through plastic produces Cerenkov radiation. This is similar to how Ice Cube and Kamiokande detect neutrinos through the fast-moving charged particles they produce - though those experiments use huge amounts of water or ice.

@johncarlosbaez Muon detectors can be very small these days:

https://mdetect.com.au

There’s no scale on that picture, but I think it’s only a few centimetres across.

@johncarlosbaez

https://www.swinburne.edu.au/research/achievements-recognition/research-impact/inventions/

“A 10cm x 10cm prototype, which will be scaled up for commercial use, contains a piece of plastic scintillator, which re-emits energy absorbed from ionizing radiation as light … ”

But I don’t think they’ll need to scale it up to a kilometre. Muons are (obviously) much more detector-friendly than neutrinos.