I'm watching some lectures on Relativistic Quantum Information and in one of them - available here - the instructor is talking about measuring quantum fields.
He then says basically that in real experiments, we don't measure quantum fields directly with projective measurements as in non-relativistic quantum mechanics, but that we rather couple the system to a non-relativistic particle detector and then measure the detector.
If I'm getting it this will ultimately lead to things like the Unruh-deWitt detector.
Now in fact in the QFT course I've taken measurements on quantum fields weren't really covered, so I'm wondering about what he said.
My question here is the following: in practice, always in real world relativistic QFT experiments (like in LHC and other particle accelerators), is what he said true?
Does a measurement of a quantum field always occurs by coupling to a first-quantized system and measuring the first-quantized system?
To be honest, I find this quite weird, because if that's the case, then there would be a limitation imposed by the detector on what we can actually measure. This seems to never be mentioned in non-relativistic QM.
So if this is true, why it must be done that way? Why can't one do a projective measurement on a quantum field directly?