What makes an object a detector in QM? By introducing a detector in one path of a Mach Zehnder MZI the state of a photon is changed from superposition in both paths to a particle. But what can serve as a detector?
For example, an impenetrable plate? Must it generate an observable signal for humans, e.g., a flash, or sound? What if there are no humans to hear or see? What if they don't notice the signal?
 A: The detector enforces decoherence, unlike, e.g., beam-splitters and mirrors. Maintaining coherence is a subtle & difficult art; while decoherence comes naturally. But consciousness is a canard!

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*The measured state is not interfering with the states excluded by the measurement.

There are people spending a lifetime on consciousness rabbit-holes, but you must be aware of the following quotes by professionals, which people start their physics education with. (I did, half a century ago.)

"Of course the introduction of the observer must not be misunderstood to imply that some kind of subjective features are to be brought into the description of nature. The observer has, rather, only the function of registering decisions, i.e., processes in space and time, and ''it does not matter whether the observer is an apparatus or a human being''; but the registration, i.e., the transition from the "possible" to the "actual," is absolutely necessary here and cannot be omitted from the interpretation of quantum theory."
Werner Heisenberg, ''Physics and Philosophy'', p. 137


"Was the wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some highly qualified measurer - with a PhD?"
J  S  Bell, 1981, ''Quantum Mechanics for Cosmologists''. In C J Isham, R Penrose and D W Sciama (eds.),
''Quantum Gravity 2: A second Oxford Symposium'', p 611 . (Oxford: Clarendon Press).


According to standard quantum mechanics, it is a matter of complete indifference whether the experimenters stay around to watch their experiment, or instead leave the room and delegate observing to an inanimate apparatus which amplifies the microscopic events to macroscopic measurements and records them by a time-irreversible process
"Speakable and Unspeakable in Quantum Mechanics: Collected Papers on Quantum Philosophy",  p 170,  J S  Bell,  Cambridge University Press (2004) ISBN 9780521523387


"Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not."
"The Feynman Lectures on Physics, Vol. III",  Ch 3.2, R P Feynman(Basic Books, 2015) ISBN 9780465040834

A: CosmasZachos provided a good answer already.
Here I note the following.
There are two ways to treat detection (aka measurement aka observation) in quantum theory. Either you just say "the system is measured in such-and-such a basis" or you explicitly include the interaction with the detector or other instrument in the calculation of the evolution of system+detector. The second way is equivalent to the first once the evolution reaches a point at which irreversible processes have caused the result of the detection or measurement to be recorded in a way that will never be totally undone (in the sense of a complete reversal of the evolution and consequent entire erasure of the record). By "equivalent" here I mean "statistically equivalent". Both methods give statistical predictions.
A: A detector is something that takes some or all of the energy of a particle and makes the energy do work on the materials of the detector.
