Within second quantization say we have an arbitrary many-particle state. What is then the expectation value of a creation-/ annihilation Operator in the case of the particles being fermions/bosons?
I am reading my Professors lecture notes on second quantization and at one point I think he uses, that in the fermionic case said expectation values vanish, but for bosons don't, but I can't find a way to prove it to myself.
Edit: After some research I reckon it has something to do with coherent states?