  A long-standing problem in quantum mesoscopic physics is which operator order
corresponds to noise expressions like <I(-\omega)I(\omega)>, where I(\omega) is
the measured current at frequency \omega. Symmetrized order describes a
classical measurement while nonsymmetrized order corresponds to a quantum
detector, e.g., one sensitive to either emission or absorption of photons. We
show that both order schemes can be embedded in quantum weak-measurement theory
taking into account measurements with memory, characterized by a memory
function which is independent of a particular experimental detection scheme. We
discuss the resulting quasiprobabilities for different detector temperatures
and how their negativity can be tested on the level of second-order correlation
functions already. Experimentally, this negativity can be related to the
squeezing of the many-body state of the transported electrons in an ac-driven
tunnel junction.
