Nonsymmetrized Correlations in Quantum Noninvasive Measurements

A long-standing problem in quantum mesoscopic physics is how to order operators in quantum noise expressions like where I(\omega) is the measured current at frequency \omega. Symmetrized ordering of operators describes a classical measurement while nonsymmetrized ordering corresponds to a quantum detector, sensitive to either emission or absorption of photons. We show that both ordering schemes can be embedded in quantum weak-measurement theory taking into account a finite interaction time between the system and the detector. Importantly, the nonsymmetrized ordering reveals its nonclassical nature already in second-order correlations, contrary to symmetrized ordering. This feature can be related to the squeezing of the many-body state of the transported electrons in a ac-driven tunnel junction