Mesoscopic quadratic quantum measurements

The quadratic quantum measurements were analyzed by using a mesoscopic detector. The output spectrum of a detector with both linear and quadratic response, continuously monitoring two qubits, was investigated. It was found that the quadratic measurements had nontrivial quantum information properties providing a simple way of entangling two noninteracting qubits. It was observed that quadratic measurement resulted in the spectral peak at frequency that was twice the frequency of individual qubit oscillations.