Quantum non-demolition measurement of a superconducting two-level system

In quantum mechanics, measurement can be understood as the interplay between extraction of information and disturbance of the state of the measured system. For projective measurements this disturbance is minimized: the post-measurement state is fully correlated to the indication of the detector. Quantum non-demolition (QND) detection is a strategy used to implement a projective measurement, which relies on a specific type of interaction between the measured system and the detector. In our experiments we apply these principles to the measurement of a superconducting flux qubit, which is an artificial two-level system built using mesoscopic Josephson junctions. Our detection method relies on probing the response of a hysteretic non-linear resonator coupled to the qubit. This setup allows for very efficient detection of the state of our system, with a measured contrast of 85{\%}. The large correlations between the results of two consecutive measurements demonstrate the QND nature of this method. This result establishes the validity of a QND strategy for projective measurement of superconducting qubits and has implications for quantum information processing.