Abstract
In quantum mechanics, the process of measurement is a subtle interplay between extraction of information and disturbance of the state of the quantum system. A quantum non-demolition (QND) measurement minimizes this disturbance by using a particular system—detector interaction that preserves the eigenstates of a suitable operator of the quantum system. This leads to an ideal projective measurement. We present experiments in which we carry out two consecutive measurements on a quantum two-level system, a superconducting flux qubit, by probing the hysteretic behaviour of a coupled nonlinear resonator. The large correlation between the results of the two measurements demonstrates the QND nature of the readout method. The fact that a QND measurement is possible for superconducting qubits strengthens the notion that these fabricated mesoscopic systems are to be regarded as fundamental quantum objects. Our results are also relevant for quantum-information processing for protocols such as state preparation and error correction.
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Acknowledgements
We thank M. Devoret, P. Bertet, T. Meunier, G. Nogues and J. M. Raimond for discussions, and R. N. Schouten for his contribution to the measurement system. Financial support was provided by the Dutch Foundation for Fundamental Research on Matter (FOM), the EU projects SQUBIT2 and EuroSQIP and the Dutch National Initiative on Nano Science and Technology NanoNed.
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Lupaşcu, A., Saito, S., Picot, T. et al. Quantum non-demolition measurement of a superconducting two-level system. Nature Phys 3, 119–123 (2007). https://doi.org/10.1038/nphys509
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DOI: https://doi.org/10.1038/nphys509
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