Quantum nondemolition measurements of a qubit.
The concept of quantum nondemolition (QND) measurement is extended to coherent oscillations in an individual two-state system. Such a measurement enables direct observation of an intrinsic spectrum of these oscillations avoiding the detector-induced dephasing that affects the standard (non-QND) measurements. The suggested scheme can be realized in Josephson-junction qubits which combine flux and charge dynamics.
Averin
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Department of Physics and Astronomy, SUNY Stony Brook, Stony Brook, New York 11794-3800, USA.