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 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.
@article{arxiv.cond-mat/0202082,
title = {Quantum nondemolition measurements of a qubit},
author = {D. V. Averin},
journal= {arXiv preprint arXiv:cond-mat/0202082},
year = {2009}
}