Quantum state detectors based on switching of hysteretic Josephson junctions biased close to their critical current are simple to use but have strong back-action. We show that the back-action of a DC-switching detector can be considerably reduced by limiting the switching voltage and using a fast cryogenic amplifier, such that a single readout can be completed within 25 ns at a repetition rate of 1 MHz without loss of contrast. Based on a sequence of two successive readouts we show that the measurement has a clear quantum non-demolition character, with a QND fidelity of 75 %.
@article{arxiv.1007.0166,
title = {QND measurement of a superconducting qubit in the weakly projective regime},
author = {T. Picot and R. Schouten and C. J. P. M. Harmans and J. E. Mooij},
journal= {arXiv preprint arXiv:1007.0166},
year = {2015}
}