title: Quantum nondemolition measurement of a superconducting qubit in the weakly projective regime.
abstract: Quantum state detectors based on switching of hysteretic Josephson junctions biased close to their critical current are simple to use but have strong backaction. We show that the backaction 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 nondemolition character, with a QND fidelity of 75%.
authors.affiliation: Kavli Institute of Nanoscience, Delft University of Technology, Post Office Box 5046, 2600GA Delft, The Netherlands.
authors.forename: T
authors.initials: T
authors.lastname: Picot
authors.forename: R
authors.initials: R
authors.lastname: Schouten
authors.forename: C J P M
authors.initials: CJ
authors.lastname: Harmans
authors.forename: J E
authors.initials: JE
authors.lastname: Mooij