title: High-fidelity adaptive qubit detection through repetitive quantum nondemolition measurements.
abstract: Using two trapped ion species ((27)Al(+) and (9)Be(+)) as primary and ancillary quantum systems, we implement qubit measurements based on the repetitive transfer of information and quantum nondemolition detection. The repetition provides a natural mechanism for an adaptive measurement strategy, which leads to exponentially lower error rates compared to using a fixed number of detection cycles. For a single qubit we demonstrate 99.94% measurement fidelity. We also demonstrate a technique for adaptively measuring multiple qubit states using a single ancilla, and apply the technique to spectroscopy of an optical clock transition.
authors.affiliation: Time and Frequency Division, National Institute of Standards and Technology, Boulder, Colorado 80305, USA.
authors.forename: D B
authors.initials: DB
authors.lastname: Hume
authors.forename: T
authors.initials: T
authors.lastname: Rosenband
authors.forename: D J
authors.initials: DJ
authors.lastname: Wineland