  Quantum non-demolition (QND) measurements improve sensitivity by evading
measurement back-action. The technique was first proposed to detect mechanical
oscillations in gravity wave detectors,and demonstrated in the measurement of
optical fields, leading to the development of rigorous criteria to distinguish
QND from similar non-classical measurements. Recent QND measurements of
macroscopic material systems such as atomic ensembles, and mechanical
oscillators, show some QND features, but not full QND character. Here we
demonstrate certified QND measurement of the collective spin of an atomic
ensemble. We observe quantum state preparation (QSP) and information-damage
trade-off (IDT) beyond their classical limits by seven and twelve standard
deviations, respectively. Our techniques complement recent work with
microscopic systems, and can be used for quantum metrology and memory, the
preparation and detection of non-gaussian states, and proposed quantum
simulation and information protocols. They should enable QND measurements of
dynamical quantum variables and the realization of QND-based quantum
information protocols.
