  We measure the sensitivity of a broadband atomic magnetometer using quantum
non-demolition spin measurements. A cold, dipole-trapped sample of rubidium
atoms provides a long-lived spin system in a non-magnetic environment, and is
probed non-destructively by paramagnetic Faraday rotation. The calibration
procedure employs a known reference state, the maximum-entropy or 'thermal'
spin state and quantitative imaging-based atom counting to identify electronic,
quantum, and technical noise in both the probe and spin system. The measurement
achieves sensitivity 2.8 dB better than the projection noise level (6dB better
if optical noise is suppressed) and will enable squeezing-enhanced broadband
magnetometry [Geremia, et al. PRL 94, 203002 (2005)].
