Measurement of Spin Projection Noise in Broadband Atomic Magnetometry

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)].