  We present an experimental demonstration of closed-loop quantum parameter
estimation in which real-time feedback is used to achieve robustness to
modeling uncertainty. By performing broadband estimation of a magnetic field
acting on hyperfine spins in a cold atom ensemble, we show that accuracy is not
compromised by fluctuations in total atom number even though the measured
signal in our canonical configuration depends only on the product of the field
and atom number. This methodology could be essential for efforts to utilize
conditional squeezing in spin-resonance measurements.
