Title: Simultaneous tracking of spin angle and amplitude beyond classical limits,
Abstract: We show how simultaneous, back-action evading tracking of non-commuting
observables can be achieved in a widely-used sensing technology, atomic
interferometry. Using high-dynamic-range dynamically-decoupled quantum
non-demolition (QND) measurements on a precessing atomic spin ensemble, we
track the collective spin angle and amplitude with negligible effects from back
action, giving steady-state tracking sensitivity 2.9 dB beyond the standard
quantum limit and 7.0 dB beyond Poisson statistics.