Simultaneous tracking of spin angle and amplitude beyond classical limits

Measurement of spin precession is central to extreme sensing in physics(1,2), geophysics(3), chemistry(4), nanotechnology5 and neuroscience6, and underlies magnetic resonance spectroscopy(7). Because there is no spin-angle operator, any measurement of spin precession is necessarily indirect, for example, it may be inferred from spin projectors at different times. Such projectors do not commute, and so quantum measurement back-action-the random change in a quantum state due to measurement-necessarily enters the spin measurement record, introducing errors and limiting sensitivity. Here we show that this disturbance in the spin projector can be reduced below N-1/2-the classical limit for N spins-by directing the quantum measurement back-action almost entirely into an unmeasured spin component. This generates a planar squeezed state(8) that, because spins obey non-Heisenberg uncertainty relations(9,10), enables simultaneous precise knowledge of spin angle and spin amplitude. We use high-dynamic-range optical quantum non-demolition measurements(11-13) applied to a precessing magnetic spin ensemble to demonstrate spin tracking with steady-state angular sensitivity 2,9 decibels below the standard quantum limit, simultaneously with amplitude sensitivity 7.0 decibels below the Poissonian variance14. The standard quantum limit and Poissonian variance indicate the best possible sensitivity with independent particles. Our method surpasses these limits in non-commuting observables, enabling orders-of-magnitude improvements in sensitivity for state-of-the-art sensing(15-18) and spectroscopy(19,20).