Cavity-aided non-demolition measurements for enhanced spin squeezing*

Projection noise sets a maximum resolution for all sensors that use population measurements of unentangled atoms to sense a quantum phase -- the ``Standard Quantum Limit.'' This limitation can be overcome through the use of entangled, ``squeezed'' ensembles of atoms, allowing for potential improvements in sensor performance. We use quantum non-demolition measurements to prepare and directly observe spin-squeezed states with phase resolution 10.2(6)dB below the SQL, with no background subtraction.