Coherence Perserving Quantum Nondemolition Measurements

We demonstrate that the collective vacuum Rabi splitting can be used to perform quantum nondemolition measurements on the clock states of $10^6$~$^{87}$Rb atoms in an optical cavity. We observe a 13(1)~dB increase in sensitivity over sampled measurements and a precision 8.6(2.6)~dB below the projection noise level. We infer the preparation of spin squeezed states with sensitivities 3.4(6)~dB below the standard quantum limit and directly observe a 1.1(4)~dB spectroscopic improvement. The measurement is enhanced using a large ensemble and may lead to more precise atomic sensors.