Entanglement-assisted atomic clock beyond the projection noise limit

We use a quantum non-demolition measurement to generate a spin squeezed state and to create entanglement in a cloud of 10⁵ cold cesium atoms, and for the first time operate an atomic clock improved by spin squeezing beyond the projection noise limit in a proof-of-principle experiment. For a clock-interrogation time of 10 the experiments show an improvement of 1. 1 dB in the signal-to-noise ratio, compared to the atomic projection noise limit.