Until now, all metrological-grade clocks have used uncorrelated atoms, characterized by a statistical noise called "quantum projection noise", which fundamentally limits their performance. It is known that this limit can be overcome by using a form of quantum entanglement called "spin squeezing".
However, the lifetime of spin squeezed states produced in the laboratory is typically several orders of magnitude shorter than that required for metrological grade instruments.
Being able to preserve the brittle quantum state for such a long time also enables us to better understand the properties of its "real life", with surprising results : the clock signal produced by the intricate state amplifies with time, reaching more than 4 times its expected value, as demonstrated by the researchers.
The effect is explained by extremely weak interactions between the atoms - so weak that they remained invisible in previous short-lived experiments. These interactions conspire with the quantum correlation created to amplify the coupling with the optical microcavity used to detect the atoms.
The microwave clock used in this experiment operates in the regime of the new generation of compact clocks for Global Navigation System satellites, triggering growing hopes for quantum improvements in compact clocks in the near future.
