Two-photon Resolved-Sideband Raman Cooling of171Yb in an optical cavity.

Atomic collision is an essential ingredient to cool trapped clouds to ultracold temperatures via an evaporative cooling process. Without it, such as the cases of cooling optical-lattice clock atoms like 171Yb and 88Sr, it is rather challenging to reach such low temperatures.

Moreover, it has been recently demonstrated that high-finesse optical cavities can significantly enhance the performances of optical lattice clocks (through quantum nondemolition measurements and/or many-body entanglement). The strong coupling between the optical cavity and the atomic ensemble represents another complication in the cooling process.

We demonstrate a resolved Raman sideband cooling of an ensemble of 171Yb coupled to a high-finesse optical cavity, without the assistance of atomic collisions. The atomic temperature in the tightly binding direction reaches1.8±0.5μK with average vibration numbern= 0.2 in 200 ms, while that of the other direction remains 8μK. Such low temperature enables us to perform an optical Rabi π-pulse with transfer probability exceeding 93% on the ultranarrow optical clock transition.

We further notice atoms self-organize into locations with maximal atom-photon-interaction. This also maximizes the metrological improvement provided by the high-finesse optical cavity.