1 Department of Physics and Astronomy, Faculty of Science, Aarhus University, Aarhus University2 Leibniz Universität Hannover3 Department of Physics and Astronomy, Science and Technology, Aarhus University4 LNE-SYRTE, Observatoire de Paris5 Laboratoire de Physique des Solides6 Department of Physics and Astronomy, Science and Technology, Aarhus University
Optically trapped ensembles are of crucial importance for frequency measurements and quantum memories but generally suffer from strong dephasing due to inhomogeneous density and light shifts. We demonstrate a drastic increase of the coherence time to 21 s on the magnetic field insensitive clock transition of 87Rb by applying the recently discovered spin self-rephasing [C. Deutsch et al., Phys. Rev. Lett. 105, 020401 (2010)]. This result confirms the general nature of this new mechanism and thus shows its applicability in atom clocks and quantum memories. A systematic investigation of all relevant frequency shifts and noise contributions yields a stability of 2.4×10-11τ-1/2, where τ is the integration time in seconds. Based on a set of technical improvements, the presented frequency standard is predicted to rival the stability of microwave fountain clocks in a potentially much more compact setup.