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ReportsSr Lattice Clock at 1 x 10–16 Fractional Uncertainty by Remote Optical Evaluation with a Ca Clock  
Optical atomic clocks promise timekeeping at the highest precision and accuracy, owing to their high operating frequencies. Rigorous evaluations of these clocks require direct comparisons between them. We have realized a high-performance remote comparison of optical clocks over kilometer-scale urban distances, a key step for development, dissemination, and application of these optical standards. Through this remote comparison and a proper design of lattice-confined neutral atoms for clock operation, we evaluate the uncertainty of a strontium (Sr) optical lattice clock at the 1 x 10–16 fractional level, surpassing the current best evaluations of cesium (Cs) primary standards. We also report on the observation of density-dependent effects in the spin-polarized fermionic sample and discuss the current limiting effect of blackbody radiation–induced frequency shifts.
1 JILA, National Institute of Standards and Technology, and University of Colorado, Department of Physics, University of Colorado, Boulder, CO 80309–0440, USA. * Present address: Columbia University, New York, NY 10027, USA.
|| Present address: Oberlin College, Oberlin, OH 44074, USA. ¶ Permanent address: Università di Firenze, Italy.
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Permanent address: Niels Bohr Institute, Copenhagen, Denmark. 
Present address: Stanford University, Palo Alto, CA 94305, USA. 
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