Andrew D. Ludlow, Nathan M. Hinkley, Jeffrey A. Sherman, Nate B. Phillips, Marco Schioppo, Nathan D. Lemke, Kyle P. Beloy, M Pizzocaro, Christopher W. Oates
Atomic clocks have been transformational in science and technology, leading to innovations such as global positioning, advanced communications, and tests of fundamental constant variation. Next-generation optical atomic clocks can extend the capability of these timekeepers, where researchers have long aspired toward measurement precision at 1 part in 1018. This milestone will enable a second revolution of new timing applications such as relativistic geodesy, enhanced Earth- and space-based navigation and telescopy, and new tests on physics beyond the Standard Model. Here, we describe the development and operation of two optical lattice clocks, both utilizing spin-polarized, ultracold atomic ytterbium. A measurement comparing these systems demonstrates an unprecedented atomic clock instability of 1.6x10-18 after only 7 hours of averaging.