Optical Frequency Measurements at 1×10−18 Uncertainty with Ytterbium Optical Lattice Clocks

We describe ytterbium optical lattice clocks under development at NIST. This includes the characterization of optical frequency stability and uncertainty, as well as measurements between two independent clocks, at the 10⁻¹⁸level. We also report on optical frequency ratio measurements with other optical clocks at NIST.