$^{87}$Sr Optical Lattice Clock

The JILA optical clock based on 1D lattice-confined $^{87}$Sr atoms operating on the ultra narrow $^1S_0$-$^3P_0$ transition has reached a fractional uncertainty of 1.4$\times 10^{-16}$.\footnote{A. D. Ludlow \textit{et al.,} Science, \textbf{319}(5871) pp. 1805-1808, 2008.} We have characterized a density dependent frequency shift, one of the largest sources of uncertainty in the JILA 1D lattice clock, at the $5\times 10^{-17}$ level.\footnote{G. K. Campbell \textit{et al.,} Science, \textbf{324}(5925) pp. 360-363, 2009.} We have implemented a 2D optical lattice confinement to further reduce this uncertainty. In addition, we report advances in characterizing and eliminating blackbody radiation-induced clock shifts, which currently dominate our systematic uncertainty. Direct optical frequency comparisons with the NIST Yb optical lattice clock demonstrate clock stability below $10^{-16}$ at 1000 s.