Blackbody radiation shift of the 27Al+ 1S0 - 3P0 transition
  The differential polarizability, due to near-infrared light at 1126 nm, of the 27Al+ 1S0 -> 3P0 clock transition is measured to be 4\pi\epsilon_0 x (1.6 +/- 0.5) x 10^{-31} m^3. This measurement is combined with experimental oscillator strengths to extrapolate the differential static polarizability of the clock transition as 4\pi\epsilon_0 x (1.5 +/- 0.5) x 10^{-31} m^3. The resulting room temperature blackbody fractional frequency shift of -8(3) x 10^{-18} is the lowest known shift of all atomic transitions under consideration for optical frequency standards. A method is presented to estimate the differential static polarizability of an optical transition, from a differential light shift measurement. 
T. Rosenband, W. M. Itano, P. O. Schmidt, D. B. Hume, J. C. J.   Koelemeij, J. C. Bergquist, and D. J. Wineland