Optical frequency divider with division uncertainty at the 10^(-21)   level
  Optical clocks with unprecedented accuracy of 10^(-18) will lead to innovations in many research areas. All the applications of optical clocks rely on the ability of precisely converting the frequency from one optical clock to another, or particularly to the frequencies in the fiber telecom band for long-distance transmission. Here, we report a low-noise, high precision optical frequency divider. It can realize accurate optical frequency conversion as well as enable precise measurement of optical frequency ratios. By comparing against the frequency ratio between the fundamental and the second harmonic of a 1064 nm laser rather than a second similar system, the optical frequency divider is demonstrated to have a frequency division instability of 6e-19 at 1 s and a fractional frequency division uncertainty of 1.4e-21, nearly three orders of magnitude better than the most accurate optical clocks. It allows optical clocks to be accessible to many precision measurement applications. 
Yuan Yao, Yanyi Jiang, Hongfu Yu, Zhiyi Bi and Longsheng Ma
physics.optics