A transportable 40Ca+ single-ion clock with $7.7\times 10^{-17}$   systematic uncertainty
  A transportable optical clock refer to the $4s^2S_{1/2}-3d^2D_{5/2}$ electric quadrupole transition at 729 nm of single $^{40}Ca^+$ trapped in mini Paul trap has been developed. The physical system of $^{40}Ca^+$ optical clock is re-engineered from a bulky and complex setup to an integration of two subsystems: a compact single ion unit including ion trapping and detection modules, and a compact laser unit including laser sources, beam distributor and frequency reference modules. Apart from the electronics, the whole equipment has been constructed within a volume of 0.54 $m^3$. The systematic fractional uncertainty has been evaluated to be $7.7\times 10^{-17}$, and the Allan deviation fits to be $2.3\times {10}^{-14}/\sqrt{\tau}$ by clock self-comparison with a probe pulse time 20 ms. 
Jian Cao, Ping Zhang, Junjuan Shang, Kaifeng Cui, Jinbo Yuan, Sijia   Chao, Shaomao Wang, Hualin Shu, Xueren Huang
physics.atom-ph