  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.

Atomic Physics