A compact, transportable single-ion optical clock with 7.8 × 10−17 systematic uncertainty

A transportable optical clock based on the 4s2S1/2-3d2D5/2 electric quadrupole transition at 729 nm of a single 40Ca+ ion trapped in a mini Paul trap has been developed. The physical system of the 40Ca+ 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. The systematic fractional uncertainty has been evaluated to be 7.8 × 10−17, and the Allan deviation has been rescaled to be $${2.3\times {{10}^{-14}}}/{\sqrt{\tau }}\;$$2.3×10-14/τ for a single clock by self-comparison with a probe pulse time of 20 ms. Apart from the electronics, the whole setup has been constructed within a volume of 0.54 m3. This size is to our knowledge currently the best achieved compactness with any type of optical clock. Moreover, this transportable clock is planned to be used for high precision measurements and it’s the first step to a space optical clock.