A transportable optical clock refer to the 4s 2 S 1/2 -3d 2 D 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 7.7×10 -17 , and the Allan deviation fits to be 14 2.3 10    by clock self-comparison with a probe pulse time 20 ms.

We achieve the sympathetic sideband cooling of a 40 Ca + -27 Al + pair. Both axial modes of the two-ion chain are cooled simultaneously to near the ground state of the motion. The center of mass mode is cooled to an average phonon number of 0.052(9), and the breathing mode is cooled to 0.035(6). The heating rates of both a single 40 Ca + and the 40 Ca + -27 Al + pair are measured and compared. This work is a fundamental step toward the implementation of a 40 Ca + -27 Al + quantum logic clock.

We report a robust, compact, and transportable optical clock (TOC-729-2) based on a trapped single 40Ca+ ion with a systematic uncertainty of 1.1×10−17, which is limited by the black-body radiation shift uncertainty at room temperature. By comparing it with the previous transportable optical clock (TOC-729-1) similar but completely independent, the instability was measured to be better than 1.2×10−14/τ. Benefiting from the modular and integrated design, this TOC was constructed in a volume of ∼0.33 m3 excluding the controlling electronics in 19-in. racks. After being moved ∼1200 km away by express delivery, the single-ion signal was restored within 24 h. With the TOC uptime of 92% in 35-day period, the absolute frequency of the 729 nm transition of 40Ca+ was measured using a satellite link to International Atomic Time (TAI) to provide traceability to the SI second, and the result is 411 042 129 776 400.15(22) Hz, corresponding to a relative uncertainty of 5.3×10−16.