Preliminary Frequency Comparison of Two ⁴⁰Ca⁺ Optical Frequency Standards

Frequency comparison is one of the most efficient ways to evaluate the performance of a frequency standard. Based on the pre-existing ⁴⁰Ca⁺ optical frequency standard, we set up the second ⁴⁰Ca⁺ optical frequency standard, which has been improved in the materials and structure of ion traps for better control of the magnetic field. After the compensation, the residual magnetic field at the position of the ion is adjusted to be ～500 nT with a long time jitter of ～10 nT, which is better than the pre-existing ⁴⁰Ca⁺ optical frequency standard. We realize the '4-point-closed-loop locking' on the second ⁴⁰Ca⁺ optical frequency standard after a series of preparatory works. Through half an hour of measurement time, the two frequency standards exhibited a stability of 2.1×10^?13τ^?1/2 and a relative frequency difference of 1.5 (2.9) Hz.