We report on Korea's first optically pumped primary frequency standard (KRISS-1) developed at the Korea Research Institute of Standards and Science. Although a relatively small microwave cavity (the cavity drift region is about 36 cm in length) has been employed in this experiment, we achieved our desired frequency accuracy by applying the regularized inverse on the Ramsey spectrum analysis and compensating the effect of the evanescent field inside the cavity. KRISS-1 typically performs at a short term stability of 1.3 × 10 −12 / √ τ with a combined uncertainty of 1.0 × 10 −14 .

The authors report on a method to determine the Rabi frequency and transit time distribution of atoms that are essential for proper operation of atomic beam frequency standards. Their method, which employs alternative regularized inverse on two Ramsey spectra measured at different microwave powers, can be used for the frequency standards with short Ramsey cavity as well as long ones. The authors demonstrate that uncertainty in Rabi frequency obtained from their method is 0.02%.