A Potassium Atom Four-Level Active Optical Clock Scheme

We present an active optical clock scheme with a four-level quantum potassium system.We calculate the population probabilities of each state using the density matrix.At the steady state,P33 and P55 are equai to 8.3%and3.5%,respectively,and the population inversion between the 5S1/2 and 4P3/2 states is built up in the thermal potassium cell with a 404.7 nm pumping laser.According to the mechanism of the active optical clock,under the action of the 404.7 nm pumping laser,the scheme can output a 1252.2 nm quantum-limited-linewidth laser,which can be directly used as an active optical frequency standard.