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, ρ33 and ρ55 are equal to 8.3% and 3.5%, 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.