A clock generator includes a hermetically sealed cavity and clock generation circuitry. A dipolar molecule in the hermetically sealed cavity has a quantum rotational state transition at a fixed frequency. The clock generation circuitry generates an output clock signal based on the fixed frequency of the dipolar molecule. The clock generation circuitry includes a detection circuit, a reference oscillator, and control circuitry. The detection circuit generates a first detection signal and a second detection signal representative of amplitude of signal at an output of the hermetically sealed cavity responsive to a first sweep signal and a second sweep signal input to the hermetically sealed cavity. The control circuitry sets a frequency of the reference oscillator based on a difference in time of identification of the fixed frequency of the dipolar molecule in the first detection signal and the second detection signal.