An Even Harmonic Eliminator for Molecular Clock

In some embodiments, a molecular clock includes: a waveguide gas cell containing gas molecules having a rotational spectral line with a first frequency; a voltage-controlled oscillator (VCO) to generate a clock signal; a transmitter referenced to the clock signal to generate a probing signal for transmission through the waveguide gas cell; and a receiver to receive the probing signal transmitted through the waveguide gas cell and interacting with gas molecules. The receiver can include: a filter circuit configured to filter out even harmonic components from the received signal; and a lock-in detector to generate an error signal indicating an offset between the first frequency and the second frequency, wherein the error signal is fed back to control generation of the VCO clock signal.