Photonic terahertz source frequency stabilized to the part per trillion level through molecular spectroscopy

Photonic sources for terahertz (THz) generation demonstrate unrivaled phase noise. The resulting THz waves can be used for precision spectroscopy of molecular rotations. Here, we show that a rotational transition of nitrous oxide (N
2
O) can be used to stabilize the THz frequency of a photonic oscillator to a few parts per trillion,, after only 10 seconds of averaging time. The core of the photonic oscillator utilizes a dissipative Kerr soliton generated in a micro-ring resonator, and the THz spectrometer is purely built in a waveguide. With these techniques combined, this demonstration represents a low-noise, low-drift THz source that is feasibly miniaturized.