Scientists have achieved SI-traceable calibration of a commercial terahertz (THz) transmitter-receiver system. By employing a Rydberg atomic sensor, they precisely measured the electric field strength in the THz range, a task previously challenging due to the limited availability of SI-traceable sensors. This breakthrough enables more rigorous characterization of THz systems, which is crucial for the development of technologies operating in this spectrum.

The method relies on the Autler-Townes (AT) effect observed in a Rydberg transition of cesium atoms (17D₅/₂ → 18P₃/₂). When a thermal atomic vapor is exposed to THz radiation, a splitting of spectral lines is induced. The magnitude of this splitting, measured as a function of the THz frequency detuning from resonance, allows for the determination of the on-resonant Rabi frequency. From this, and knowing the atomic transition dipole moments and fundamental constants, the absolute THz electric field strength can be extracted.

This technique has enabled the calibration of both the transmitter and receiver of the photomixer system. The atomically measured electric field was used to calibrate the power and field emitted by the transmitter. Simultaneously, measurements with a commercial InGaAs photomixer receiver allowed for the calibration of its current-to-field responsivity, and the conversion of its current-noise floor into an absolute noise-equivalent THz electric-field sensitivity. This work establishes a quantitative link between state-of-the-art and absolute atom-based THz sensors.