Absolute Terahertz Frequency Comb Sensing via Rydberg Atom Upconversion

Rydberg atoms have emerged as a transformative platform for electrometry, offering high sensitivity and SI-traceable measurements based on fundamental atomic constants. While these sensors have been successfully deployed in the microwave regime, their application to the terahertz (THz) domain—a notoriously difficult spectral frontier—offers significant potential for bridging the gap between electronic and optical metrology. Here we demonstrate the first high resolution measurement of a THz frequency comb, extending the revolutionary benefits of optical comb spectroscopy to the THz domain.

In this work, we assess the benefits of Rydberg atoms for THz sensing and address the critical limitations of existing detection methodologies. We show that while Autler-Townes (AT) splitting provides a direct route to absolute calibration, it is fundamentally limited by spectral feature resolution, hindering precision measurements of complex signals. Furthermore, while superheterodyne detection offers enhanced sensitivity, its reliance on a stable, tunable THz local oscillator (LO) renders it technologically infeasible due to the lack of mature sources in this frequency band.

To overcome these barriers, we demonstrate a hybrid detection scheme utilizing thz-to-optical upconversion. This approach converts THz fields into detectable optical photons, bypassing the need for a THz LO while providing high sensitivity. Crucially, we retain the benefits of absolute calibration by cross-referencing our upconversion signal with AT splitting measurements in the regimes where they overlap. Using a THz frequency comb, we demonstrate the capability to resolve and measure the electric field amplitudes of individual comb lines over an octave-spanning range. We present a theoretical model of the atom-light-field interaction that has been independently verified against our experimental data, confirming the validity of our approach. Additional results include the achievement of sensitivities down to the thermal radiation level, the successful determination of absolute mode numbers for the comb, and the characterization of the comb's interaction with multiple distinct Rydberg transitions. This work establishes a robust framework for wideband, absolutely calibrated THz electrometry.