Terahertz (THz) radiation, being part of the electromagnetic spectrum, lies at the boundary of electronics and optics, positioned between microwaves (used, for example, in Wi-Fi) and infrared. Although it holds immense potential for applications that include inspecting packages without harmful X-rays, superspeed 6G communication, and spectroscopy and imaging of organic compounds, turning this potential into precise and sensitive measurements has been technically very difficult. In the past few years, scientists have made major strides in both generating and detecting terahertz radiation, yet until now they had not been able to measure a frequency comb in this region with the required precision.

Frequency combs as ultra-precise electromagnetic rulers

Why is this so important? Frequency combs, which earned a Nobel Prize in 2005, can be imagined as an incredibly accurate ruler made not from a solid material, but from light or radio waves. Instead of millimeter markings, there is a sequence of evenly spaced lines ("teeth") at strictly defined frequencies. This "electromagnetic ruler" lets physicists determine the frequency of an unknown signal with extraordinary accuracy simply by seeing which "tooth" it matches. Because of this, frequency combs act as reference standards that can be used to calibrate and stabilize many types of instruments over a wide range of frequencies. Depending on where in the electromagnetic spectrum this ruler sits, scientists talk about optical, radio, or terahertz frequency combs.

Terahertz frequency combs are particularly attractive because they can support calibration and highly accurate measurements in a band where the oscillation frequencies are higher than typical radio waves but lower than optical waves (light). However, such combs are notoriously difficult to measure with high precision, since their oscillations are too rapid for conventional electronics and cannot be directly captured with standard optical methods. Researchers have been able to determine the spacing between the comb "teeth" and to measure the total power spread across the spectrum, but determining how much power belongs to a single tooth has remained a major challenge.

Rydberg atoms turned into quantum antennas

The scientists from the Faculty of Physics and the Centre for Quantum Optical Technologies at the Centre of New Technologies, University of Warsaw have now overcome this obstacle and, for the first time, measured the signal emitted by a single terahertz comb tooth. To achieve this, they employed a gas of rubidium atoms prepared in a Rydberg state. A Rydberg atom is defined as having a single electron excited to a very high orbit by being illuminated with precisely tuned lasers. This "swollen" atom acts as a quantum antenna that is extremely sensitive to external electric fields. In addition, by using tunable lasers, the detector can be adjusted to respond to one specific frequency within such a field, across a range that extends up to terahertz waves.

Traditionally, in Rydberg electrometry, the phenomenon of Autler-Townes splitting is used to measure the electric field. Its huge advantage is that the measurement result depends only on fundamental atomic constants, providing an absolutely calibrated readout. Unlike classical antennas, which require laborious calibration in specialized radio laboratories, the atomic-based system is, in a sense, a standard unto itself. Moreover, thanks to the richness of energy states in the atom, such a sensor can be tuned almost continuously over an enormous range, from a direct current (DC) signal up to the aforementioned terahertz.

Hybrid terahertz to light conversion for extreme sensitivity

However, this method has a limitation: on its own, it is not sensitive enough to record very weak terahertz signals. To remedy this, the research team additionally applied a radio wave-to-light conversion technique invented at the University of Warsaw and adapted it to the needs of terahertz radiation. In this process, the weak terahertz signal is converted into optical photons, which can then be detected with immense sensitivity using single-photon counters. This hybrid approach is the key to success: it combines the extreme sensitivity of photon detection with the ability to "recover" the calibration capabilities of the Autler-Townes method even for the weakest signals.

The sensor based on Rydberg atoms possesses all the features needed to perform precise frequency comb calibration: it can be tuned to a single tooth of the comb, and then retuned to the next, and the next. The scientists managed to observe several dozen teeth in a very wide frequency range this way. Additionally, thanks to the knowledge of the fundamental properties of atoms, the comb was directly calibrated, precisely determining its intensity.

New path for terahertz metrology and future technologies

The results obtained by the physicists from the University of Warsaw Wiktor Krokosz, Jan Nowosielski, Bartosz Kasza, Sebastian Borówka, Mateusz Mazelanik, Wojciech Wasilewski, and Michał Parniak represent much more than the development of another sensitive detector. Their work lays the groundwork for a new area of metrology. With the help of Rydberg atoms, the transformative uses of optical frequency combs can now be extended into the previously difficult terahertz region. Importantly, unlike many quantum technologies that require extremely low temperatures, this system functions at room temperature, which greatly lowers costs and makes future commercialization more realistic. This creates an opportunity to build reference measurement standards for the coming generation of terahertz technologies.

The project "Quantum Optical Technologies" (FENG.02.01-IP.05-0017/23) is implemented as part of Measure 2.1 International Research Agendas of the Foundation for Polish Science, co-financed by the European Union from Priority 2 of the European Funds for Modern Economy Programme 2021-2027 (FENG). The research is also one of the results of the SONATA17 and PRELUDIUM23 projects funded by the National Science Centre.)