Rydberg atomic radio-optical measurement and spectrum processing techniques (invited)

  Significance   Rydberg atoms are highly excited atoms with large electric dipole moments. The energy difference between adjacent levels can cover an ultra-wide frequency spectrum range from DC to THz, making it possible to achieve high-sensitivity and ultra-wideband reception of electromagnetic fields. Radio-optical measurements based on Rydberg atoms are achieved by precisely controlling two laser beams, the probe laser and the control laser, to transform ground state alkali metal atoms into Rydberg atoms and induce Electromagnetic Induced Transparency (EIT) in the transmitted spectrum of the probe laser. Under the interaction of the input radio signal, Autler-Townes (AT) splitting occurs in the transparent EIT spectrum, completing the conversion of radio signals to optical signals (Fig.2-3), thereby extracting information such as frequency, amplitude, and phase of the radio signal. This technology has attracted great attention in electronic information fields such as electric field metrology, electromagnetic spectrum detection, communication, and radar in recent years. The physical implementation of this technology is simple and does not require strict physical conditions as usual quantum technologies such as single-photon sources or ultra-cold and superconducting conditions. It can be achieved at room temperature without being limited by the level of production technology. It is considered one of the fastest applicable quantum technologies with its high stability, accuracy, and repeatability that could partially replace existing radio reception technologies in the near future.

  Progress  In the past decade, researchers have made significant progress in the study of radio-optical measurement techniques based on Rydberg atoms, from precise measurements of single-frequency static radio signals in electric field metrology applications to real-time reception of single-frequency dynamic radio signals in communication applications, and to spectrum detection and communication reception of complex multi-frequency radio signals. The key to this technology is how to quickly and accurately extract information about the radio signal from the output EIT spectrum of the atomic system. Different types of radio signals, such as static, dynamic, single-frequency, and multi-frequency radio signals, require different information extraction and spectral processing methods, as well as different experimental designs and implementations. For single-frequency static radio signals, researchers have already used Rydberg atoms in experiments to measure field strengths in the 0-320 GHz frequency range with a maximum coverage range of 780 pV·cm⁻¹ to 50 V·cm⁻¹. By using heterodyne technology (Fig.8) and critical phenomena in many-body Rydberg atomic system, the current sensitivity can reach as low as 49 nV·cm⁻¹·Hz^−1/2. Unlike measuring single-frequency static radio signals, for single-frequency dynamic radio signals, Rydberg atom systems are required to track and respond to rapidly changing radio signals in real-time and quickly read EIT spectral changes at the end point. Its primary application scenario is the communication reception. Since 2018, a large number of verification experiments on wireless communication reception principle have been carried out based on Rydberg atoms. This technology can directly convert intermediate frequency or baseband signals on the carrier into optical signals for direct demodulation. Verified communication methods include amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). When the input wireless signal becomes complex, especially for multi-frequency wireless signal input, the output EIT spectrum of the probe laser will become complicated. It will be a big challenge to quickly and effectively read and distinguish information from different frequency wireless signals. Currently, a small amount of research is focused on verifying dynamic multi-frequency wireless signals for communication reception, including using multi-harmonic Rydberg atomic level structures to finely adjust and optimize system parameters at the front end or using post-processing techniques such as deep learning at the back end to achieve recognition and reading of multiple frequency information. People have experimentally achieved simultaneous reception of five completely different frequency signals within a spectrum range of over 100 GHz (Fig.18) or 20 similar frequency signals within a range of 100 kHz (Fig.23).

  Conclusions and Prospects  Through continuous research over the past 10 years, it has been experimentally verified that radio-optical measurements based on Rydberg atoms have unique quantum advantages in spectrum range, sensitivity, minimum field strength, signal demodulation mechanism, and other aspects. This technology has demonstrated promising prospects in applications such as electric field metrology, electromagnetic spectrum detection, communication, radar, and more. In order to further develop this technology to fully leverage the unique quantum advantages of Rydberg atoms and achieve practical applications as soon as possible, researchers need to deepen their research on the comprehensive performance improvement, anti-interference ability enhancement, miniaturization integration and simultaneously reduce the costs of radio-optical measurements based on Rydberg atoms.