Microwave electric field (MW E-field) sensing is important for a wide range of applications in the areas of remote sensing, radar astronomy and communications. Over the past decade, Rydberg atoms have been used in ultrasensitive, wide broadband, traceable, stealthy MW E-field sensing because of their exaggerated response to MW E-fields, plentiful optional energy levels and integratable preparation methods. This review first introduces the basic concepts of quantum sensing, the properties of Rydberg atoms and the principles of quantum sensing of MW E-fields with Rydberg atoms. An overview of this very active research direction is gradually expanding, covering the progress of sensitivity and bandwidth in Rydberg atom-based microwave sensing, superheterodyne quantum sensing with microwave-dressed Rydberg atoms, quantum-enhanced sensing of MW E-field and recent advanced quantum measurement systems and approaches to further improve the performance of MW E-field sensing. Finally, a brief outlook on future development directions is provided.

The polarization spectra of electromagnetically induced transparency (EIT) for Zeeman sublevels in a cascade system with Rydberg state are demonstrated. The magnitude dependence of Rydberg-EIT on the polarizations of probe and coupling laser fields is studied, and shown mainly due to the strengths of relative dipole matrix elements between degenerate Zeeman sublevels. We further investigate the polarization spectra of Rydberg-EIT in the optimal polarization combinations of left-handed and right-handed circularly polarized fields when an external magnetic field is applied. The existence of nondegenerate Zeeman sublevels in an external magnetic field results in the splitting of Rydberg-EIT. The theoretical calculations are very consistent with the experimental spectra.

We perform Zeeman spectroscopy on a Rydberg electromagnetically induced transparency (EIT) system in a room-temperature Cs vapor cell, in magnetic fields up to 50 Gauss. The magnetic interactions of the |6S_1/2 F_g = 4> ground, |6P_3/2 F_e = 5> intermediate, and |33S_1/2> Rydberg states that form the ladder-type EIT system are in the linear Zeeman, quadratic Zeeman, and the Paschen-Back regimes, respectively. We explain the dependence of Rydberg EIT spectra on the magnetic field and polarization. The asymmetry of the EIT spectra, which is caused by the quadratic Zeeman effect of the intermediate state, becomes paramount in magnetic fields ≥40 Gauss. We investigate the interplay between Rydberg EIT, which reduces photon scattering, and optical pumping, which relies on photon scattering. We employ a quantum Monte Carlo wave-function approach to quantitatively model the spectra and their asymmetry behavior. Simulated spectra are in good agreement with the experimental data.

The Rydberg atomic receiver, sensing microwave electric field with high sensitivity and broad bandwidth, possesses the potential to be the staple for precise navigation and remote sensing. In this Letter, a Ku-band three-dimensional location system using an L-shaped array of Rydberg atomic receivers is theoretically proposed and experimentally demonstrated, and the proof of principle results show excellent consistency between the location-derived and the setting coordinates. The novel L-shaped array, together with the triangulation method, gives both phase difference and angle of arrival, achieving location of the horn for a signal microwave field in three-dimensional space. The concluded validity of this location system in the testing scene remains at approximately 90% with a theoretical maximum location tolerance of 5.7 mm. Furthermore, the estimation of two different spatiotemporal coordinates for the moving target confirms the velocity measurement capability of the system with errors less than 0.5 mm/s. The proposed location system using a Rydberg atomic receiver array is a verification for the most basic element and can be extended through repetition or nesting to a multi-input–multi-output system as well as multi-channel information processing.