Rydberg atoms, where one valence electron is promoted to an orbit far away from the nucleus, have very large dipole moments, making them good candidates to be used as very sensitive probes of electromagnetic fields (Fancher et al., IEEE Transactions on Quantum Engineering, 2, 1–13, 2021). Compared to classical antennae, they can be made dielectric, their size is independent of the wavelength to be measured, and they offer the potential to perform self-calibrated measurements, referenced on the properties of the atoms themselves. So far, most of the research in this field has focused on sensing electromagnetic fields with room-temperature (or hotter) Rydberg atoms, where the absorption of a probe laser beam can be made dependent of the microwave field to be measured via the phenomenon of electromagnetically - induced transparency.
Rydberg atoms, where one valence electron is promoted to an orbit far away from the nucleus, have very large dipole moments, making them good candidates to be used as very sensitive probes of electromagnetic fields (Fancher et al., IEEE Transactions on Quantum Engineering, 2, 1–13, 2021). Compared to classical antennae, they can be made dielectric, their size is independent of the wavelength to be measured, and they offer the potential to perform self-calibrated measurements, referenced on the properties of the atoms themselves. So far, most of the research in this field has focused on sensing electromagnetic fields with room-temperature (or hotter) Rydberg atoms, where the absorption of a probe laser beam can be made dependent of the microwave field to be measured via the phenomenon of electromagnetically - induced transparency.
Atoms form the basis of precise measurement for many quantities (time, acceleration, rotation, magnetic field, etc.). Measurements of microwave frequency electric fields by traditional methods (i.e. engineered antennas) have limited sensitivity and can be difficult to calibrate properly. Highly-excited (Rydberg) neutral atoms have very large electric-dipole moments and many dipole allowed transitions in the range of 1 - 500 GHz. It is possible to sensitively probe the electric field in this range using the combination of two quantum interference phenomena: electromagnetically induced transparency and the Autler-Townes effect. This technique allows for very sensitive field amplitude, polarization, and sub-wavelength imaging measurements. These quantities can be extracted by measuring properties of a probe laser beam as it passes through a warm rubidium vapor cell. Thus far, Rydberg microwave electrometry has relied upon the absorption of the probe laser. We report on our use of polarization rotation, which corresponds to the real part of the susceptibility, for measuring the properties of microwave frequency electric fields. Our simulations show that when a magnetic field is present and directed along the optical propagation direction a polarization rotation signal exists and can be used for microwave electrometry. One central advantage in using the polarization rotation signal rather than the absorption signal is that common mode laser noise is naturally eliminated leading to a potentially dramatic increase in signal-to-noise ratio.
Significant progress has been made in atom-based measurements of length, time, gravity and electromagnetic fields in recently years. Rydberg atom-based microwave electric field measurement, using electromagnetically induced transparency (EIT) in room temperature alkali-metal vapors, has been extensively investigated and aroused the broad interest. This approach may establish a new standard for the measurements of microwave (MW) and radio frequency (RF) electric fields.In this review, we describe the work on a new method of measuring electric fields based on quantum interference by using either cesium or rubidium atoms contained in a dielectric vapor cell. Rydberg atoms with principal quantum number n >>1 have large direct current (DC) polarizabilities and microwave transition dipole moments, thereby making them extremely sensitive to external electric fields. Using the Rydberg three-level EIT to detect the level splitting and shift that is induced by the external field, we can realize a rapid and robust self-calibration method of measuring the electric field in a frequency range from 0.01 GHz to 1000 GHz. For the MW electric field (frequency range > 1 GHz), the MW field causes the Rydberg states to split, known as an Autler-Townes splitting (A-T) effect when the applied microwave can resonate with adjacent Rydberg states. The MW coupled A-T splitting is proportional to the applied electric field strength, from which the field strength is measured. Using the EIT window, a high sensitivity of 3 μV·cm-1·Hz-1/2 and small electric field of 1 μV/cm are expected to be achieved with a modest setup, and the limitations of the sensitivity are also addressed in the review. For the RF field at frequency mj EIT lines, and avoided crossings formed with the fine-structure levels of equal mj and different J's, which is used to calibrate and measure the RF field amplitude. On the other hand, the dependence of the EIT-line strength on the RF field polarization provides a fast and robust polarization measurement of RF fields based on matching experimental data with a theoretical simulation. The measurements of minimum strengths and sensitivity of RF fields based on Rydberg atoms are one order magnitude below the values obtained by traditional antenna methods. The atom-based field measurement paves the way for determining fields through calibration-free, invariable atomic properties and miniaturization. We also propose its various potential applications in the future.
While most experimental developments have focused on room-temperature vapors so far, using cold atoms has the advantage of strongly reducing the frequency broadening associated with Doppler effect, while increasing the interaction time between the atoms and the probing beams, offering prospects to make the most of the intrinsic narrow linewidth of Rydberg atoms and perform long-term, stable measurements. Scientists researchers at ONERA have previously used cold atoms to perform absolute measurements of the gravity field onboard moving carriers, showing that this technology is now compatible with practical applications out of the lab. We have recently developed at ONERA a novel approach to sense electromagnetic fields with cold
Rydberg atoms are highly promising for microwave electric field sensing owing to their large dipole matrix elements. While most experimental developments have focused on room-temperature vapors so far, utilizing cold atoms in this context could open new possibilities for applications where accuracy, long term stability and highresolution at large integration times are required, such as calibrating blackbody shifts in state-of-the-art optical clocks or measuring the cosmic MW background. Here, we report a novel approach [1] for the metrology of microwave fields with cold 87 Rubidium Rydberg atoms based on trap-loss-spectroscopy in a magneto-optical trap (MOT).
The detection of microwave (MW) fields with room-temperature Rydberg atomic vapors is a novel technique of optical sensing, introducing a range of merits typically not possible to achieve with conventional antennas [1]. Firstly, the atomic response to MW fields is dependent on a given transition dipole moment between Rydberg energy levels - as this value can be precisely derived from the theory of alkali atoms, e.g. rubidium, Rydberg sensors are intrinsically calibrated and there is no dependency on particular device. Secondly, the detector does not contain any metal parts (apart from trace amount of rubidium), introducing very little distortion to the measured field, so the detection is stealthy, and the detector is practically invulnerable even to strong fields. Typically, however, the best state-of-the-art performance is achieved with an additional local oscillator MW field [2], emitted from the antenna, in which case the scheme is no longer all-optical and some advantages are lost.
Rydberg atoms are neutral atoms excited to high principal quantum number states, which endows them with exaggerated properties such as large electric dipole moments, long lifetimes, and extreme sensitivity to external electromagnetic fields. These characteristics form the foundation of Rydberg atom-based sensors, an emerging class of quantum devices capable of optically detecting electric fields across frequencies from DC to the terahertz regime. Rydberg-based electrometry operates through both Autler–Townes (AT) splitting of resonant Rydberg transitions and Stark-shift measurements for high-frequency or far-detuned fields, enabling broadband field sensing from DC to the THz regime. Using ladder-type electromagnetically induced transparency (EIT) and AT splitting, these sensors enable non-invasive, SI-traceable measurements of field amplitude, frequency, phase, and polarization. Recent developments have demonstrated broadband electric field probes, voltage calibration standards, and compact RF receivers based on thermal vapor cells and integrated photonic architectures. Furthermore, innovations in multi-photon EIT, superheterodyne readout, and multi wave mixing have expanded the dynamic range and bandwidth of Rydberg-based electrometry. Despite challenges related to environmental perturbations, linewidth broadening, and laser stabilization, ongoing advances in atomic control, hybrid photonic integration, and EIT-based readout promise scalable, chip-compatible sensors. This review summarizes the physical principles, experimental progress, and emerging applications of Rydberg atom-based sensing, emphasizing their potential for next generation quantum metrology, wireless communication, and precision field mapping.