Rydberg atoms are becoming promising platform for electric field measurements. They are special because their valence electron is in a highly excited energy state and thus has the orbit with large radius and, correspondingly, dipole moment. This makes such electrons very sensitive to electric fields that changes their energy. Since such energy changes can be detected in real time using laser excitation, one can determine the value of the electric field by monitoring the optical field transmission. Given that atoms are the same everywhere, measurements based on Rydberg atoms are absolute and there is no worry that an atom based standard for measurement will change like an classical standard will over time. That makes Rydberg atoms promising tools for accurate measurements.

Electromagnetically induced transparency (EIT) is a well studied and known phenonena. In Rydberg atoms two lasers in a ladder configurations are needed to connect a ground state |g ⟩ and a highly excited Rydberg state |r ⟩ through an intermediate excited state |e ⟩. The ground and excited state are linked by a 780 nm red laser, and the excited and Rydberg states are linked by a 480 nm blue laser. When the laser frequency is on resonence with both transitions, the atoms are in a superpositon of the ground and Rydberg states known as a dark state. In this case the optical absorption is reduced. If the frequency of one of the lasers is swept, a relatively narrow EIT resonance is observed. The position of this resonance depends on exact Rydberg level energy, and sensitive to the electric field.

Rydberg Raman-Ramsey (R³) EIT

The goal of this collaborative project between our group and Draper Labs was to enhance the sensitivity of Rydberg atom measurements. In room temperature atomic experiments, the motion of the atoms causes a broadening of the transmission peak. The sensitivity of the detector is ultimately limited to how narrow the transmission peak is to take measurements. Our approach employs a Ramsey interrogation of the atoms. The Ramsey interrogation consists of a light pulse that prepares the Rydberg atoms, after which atomic superposition is left to evolve in the dark for potentially long time before it is probed with a second light pulse. In a recent theoretical study we found that such interrogation method can potentially produce much narrower resonances and significantly improve the electric field sensitivity. Our preliminary experimental studies have found clear indication of Rydberg state population transfer, but so far we were notable to observe coherent Raman-Ramsey effect with eather a time pulsed light, and a spatially separated beams. This work has since concluded, and has been published here.

The figure illustrate the potential advantage of the R³ resonances for precision metrology by showing the variation if the probe laser transmission when the blue laser frequency is scanned across the EIT resonance. The large envelope is the standard EIT transmission that is several MHz wide. The small narrow feature on top of it is due to Raman-Ramsey interaction. If we zoom in on it (the inset), one can see that its width can be reduce down to tens of kHz, limited by the laser frequency and phase stability. If this narrow feature is used as a frequency marker for any electric-field induced shifts, it provides superior sensitivity. Experimentally, we have been trying to resolve this feature in the lab, but it is small and there are technical limitations that have prevented us from seeing this feature.

Rydberg Quantum Enhanced Tracker (QET).

A project with a collaboration between William & Mary and Thomas Jefferson National Lab in Newport News. This project is a direct application of Rydberg electric field sensing. An electron beam provides a DC electric field gradient that can be detected by Rydberg atoms. When Rydberg atoms are subject to a DC electric field, the EIT transmission peak created from optical detection will have a frequency shift. The Rydberg QET project aims to use the shift of the transmission peak to measure electron beam current and give dimenstions of beam width and position.

In a uniform DC electric field, the EIT peak will have a total shift in frequency as shown here, and the motion along the frequency axis is corelated to only the strength of the electric field, and the polarizability of the atomic state. In our experiment, we do not have a uniform DC electric field, so we must look at distortions to the EIT peak that are presented as braodening rather than relative shifts. To do this we pulse the electron beam, and see how the field changes to avoid unwanted charging effects and have a clean signal due to only the presence of the electron beam.

So far in this project, we have found a spatial dependent signal that makes the EIT respond in similar to how a simplified model shows the peak should be distorted. Pulsing the electron beam has mitigated much of the charging effects that have been problematic, but we still are sensitive to a spatial electric gradient not due to the electron beam within our sensing cell. To map the spatial gradient we have attempted moving the electron beam around the lasers, moving the lasers around the electron beam, and also angling the laser beams and moving the overlapped-crossed beam region around the cell. The most promising result we have gotten is a spatially symmetric signal that changes with increasing current shown in the image below. With further analysis, we hope to extract the beam position, diameter and current for a nearly non-invasive electron beam diagnostic tool. This work has been presented at multiple high-level conferences within the optics field including CLEO, DAMOP, and FiOLS.