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.

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 so far.

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 is practically a DC electric field 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 image the electron beam.