Very sensitive vapor-cell quasi-DC atomic E-field sensor

Abstract

We report several technical approaches that significantly improve the performance of a vapor-cell atomic electrometer operating in the quasi-DC frequency domain ($\ll$ 1 kHz). With a very small active volume of approximately 11 mm$^3$ inside the vapor cell, we demonstrated a noise floor for electric field (E-field) sensitivity ranging from 0.2 to 7.7 mV/m$\sqrt{\rm Hz}$ for a frequency band of 1--100 Hz. Our work utilizes only a bare vapor cell for electrometry, without any metal parts or electrodes, to ensure minimal distortion of the measured E-field and to minimize the effective sensing volume for high spatial resolution. The E-field-sensitive atomic state (Rydberg state) is excited and read out optically, maximizing the simplicity of the system design and enabling the miniaturization of quasi-DC E-field sensors for potential applications, such as diagnostics of electronics without physical contact, communications in and below the super-low frequency (SLF) band, proximity detection, remote activity surveillance, tracing charge signatures, and research in bioscience and geoscience.

Figures (16)

• Figure 1: The two major geometries of the vapor cells used in our experimental work are shown here. On the left is the cylindrical Rb vapor cell made with quartz or Pyrex glass, with and without coatings, and on the right is the cubic Rb vapor cell made with monocrystalline sapphire. The cylindrical cells have an external diameter of 25 mm and a length of 25 mm, and the window thickness is 3 mm. The cubic cells have external dimensions of $20\times20\times20$ mm$^3$ with a wall thickness of 3 mm.
• Figure 2: Sheet resistance $R_\Box$ for different surface materials over 480-nm laser power with these measurement conditions: diameter of the laser beams =1.6 mm; 780-nm laser power = 25 $\mu$W; Rb number density $\approx1.4\times10^{10}$ cm$^{-3}$ (measured); cell-body temperature $\sim45$$^\circ$C; bias magnetic field $\sim7$ G.
• Figure 3: Example data of the experimentally verified B-field suppressed E-field screening. The screening time constant $\tau$ quadratically depends on the B-field coil current. The B-field amplitude vs. the coil current ratio is 9.7 G/A. The corresponding experimental conditions are: laser beam diameter = 1 mm; 480-nm laser power = 100 mW; 780-nm laser power = 25 $\mu$W; Rb number density $\approx4\times10^{10}$ cm$^{-3}$; cell-body temperature $\sim35$$^\circ$C.
• Figure 4: Rb energy-level diagram with some possible three-photon excitation methods to reach Rydberg $P$ states. The highlighted wavelengths are used in this work.
• Figure 5: (a) Energy levels as functions of electric field with a bias B-field at 5 G for $n=100$, $l=0,1$ and $n=97$ and $l=3\ldots96$. (b) Relative coupling strengths (expressed in darkness) of the $100S$ and $100P$ orbitals to the interrogation lasers. (c) Zoom-in of the $100P_{3/2}$ and $100P_{1/2}$ E-field induced shift states. (d) Zoom-in of the $100S_{1/2}$ E-field induced shift states. One can clearly see that the $100S_{1/2}$ state is less sensitive to the electric field and experiences interference from high-order orbitals at a lower electric-field strength compared to the $100P_{3/2}$ and $100P_{1/2}$ states.