Probe-assisted Depopulation Pumping in Low-pressure Alkali-metal Vapor Cells for Magnetometry

Abstract

For precision atomic magnetometry, inert buffer gas is included in alkali-metal vapor cells to significantly broaden hyperfine transitions, which facilitates optical pumping and reduces diffusive relaxation, while also providing non-radiative excited state quenching. We show low-buffer gas pressure (below 50 Torr) alkali vapor cells with resolved hyperfine manifolds can also yield high-performance magnetometers. For high polarization in $^{87}$Rb, we optically pump $F=2$ states with narrow linewidth $σ_+$ light, while tuning a probe beam to depopulate $F=1$ states ($Δν= 6.8$ GHz from $F=2$). The probe tuning then also provides $F=2$ detection with high optical rotation and low probe broadening; we demonstrate top-bottom gradiometry, within a single 25 Torr, 0.5 cc cell, that yields an Earth's field free-precession magnetometer sensitivity of 18 fT/$\sqrt{\text{Hz}}$ with a 1 kHz bandwidth, as well as RF magnetometer sensitivity of 12 fT/$\sqrt{\text{Hz}}$ in a small band about 110 kHz.

Figures (4)

• Figure 1: (Top) Grotrian diagram for several regimes, along with representative data. (Bottom) Dominant rates for probe-assisted depopulation pumping; $N_2$ quenching overcomes radiative trapping, with $(\star)$ denoting experimental conditions being wall-relaxation and probe-broadening limited.
• Figure 2: (Top) Schematic of $\sigma_+$ light optically pumping a $^{87}$Rb cell along a 44 µT field. After pumping periods, tipping pulses are applied, and spin precession is detected by optical rotation of a linearly polarized probe. (Bottom) For a 25 Torr buffer gas $^{87}$Rb cell at 90$^\circ$C, the scalar magnetometer amplitude responds to variation of probe and pump wavelengths. The probe detuning is shown along the x-axis, with pump detuning plotted from violet to red (low to high $\nu$) in steps of 1.46 GHz. Labeled are mid ($\circ$), assisted ($\star$), and suppressed ($\times$) regimes, and a guide along the x-axis $F\! \rightarrow \!F' : 2\! \rightarrow \! 1, 2 \! \rightarrow \! 2, 1 \! \rightarrow \! 1, 1 \! \rightarrow \! 1$. Inset: Linearly polarized probe absorption cross-section $\sigma(\nu)$.
• Figure 3: Top/bottom optical rotation and spectral densities for 0.4 ms shots are shown. Frequencies are extracted each shot for vapor cell top/bottom, and repeated with 2 kHz. A probe-assisted depopulation pumping scalar magnetometer measures 44 µT with current supply noise $200$ fT/$\sqrt{\text{Hz}}$. Top/bottom gradiometry projects magnetic field sensitivity of $17.8\pm 0.3$ fT/$\sqrt{\text{Hz}}$ with 1 kHz bandwidth.
• Figure 4: An RF magnetometer is made with the same geometry as the scalar sensor, where the resonant frequency is determined by the 15.7 µT scalar field strength.