Imaging of Relaxation Times and Microwave Field Strength in a Microfabricated Vapor Cell

TL;DR

This paper introduces a novel imaging technique combining time-domain measurements and absorption imaging to spatially resolve relaxation times and microwave fields in microfabricated vapor cells, aiding their optimization for quantum technologies.

Contribution

The authors develop a new characterization method that provides high-resolution spatial images of atomic relaxation times and microwave fields in vapor cells, enhancing understanding of cell dynamics.

Findings

Uniform $T_1$ times of 265 μs at 90°C across the cell center.

Peak $T_2$ times of around 350 μs observed.

Identification of a 'skin' region with reduced relaxation times at the cell edges.

Abstract

We present a new characterisation technique for atomic vapor cells, combining time-domain measurements with absorption imaging to obtain spatially resolved information on decay times, atomic diffusion and coherent dynamics. The technique is used to characterise a 5 mm diameter, 2 mm thick microfabricated Rb vapor cell, with N$_2$ buffer gas, placed inside a microwave cavity. Time-domain Franzen and Ramsey measurements are used to produce high-resolution images of the population ($T_1$) and coherence ($T_2$) lifetimes in the cell, while Rabi measurements yield images of the $\sigma_-$, $\pi$ and $\sigma_+$ components of the applied microwave magnetic field. For a cell temperature of 90$^{\circ}$C, the $T_1$ times across the cell centre are found to be a roughly uniform $265\,\mu$s, while the $T_2$ times peak at around $350\,\mu$s. We observe a `skin' of reduced $T_1$ and $T_2$ times around the edge of the cell due to the depolarisation of Rb after collisions with the silicon cell walls. Our observations suggest that these collisions are far from being 100$\%$ depolarising, consistent with earlier observations made with Na and glass walls. Images of the microwave magnetic field reveal regions of optimal field homogeneity, and thus coherence. Our technique is useful for vapor cell characterisation in atomic clocks, atomic sensors, and quantum information experiments.