Fig. 3: Experimental setup and main results.

a Schematic of the experimental setup: Central to the experiment is a microwave cavity supporting a TE₀₁₁ mode, polarized along the y-axis. The light incident on the cavity is polarized along the z-axis and propagates along the x-axis. The vapor cell inside the cavity has enriched ⁸⁷Rb. Unshifted light from the external cavity diode laser (ECDL) is used as a local oscillator for heterodyne detection; it is combined with the other beam in a fiber 50:50 beam splitter (BS), producing two beat signals at f_het,P and f_het,S. AOM - Acousto-optic modulator; EOM - Electro-optic modulator; SG - signal generator; SA - spectrum analyzer; PD - photodiode. b The generated signal measured with the spectrum analyzer in zero-span mode, showing a FWHM of 550(25) MHz. The dashed curve shows the calculated Doppler-broadened absorption coefficient of the pump light; the individual components due to the \({F}^{{\prime} }=1\) and \({F}^{{\prime} }=2\) excited hyperfine levels are shown as dotted curves. Without any fitting or free parameters, the generated signal data is proportional to the MB absorption distribution of the pump transitions. c The generated signal amplitude responds roughly linearly to increased pump optical power. d The response of the generated signal amplitude to microwave powers from SG1. The largest pump and idler powers measured correspond to intensities of about 3 & 7 mW/cm². Error bars represent the standard deviation.