Fig. 2: Transmission spectrum of a unitary Fermi gas strongly coupled to light.

a, b Schematic view of the experiment, depicting the three relevant energy levels and cavity transmission signal, averaged over three realizations, as a function of Δ_c/2π, for Δ_a/2π = 38.2 MHz for a spin-polarized Fermi gas with N_↑ = 9.1(7) × 10⁴ and N_↓ = 2(1) × 10³ (a) and for Δ_a = 0 for a spin-balanced Fermi gas of 2.0(1) × 10⁵ atoms equally populating each spin state (b). The values of Δ_a are centered on the anticrossing feature for both cases. c, d Coupled-system transmission spectrum as a function of Δ_c/2π and Δ_a/2π for the spin-polarized (c) and spin-balanced Fermi gas (d). The transmissions normalized by the maximum observed in each case are displayed in logarithmic scale. The bare atomic transitions for \(\left|\uparrow \right\rangle\) and \(\left|\downarrow \right\rangle\) are shown with red and blue dashed lines, respectively. The grey dashed lines indicate a weak contribution attributed to molecular states. e, f Transmission spectra obtained from ab initio calculation accounting for the higher-order transverse mode contributions for the spin-polarized (e) and the spin-balanced gases (f).