Fig. 2: Cooperative response in the absence and presence of the Rydberg ancilla.

a, The cooperative response of the atomic array with (blue) and without (red) the control beam for a probe duration of t_p = 20 μs and the ancilla prepared in \(\left\vert {g}^{{\prime} }\right\rangle\). Without the control laser, we reproduce the cooperative subradiant response of the mirror with a transmission and reflection linewidth Γ_M/2π of 4.40(32) or 3.75(14) MHz, respectively. With the control laser present, we observe a splitting of the single peak into an EIT doublet, where the width of each peak Γ_EIT/2π amounts to 2.95(17) and 3.01(28) MHz in transmission and reflection, respectively, and a minimal transmittance and maximal reflectance of 0.35(2) and 0.37(2), respectively, is observed. b, Preparing the ancilla atom in the Rydberg state \(\left\vert P\right\rangle\), the spectra change dramatically, and reveal a triple-peak structure, featuring contributions of both the cooperative mirror and EIT spectrum. Superimposing both spectra while having the ancilla Rydberg fraction \({P}_{\left\vert P\right\rangle }\) and a global offset as free fit parameters (solid orange lines), we find excellent agreement with our data set with \({P}_{\left\vert P\right\rangle }\) of 0.61(2) and 0.45(2) in transmittance and reflectance, respectively, in good agreement with an independent reference measurement of \({P}_{\left\vert P\right\rangle }=0.52(8)\) (Supplementary Information). The dashed orange lines illustrate the expected spectra assuming ideal ancilla preparation and substantially shorter probe duration than the Rydberg lifetime (t_p = 2 μs), improving the ancilla Rydberg fraction to \({P}_{\left\vert P\right\rangle }=0.96\). The insets in each figure illustrate the atomic array and beam directions for each experimental configuration. The red (blue) arrows indicate the incident and scattered probe (control) beam directions. The measurements are averages over 70–125 independent repetitions. Error bars denote the s.e.m.