Preparing the ancilla atom in the Rydberg state |P>, 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|P> and a global offset as free fit parameters (solid orange lines), we find excellent agreement with our data set with P|P> of 0.61(2) and 0.45(2) in transmittance and reflectance, respectively, in good agreement with an independent reference measurement of P|P>=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|P>=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.
