Resonance fluorescence of two asymmetrically pumped and coupled two-level systems

We study a driven-dissipative duo of two-level systems in an open quantum systems approach, modeling a pair of atoms or (more generally) meta-atoms. Allowing for complex-valued couplings in the setup, which are of both a coherent and incoherent character, gives rise to a diverse coupling landscape. We consider several points on this landscape, for example where the coupling between the two coupled two-level systems is dominated by coherent, incoherent, unsymmetrical, and even unidirectional interactions. Traversing the coupling terrain leads to remarkable features in the populations of the pair, correlations, and optical spectra. Most notably, the famous Mollow triplet spectrum for a single atom may be superseded for a pair by a Mollow quintuplet (or even by a spectral singlet) and the setup allows for population trapping to arise, all depending upon the precise nature of the coupling between the two-level systemsCAD is supported by a University Research Fellowship (URF/R1/201158) from the Royal Society. EdV acknowledges the CAM Pricit Plan (Ayudas de Excelencia del Profesorado Universitario), the TUMIAS Hans Fischer Fellowship, and the MCIN/AEI via Grant No. PID2020- 113415RB-C22. AIFD acknowledges sponsorship from the Spanish MCIN/AEI via the Grant Nos. PID2021- 126964OB-I00 and TED2021-130552B-C22, as well as the European Union’s Horizon Europe Research and Innovation Programme under agreement 101070700