Nonperturbative Treatment of Giant Atoms Using Chain Transformations

Giant atoms provide a fascinating example of how superconducting circuit implementations extend the range of quantum optical phenomena that can be experimentally studied. In particular, giant atoms permit the investigation of systems beyond the dipole approximation and exhibit pronounced non-Markovian effects. For example, polynomial decay, the possibility to design frequency-dependent coupling or the emergence of so-called oscillating bound states have been predicted for giant atoms.



While previous studies of giant atoms focused on the realm of the rotating-wave approximation (RWA), we go beyond this and perform a numerically exact analysis of giant atoms strongly coupled to their environment, in regimes where counter-rotating terms cannot be neglected. To achieve this, we use Lanczos transformations to cast the field Hamiltonian into the form of a one-dimensional chain and employ matrix-product state simulations. This yields access to all system-bath observables, in yet unexplored parameter regimes. We confirm the emergence of oscillating bound states and demonstrate the breakdown of the RWA in the strong coupling regime.