Atoms, ions, molecules, or artificial atoms are all considered atomic systems that play a central role in quantum optics, quantum metrology, quantum computing, and simulation. Especially, trapped ions and neutral atoms serve as the best qubits as they are identical in nature and have extremely long-lived coherence.

Superconducting qubits are sometimes referred to as artificial atoms because they share similar energy structures and behavior with atoms (and lasers) while interacting with microwave photons. Since these qubits are typically coupled to microwave fields guided by transmission lines, such systems serve as a good testbed for studying quantum optics in lower dimensions, where strong coupling can be achieved.

Cooperativity arises in many-body systems through exchange interactions. Typical examples include super- and subradiance, where emitters contribute to the collective (transition) dipole either constructively or destructively, leading to enhanced or suppressed decay rates, respectively. Cooperative effects are highly nontrivial and nonlinear, depending on factors such as the degree of excitation, system geometry, and ensemble density.