Collective states in highly symmetric atomic configurations, and   single-photon traps
  Abbreviated Abstract: We study correlated states in a circular and linear-chain configuration of identical two-level atoms containing the energy of a single quasi-resonant photon in the form of a collective excitation, where the collective behaviour is mediated by exchange of transverse photons between the atoms. For a circular configuration of atoms the effective Hamiltonian on the radiationless subspace of the system can be diagonalized analytically. In this case, the radiationless energy eigenstates carry a $\mathbb{Z}_N$ quantum number $p=0,1, ..., N$ which is analogous to the angular momentum quantum number $l= 0, 1, ...$, carried by particles propagating in a central potential, such as a hydrogen-like system. Just as the hydrogen s-states are the only electronic wave functions which can occupy the central region of the Coulomb potential, the quasi-particle corresponding to a collective excitation of the circular atomic sample can occupy the central atom only for vanishing $\mathbb{Z}_N$ quantum number $p$. For large numbers of atoms in a maximally subradiant state, a critical interatomic distance of $\lambda/2$ emerges both in the linear-chain and the circular configuration of atoms. The spontaneous decay rate of the linear configuration exhibits a jump-like "critical" behaviour for next-neighbour distances close to a half-wavelength. Furthermore, both the linear-chain and the circular configuration exhibit exponential photon trapping once the next-neighbour distance becomes less than a half-wavelength, with the suppression of spontaneous decay being particularly pronounced in the circular system. In this way, circular configurations containing sufficiently many atoms may be natural candidates for {\it single-photon traps}. 
Hanno Hammer