Quantum Atomic Arrays: Fractional Filling and Trapping

Quantum emitters, in particular, atomic arrays with subwavelength lattice constant, have been proposed to be an ideal platform for study the interplay between photons and electric dipoles. Previous theoretical studies are based on spin models, where each site is occupied by a point-like atom. In this work, motivated by the recent experiment [1], we develop a full quantum treatment using annihilation and creation operator of atoms in deep optical lattices. We use a diagrammatic approach on the Keldysh contour to derive the cooperative scattering of the light and obtain the general formula for the S matrix. We apply our formulism to study two effects beyond previous treatment with spin-operators, the effect of fractional filling and trapping. Both effects can lead to imperfectness of atomic mirrors. For the fractional filling case, we find the cooperative linewidth is linear in filling fraction n. When there is a mismatch between the trapping potentials for atoms in the ground state and the excited state, multiple resonances can appear in the response function. Our results are consistent with existing experiments