Tunable bandwidth and nonlinearities in an atom-photon interface with subradiant states

Optical nonlinearities at the single-photon level are key features to build efficient photon-photon gates and to implement quantum networks. Such optical nonlinearities can be obtained using an ideal two-level system such as a single atom coupled to an optical cavity. While efficient, such atom-photon interface however presents a fixed bandwidth, determined by the spontaneous emission time and thus the spectral width of the cavity-enhanced two-level transition, preventing an efficient transmission to bandwidth-mismatched atomic systems in a single quantum network. In the present work, we propose a tunable atom-photon interface making use of the direct dipole-dipole coupling of two slightly different atomic systems. We show that, when weakly coupled to a cavity mode and directly coupled through dipole-dipole interaction, the subradiant mode of two slightly detuned atomic systems is optically addressable and presents a widely tunable bandwidth and single-photon nonlinearity.