  Optical non-linearities at the single photon level are key features to build
efficient photon-photon gates and to implement quantum networks. Such optical
non-linearities 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.
