Beam splitter for dark and bright states of light

Beam splitters are indispensable elements in optical and photonic systems, and are therefore employed in both classical and quantum technologies. Depending on the intended application, these devices can divide incident light according to its power, polarization state, or wavelength. In this work, we theoretically present a novel type of beam splitter capable of separating a light beam into its two-mode bright and dark components. We propose a prototype based on an optical cross-cavity system resonantly coupled to a $\Lambda$-type three-level atom. The dark component of the incoming light is transmitted because the antisymmetric collective mode of the cavity setup is decoupled from the atom. Meanwhile, in a high-cooperativity regime, the bright component is reflected due to Autler-Townes splitting, which arises from the strong coupling between the atom and the symmetric collective mode of the cavity setup. Although the device requires only a two-level atom to operate, using a three-level atom allows the device to be turned on or off by controlling the atomic ground state. Our results pave the way for new applications of beam splitters that leverage the collective properties of light. Manipulating and exploiting this additional degree of freedom can advance the field of quantum optics and contribute to the development of quantum technologies.