A quantum switch for the world’s lightest mirror

One of the most fundamental processes in quantum optics is the coupling of a single atom to a single quantum of the light field, the photon. Usually, most of the photons in a visible light beam sent towards an atom pass without noticing the presence of the atom. This is a consequence of the weak light-matter coupling. Achieving strong coupling is thus one of the great challenges of modern quantum optics. Recently, scientists at MPQ demonstrated a new way to enhance light-matter interaction by arranging many atoms in a specific and controlled way to achieve very strong light-matter coupling at a single-photon level.

Concretely, the research team assembled Rubidium atoms in a fixed two-dimensional regular pattern at distances below the wavelength of an incoming photon: “We want the atoms to be arranged at a sub-wavelength distance because that is when they act “cooperatively”. That means that the atomic array as a whole inherits some properties of single atoms, but at much higher effective light-matter coupling strength”, Kritsana Srakaew, doctoral student and first author of the paper, explains. As a consequence of the increased light-matter coupling, even a single photon that would pass a single atom is likely to be reflected very efficiently. This realises the world’s lightest mirror, which consists of just a single layer of atoms.