Strong coupling between photons of two light fields mediated by one atom

All-optical sensing of the number of photons in one light field with another light field is a longstanding goal with intriguing prospects for various quantum applications 1 . A suitable system must be capable of strongly coupling individual photons of the two fields 2 . Here we report on the realization of such a system for two fields at wavelengths 780 nm and 795 nm. These fields drive two modes of an optical cavity, each strongly coupled to separate transitions of a single rubidium atom. An additional control laser addresses the atom and induces a tunable coupling between the modes, resulting in a doubly nonlinear energy-level structure of the photon–photon–atom system3–6. We observe strong correlations between the light fields, with photons either mutually blocking each other or transiting the system conjunctly, and demonstrate all-optical switching at the single-photon level as a first application. In this new setting of strongly coupled light fields, nondestructive counting of photons with photons 7 and heralded n-photon sources 8 might be within reach.