Vacuum-induced transparency

Engineered strong photon-photon interactions would enable a broad range of applications in quantum information processing. We demonstrate, using an ensemble of atoms strongly coupled to an optical cavity, that light transmission through an opaque medium may be controlled with a few photons and even with the electromagnetic vacuum field. The vacuum field induces 40% transmission, accompanied by a group delay of 25 ns, which corresponds to the light pulse propagating at 1600 m/s through the medium. The transmission increases to 80% when ~10 photons are injected into the cavity. The strong optical nonlinearity at a few-photon level observed in this system opens prospects for advanced quantum devices such as photon-number-state filters.