Few-photon all-optical modulation in a photonic band-gap fiber.

We demonstrate 25% all-optical modulation with <20 photons, i.e., a few attojoules of energy, using nondegenerate two-photon absorption in rubidium atoms confined to a hollow-core photonic band-gap fiber. An attenuation of up to 3 dB is induced on an optical field with a switching energy density of less than one photon per (λ(2)/2π). We show that the temporal response of the system is determined by the 5-ns transit time of the atoms across the optical mode of the fiber, which results in a modulation bandwidth up to 50 MHz.