Single-mode operation of a high-brightness narrow-band single-photon source

We demonstrate a narrow-band single-photon source based on cavity-enhanced parametric downconversion for efficient coupling to atomic quantum memories. Selecting a single longitudinal cavity mode by spectral filtering, we achieve a spectral brightness of ∼330 biphotons/(s mW MHz) and an overall bandwidth of 2.7 MHz, which suits the width of naturally broadened transitions in alkali gases like rubidium or cesium.