Abstract: There is presented an optical apparatus comprising first and second photon pair sources configured to convert at least one pump light photon into a first and second correlated signal and idler photon pairs. In one example, the apparatus is configured to use one of the signal and idler photons from the first correlated photon pair for controlling the conversion of the pump light photon in the second photon pair source. The apparatus may configured such that, at least one of the signal and idler photons from the first correlated photon pair is output from the first photon pair source onto an optical path wherein at least one of the signal and idler photons from the second correlated photon pair is output from the second photon pair source onto the optical path. A method is also provided for outputting one or more photons using the optical apparatus.