1. An interferometer for controlling the coalescence of a pair of photons, comprising: an optical source configured to generate a first pump pulse and a second pump pulse coherent with each other and shifted in time by a delay; first interferometric means configured to receive the first pump pulse and generate an antisymmetric state with two coalescent photons (1/√{square root over (2)}(|2 1 ,0 2 −|0 1 ,2 2 )); second interferometric means configured to receive the second pump pulse and to generate a symmetric state with two coalescent photons (1/√{square root over (2)}·(|2 1 ,0 2 +|0 1 ,2 2 )); and wherein the first and second interferometric means are connected so that the interferometer is configured to output a final state equal to a weighted sum of the antisymmetric state and the symmetric state (1/√{square root over (2)}·(|2 1 ,0 2 +|0 1 ,2 2 )+e iτ ·1/√{square root over (2)}·(|2 1 ,0 2 −|0 1 ,2 2 )), the weights of the sum being a function of the delay.