Abstract: Apparatus for producing a multiplicity of photons having quantum-entangled single-photon states is provided. The single photon includes two quantum-entangled degrees of freedom, and the apparatus includes a source apparatus of the multiplicity of photons having quantum-entangled single-photon states including first- and second-generation stages. The first stage includes a first element having a source generating a multiplicity of photons, the first element selects a first degree of freedom of two degrees of freedom of the single photon, which includes only one pair of values, and a second element that selects a second degree of freedom of two degrees of freedom of the single photon. The second degree of freedom includes only one pair of values, the second stage: generates a coherent superposition of the two degrees of freedom of the single photon; selects one value of a first and a second of said two degrees of freedom.