A device for achieving multi-photon interference, said device comprising: at least two solid state photon emitters, each solid state photon emitter comprising nuclear and electron spin states coupled together, each solid state photon emitter being configured to produce photon emission comprising a photon emission peak, wherein the photon emission peaks from different solid state photon emitters have a first frequency difference between peak intensities, and wherein the electron spin states of each solid state photon emitter are resolvable; an excitation arrangement configured to individually address the at least two solid state photon emitters; a plurality of optical outcoupling structures wherein each solid state photon emitter is provided with an associated optical outcoupling structure; a tuning arrangement configured to reduce the first frequency difference between the peak intensities of the photon emission peaks from the at least two solid state photon emitters to a second frequency difference which is smaller than the first frequency difference; a photon interference arrangement configured to overlap photon emissions from the at least two solid state emitters after tuning; and a detector arrangement configured to detect photon emissions from the at least two solid state emitters after tuning and passing through the photon interference arrangement, wherein the detector arrangement is configured to resolve small differences in photon detection times such that tuned photon emissions from the at least two solid state emitters are quantum mechanically indistinguishable resulting in quantum interference between indistinguishable photon emissions from different solid state photon emitters, wherein the at least two solid state photon emitters are formed by photon emissive defects in a diamond material, and wherein the photon emissive defects are nitrogen vacancy defects; wherein the diamond material has a sufficiently low concentration of impurities such that the photon emissive defects have a decoherence time T 2 equal to or greater than 0.05 ms, and wherein the photon emissive defects are oriented relative to a crystallographic orientation of the diamond material. at least two solid state photon emitters, each solid state photon emitter comprising nuclear and electron spin states coupled together, each solid state photon emitter being configured to produce photon emission comprising a photon emission peak, wherein the photon emission peaks from different solid state photon emitters have a first frequency difference between peak intensities, and wherein the electron spin states of each solid state photon emitter are resolvable; an excitation arrangement configured to individually address the at least two solid state photon emitters; a plurality of optical outcoupling structures wherein each solid state photon emitter is provided with an associated optical outcoupling structure; a tuning arrangement configured to reduce the first frequency difference between the peak intensities of the photon emission peaks from the at least two solid state photon emitters to a second frequency difference which is smaller than the first frequency difference; a photon interference arrangement configured to overlap photon emissions from the at least two solid state emitters after tuning; and a detector arrangement configured to detect photon emissions from the at least two solid state emitters after tuning and passing through the photon interference arrangement, wherein the detector arrangement is configured to resolve small differences in photon detection times such that tuned photon emissions from the at least two solid state emitters are quantum mechanically indistinguishable resulting in quantum interference between indistinguishable photon emissions from different solid state photon emitters, wherein the at least two solid state photon emitters are formed by photon emissive defects in a diamond material, and wherein the photon emissive defects are nitrogen vacancy defects; wherein the diamond material has a sufficiently low concentration of impurities such that the photon emissive defects have a decoherence time T 2 equal to or greater than 0.05 ms, and wherein the photon emissive defects are oriented relative to a crystallographic orientation of the diamond material.