Direct observation of the particle exchange phase of photons

Quantum theory stipulates that if two particles are identical in all physical aspects, the allowed states describing system either symmetric or antisymmetric with respect to permutations of single-particle states1–5. Experimentally, symmetry can be inferred indirectly from fact neglecting correct exchange theoretical analysis leads dramatic discrepancies observations6–13. The only way directly unveil for, say, is through interference state itself and its physically permuted version, measuring phase associated permutation process, so-called particle phase14. Following this idea, we have observed indistinguishable photons, providing direct evidence their bosonic character. An interferometric technique developed measure photons. obtained (?0.04 ± 0.07) radians, which clearly demonstrates nature two-photon wave functions.