Two-photon correlations: from Young experiments to heavy-ion collision dynamics

Two-photon correlations are discussed within the formalism of Hanbury-Brown and Twiss interferometry and Bose-Einstein correlations. The technique is presented as a universal tool to study the properties of any boson source - light sources such as stars, or photon and meson sources in the early phase of heavy-ion collisions. The formalism is developed starting from optics and quantum statistics and is finally adapted to the dynamics of heavy-ion collisions. Emphasis is put on the experimental methods derived to display the interference between photons from nuclear reactions. The influence of one-dimensional projections and the detector response on the interpretation of the source properties are discussed. The method is illustrated using experimental data, available only in the intermediate (several tens of A MeV) energy domain. The observed interference signal is interpreted, guided by dynamical phase-space calculations, in terms of source size and reaction dynamics. It is found that photons are emitted as brief light Bashes, the relative intensity of which can be linked via model calculations to the incompressibility modulus of nuclear matter. At ultrarelativistic energies, two-photon correlations are presented as a tool to observe the phase transition towards the quark-gluon plasma.