Photon counting as a tool for basic research

Two experiments of fundamental interest utilizing photon counting techniques are discussed. The first, not carried out as yet, aims to demonstrate an unusual phenomenon known as photon antibunching. Particular consideration is given to the experimental realization of the specific essentially nonclassical states of the electromagnetic field in which antibunching is observed. The second experiment, already carried out successfully, is concerned with the old quantum mechanical problem of the existence of hidden variables. This provides a very sensitive test of the fundamental quantum-mechanical concepts, and the results clearly confirm the quantum-mechanical prediction, and rule out any theories of local hidden variables.