Quantum Optical Tests of the Foundations of Physics

Quantum mechanics began with the solution of the problem of blackbody radiation by Planck's quantum hypothesis: in the interaction of light with matter, energy can only be exchanged between the light in a cavity, and the atoms in the walls of the cavity by the discrete amount E = hν, where h is Planck's constant, and ν is the frequency of the light. Einstein, in his treatment of the photoelectric effect, reinterpreted this equation to mean that a beam of light consists of particles (“light quanta”) with energy hν. The Compton effect supported this particle viewpoint of light by demonstrating that photons carried momentum, as well as energy. In this way, the wave-particle duality of quanta made its first appearance in connection with the properties of light. It might seem that the introduction of the concept of the photon as a particle would necessarily also introduce the concept of locality into the quantum world. However, in view of observed violations of Bell's inequalities, exactly the opposite seems to be true. Here, we review some recent results in quantum optics, which elucidate nonlocality and other fundamental issues in physics.