Quantum optics
''These days, every Tom, Dick and Harry thinks he knows what a photon is, but he is wrong.'' – Albert Einstein (c.1950)
Einstein's blunt statement still true. Investigations using quantum optical approaches are aimed at better understanding the basic nature of reality that is implied by quantum theory. Quantum mechanics, with its extension to quantum electrodynamics, is arguably the most successful theory in physics, with unrivalled precision in its verified mathematical predictions. And yet, the question of how we should interpret quantum theory remains an issue of considerable philosophical debate 100 years after the discovery of the Schrödinger equation.
The nature of the photon is a central element quantum theory, and quantum optics experiments allow us to probe the non-classical behaviour of light. Investigations have been undertaken globally over the last thirty years with pairs of entangled photons, which must be considered as having joint descriptions only, rather than individual properties. These have demonstrated that the quantum world is essentially non-local, and have led to recent applications in quantum communications and computation.
