A magic mirror - Quantum applications of the optical beamsplitter

Mirrors are some of the simplest optical components, and their use in optical imaging is well known. They have many other applications, such as the control of laser beams or in optical communication. Indeed they can be found in most optical instruments. It is the partially reflecting mirror, better known as the beam splitter, that is of particular interest to us. It lies at the centre of a number of recent scientific discoveries and technical developments that go beyond the limits of classical optics and make use of the quantum properties of light. In this area Australian and New Zealand researchers have made major contributions in the last two decades. I will discuss how a mirror modifies the light itself and the information that can be sent by a beam, and summarise the recent scientific achievements. This article combines the idea of photons, where the idea of quantisation is immediately obvious, with the idea of modulating continuous laser beams, which is practical and similar to the engineering description of radio communication.