Quantum Enhanced Interferometry

We review the field of quantum enhanced interferometry with an emphasis on optical scenarios. The topic became popular in preparing for detecting gravitational waves in the 1970ies and 1980ies with pioneering theoretical work, first by Braginsky and later by Thorne, Unruh and Caves. They showed that quantum correlations can modify and reduce the quantum uncertainty ultimately limiting the sensitivity of an interferometer. The first experimental demonstration was the experiment by Xiao, Wu and Kimble in 1987. They showed a sensitivity improvement of a Mach-Zehnder interferometer by using squeezed light, which is one way of using quantum correlations. In 1987, Yurke, McCall and Klauder set the stage in a more general sense, reviewing the standard SU(2) interferometer and introducing the SU(1,1) interferometer. Recent years, much progress was made in all these aspects