  According to quantum theory the interactions between physical systems are
quantized. As a direct consequence, measurement sensitivities are fundamentally
limited by quantization noise, or just `quantum noise' in short. Furthermore,
Heisenberg's Uncertainty Principle demands measurement back-action for some
observables of a system if they are measured repeatedly. In both respects,
squeezed states are of high interest since they show a `squeezed' uncertainty,
which can be used to improve the sensitivity of measurement devices beyond the
usual quantum noise limits including those impacted by quantum back-action
noise. Squeezed states of light can be produced with nonlinear optics, and a
large variety of proof-of-principle experiments were performed in past decades.
As an actual application, squeezed light has now been used for several years to
improve the measurement sensitivity of GEO600 -- a laser interferometer built
for the detection of gravitational waves. Given this success, squeezed light is
likely to significantly contribute to the new field of gravitational-wave
astronomy. This Review revisits the concept of squeezed states and two-mode
squeezed states of light, with a focus on experimental observations. The
distinct properties of squeezed states displayed in quadrature phase-space as
well as in the photon number representation are described. The role of the
light's quantum noise in laser interferometers is summarized and the actual
application of squeezed states in these measurement devices is reviewed.
