Gravitational-wave interferometers measure optical signals generated by motion of the interferometer mirrors due to a passing gravitational wave (GW). Since the GW-induced motion is extremely small, quantum mechanical noise on the laser light can pose a serious limitation to the detector sensitivity. This quantum noise arises from two effects: (i) uncertainty in the number of photons at the interferometer output due to quantum mechanical fluctuations (known as shot noise); and (ii) light pressure which exerts forces that move the mirrors of the interferometer (known as radiation pressure noise or back action noise). The Heisenberg Uncertainty Principle sets a minimum for the product of the shot noise and back action noise, but it also allows the minimum shot noise to be lowered below the standard level, provided the back action noise is increased, or vice versa. This process is sometimes called "squeezing" because the noise from one process is "squeezed" into the other. For example, previous experiments have shown how laser light can be squeezed by making its amplitude fluctuations small, but giving greater uncertainty in its phase.
Experiments will be carried out to generate and study squeezed states of light that are suitable for injection into a gravitational-wave interferometer. The effort will concentrate on the aspects of squeezed light most important for improving the sensitivity of future GW interferometers: vacuum squeezing at lower frequencies than have previously been explored. Two methods are being developed in parallel: (i) use of nonlinear optical media, such as crystals of lithium niobate, where the output light is squeezed due to correlations created by interaction of light beams in the crystal; and (ii) use of the coupling between the motion of a low-mass mechanical oscillator and intense laser light, which causes the light to be squeezed because the motion of the oscillator induced by forces due to amplitude fluctuations of the light couples to the phase of the light. The goal with both these experiments is to yield up to 6 dB of vacuum squeezing at a few hundred Hertz. In addition to improved sensitivity for gravitational wave detectors, the long-term technical advances necessary to achieve this goal will have applications in quantum optics, quantum information, (sub-)nanoscale mechanical systems and precision measurement.
