Development of Technologies for Sub-Quantum-Noise-Limited Gravitational-wave Interferometers

引力波干涉仪测量由于引力波（GW）通过而引起的干涉仪反射镜运动所产生的光信号。由于gw诱导的运动极小，激光上的量子力学噪声会对探测器的灵敏度造成严重的限制。这种量子噪声产生于两种影响：(i)由于量子力学波动导致的干涉仪输出处光子数的不确定性（称为散粒噪声）；（ii）施加使干涉仪反射镜移动的力的光压（称为辐射压力噪声或反作用噪声）。海森堡测不准原理为射击噪声和反动作噪声的乘积设定了最小值，但如果反动作噪声增加，则允许将最小射击噪声降低到标准水平以下，反之亦然。这个过程有时被称为“挤压”，因为来自一个过程的噪声被“挤压”到另一个过程中。例如，以前的实验已经表明，激光可以通过使其振幅波动小而使其相位不确定性更大而被压缩。将进行实验来产生和研究适合注入引力波干涉仪的光的压缩状态。这项工作将集中在压缩光的各个方面，这对提高未来GW干涉仪的灵敏度最为重要：比以前探索过的更低频率的真空压缩。两种方法正在并行开发：(i)使用非线性光学介质，如铌酸锂晶体，其中由于晶体中光束相互作用产生的相关性，输出光被压缩；（二）利用低质量机械振荡器的运动与强激光之间的耦合，这使得光被压缩，因为由于光的振幅波动与光的相位耦合而引起的力引起的振荡器的运动。这两个实验的目标是在几百赫兹的频率下产生高达6db的真空压缩。除了提高引力波探测器的灵敏度外，实现这一目标所需的长期技术进步将应用于量子光学、量子信息、（亚）纳米级机械系统和精密测量。

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, wherethe 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.