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

引力波干涉仪测量由于经过的引力波(GW)引起干涉仪反射镜的运动而产生的光学信号。由于GW引起的运动非常小，激光上的量子力学噪声会严重限制探测器的灵敏度。这种量子噪声产生于两种影响：(I)由于量子力学波动(称为散粒噪声)导致干涉仪输出的光子数不确定；(Ii)施加移动干涉仪反射镜的力的光压(称为辐射压力噪声或背作用噪声)。海森堡不确定度原理为散粒噪声和背动噪声的乘积设定了最小值，但它也允许在背动噪声增加的情况下将最小散粒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音被“挤压”到另一个过程中。例如，以前的实验已经展示了如何通过减小激光的幅度波动来压缩激光，但在相位上提供了更大的不确定性。将进行实验来产生和研究适合注入引力波干涉仪的压缩光。这项工作将集中在压缩光方面，这对提高未来GW干涉仪的灵敏度最重要：比以前探索的频率更低的真空压缩。同时开发了两种方法：(I)使用非线性光学介质，如Nb酸锂晶体，其中输出光由于晶体中光束的相互作用而被压缩；以及(Ii)使用低质量机械振荡器的运动与强激光之间的耦合，这导致光被压缩，因为振荡器的运动由光的幅度波动引起的力耦合到光的相位。这两个实验的目标都是在几百赫兹的频率下产生高达6分贝的真空压缩。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.