Nonclassical Interferometry towards Gravitational-Wave Detectors at a Laser Wavelength of 2.1um

最近对双黑洞合并引力波的直接观测标志着引力波天文学的开始。为了实现高信噪比的连续检测，基于激光干涉测量原理的下一代引力波探测器将致力于将应变灵敏度提高至少一个数量级。在大部分检测频带中，限制噪声源是由反射镜涂层和基底中的热噪声以及激光光场的量子噪声给出的。未来的探测器预见到晶体硅作为镜子材料和在低温下运行的变化。这需要将激光波长从目前使用的 1μm 更改为 2μm 左右。与此同时，光的压缩态已被成功证明可以减少引力波探测器中的量子噪声。因此，将这两项进步结合起来是迈向引力波天文学成功时代的重要一步。到目前为止，激光雷达和医疗应用推动了 2μm 左右激光器的发展，因此对于引力波探测器中高功率激光器的严格稳定性要求还缺乏经验。此外，压缩光尚未在2μm处得到证实，并且具有接近统一量子效率的光电探测器（以便不破坏压缩场中脆弱的非经典相关性）尚未可用。项目团队将开发针对引力波探测的2μm激光技术的完整解决方案，该解决方案仅基于现有高度稳定的1064nm激光源的简并参数下转换。在该项目中，我们将开发 2.128μm 的压缩光源，首次证明该波长与先进的量子降噪技术兼容。此外，我们将展示通过光学参量放大对检测损失的补偿，部分消除了对具有几乎完美量子效率的光电探测器的需求。因此，这项工作的结果将在规划和实现未来引力波探测器、突破可观测宇宙的边界方面发挥重要作用。

The recent direct observation of gravitational waves from a binary black-hole merger has marked the beginning of gravitational-wave astronomy. To enable a continuous stream of detections with a high signal-to-noise ratio, upcoming generations of gravitational-wave detectors, which are based on the principle of laser interferometry, will aim at an increase of strain sensitivity by at least an order of magnitude. Through most of the detection band, the limiting noise sources are given by thermal noise in the mirror coatings and substrates, as well as quantum noise of the laser light field. Future detectors foresee a change to crystalline silicon as mirror material and operation at cryogenic temperatures. This will need to be accompanied by a change in laser wavelength to around 2µm, from the currently used 1µm. At the same time, squeezed states of light have been successfully shown to reduce the quantum noise in gravitational-wave detectors. Combining these two advancements is therefore a major step towards a successful era of gravitational-wave astronomy. So far, laser development at around 2µm has been driven by LIDAR and medical applications, therefore little experience exists with the demanding stability requirements for high-power lasers in gravitational-wave detectors. Furthermore, squeezed light has not been demonstrated at 2µm, and photo detectors with a near-unity quantum efficiency - so as to not destroy the fragile nonclassical correlations in the squeezed field - are not yet available.The project team will develop a complete solution for 2µm laser technology aimed at gravitational-wave detection that is solely based on degenerate parametric down-conversion of the existing highly stable 1064nm laser sources. Within this project, we will develop a squeezed-light source at 2.128µm, demonstrating for the first time that this wavelength is compatible with advanced quantum-noise reduction techniques. In addition, we will show compensation of detection loss by optical parametric amplification, partly removing the need for photo detectors with almost perfect quantum efficiency. The results of this work will thus play a significant role in planning and enabling future gravitational-wave detectors, pushing the boundaries of the observable universe.