Quantum Optomechanics on Multiple Mass Scales

经典力学定律很好地描述了人类、网球和汽车等日常物体的运动，该定律最早由艾萨克·牛顿提出。但量子力学更准确地描述了原子及其组成粒子的微观世界。这两种物理机制--经典和量子--是众所周知的，但在这两种机制之间的边界会发生什么呢？甚至有一个清晰、明确的边界吗？物体什么时候从经典行为转变为量子行为？我们为什么要关心呢？科学家们已经认识到，量子行为存在于各种大小、形状和组成的物体中，但通常被热噪声掩盖--组成物体的原子由于储存和释放的热能而不断抖动。消除热噪声以揭示量子力学行为是非常困难的，即使对于一小部分原子也是如此，对于更大的物体来说肯定是非常困难的。但这是可以做到的，最初只针对几个特殊的原子，最近也可以针对越来越大的物体。表现出量子行为的宏观物体不仅对研究经典-量子边界很有用，而且还具有一些非常实际的应用。在这些应用中，有一种是产生奇异的光的量子态，称为“压缩”态，可以用来进行比普通光更精确的测量。干涉测量引力波探测器，如先进的LIGO探测器，发现了来自碰撞黑洞的引力波，依赖于以低于万亿分之一米(不到十亿分之一米)精度测量镜子的位置。为了进一步提高它们的灵敏度，人们可以向这些仪器注入压缩状态的光。这项工作涉及通过将激光强烈耦合到运动由量子力学效应而不是热噪声主导的可移动反射镜来制造这些特殊的光状态。为了形成压缩状态，有必要找到一种方法将光的两个通常不相关的属性--幅度和相位的组合--联系起来。这通常是通过让光通过一种非线性光学材料来实现的，该材料的折射率取决于电场的强度(光的幅度)，这样当光通过该材料时，幅度的波动就会印在光的相位上。在这项工作中，PI使用了另一种相对未被探索的使用光学机械耦合的方法。一束强光入射到一面可移动的镜子上。由于辐射压力，光的幅度波动驱动反射镜的位置。镜面位置的起伏印在从镜面反射的光的相位上，从而将幅度和相位起伏联系起来。该装置包括一个光学腔，其中一个反射镜是由GaAlAs异质结构布拉格反射镜制成的纳米悬臂振荡器。另一面镜子被烧蚀并涂覆在光纤尖端上。其目的是使用该装置在较宽的音频频段内产生压缩光。要做到这一点，必须降低镜面振荡器的热噪声，使其运动受到量子涨落的支配，这需要高度优化的振荡器设计，也可以是低温冷却的。与任何量子限制实验一样，必须减少和控制各种其他经典噪声，如地震和声学振动、光学系统的经典强度和相位噪声。

The motion of everyday objects like humans, tennis balls and cars is very well described by the the laws of classical mechanics, first laid out by Isaac Newton. But the microscopic world of atoms and their constituent particles is more accurately described by quantum mechanics. These two regimes of physics - classical and quantum - are well understood, but what happens at the boundary between the two regimes? Is there even a clear, well-defined boundary? When does an object transition from classical to quantum behavior? And why should we care? Scientists have come to understand that quantum behavior is present in objects of all sizes, shapes and compositions, but is usually masked by thermal noise - the constant jittering of atoms that make up the objects due to heat energy stored and released. Removing thermal noise to reveal quantum mechanical behavior is very difficult even for a small collection of atoms, and certainly very hard for much larger objects. But it can be done, initially just for a few special atoms, and recently for increasingly larger objects. Macroscopic objects that exhibit quantum behavior are useful not only for studying the classical-quantum boundary, but also have some very practical applications. Among these applications is generating exotic quantum states of light, called "squeezed" states, that can be used to make more precise measurements than ordinary light. Interferometric gravitational wave detectors, such as the Advance LIGO ones that discovered gravitational waves from colliding black holes, rely on measuring the positions of mirrors with sub-attometer (less than one billionth of a billionth of a meter) precision. To further improve their sensitivity, one can inject squeezed states of light into these instruments. This work pertains to making these special states of light by strongly coupling laser light to movable mirrors whose motion is dominated by quantum mechanical effects rather than thermal noise. To make a squeezed state, it is necessary to find a way to correlate two usually uncorrelated properties of the light - a combination of the amplitude and phase. This is usually done by passing light through a nonlinear optical material whose refractive index depends on the strength of the electric field (amplitude of the light), such that amplitude fluctuations get imprinted on the phase of the light as it passes through that material. In this work the PI uses an alternative, and relatively unexplored, method of using optomechanical coupling. An intense light beam is incident on a movable mirror. The amplitude fluctuations of the light drive the mirror position due to radiation pressure. The mirror position fluctuations are imprinted on the phase of the light reflected from the mirror, thus correlating amplitude and phase fluctuations. The setup comprises an optical cavity where one mirror is a nano-fabricated cantilevered oscillator made of a GaAlAs heterostructure Bragg reflector. The other mirror is ablated and coated on to an optical fiber tip. The aim is to generate squeezed light in a broad audio frequency band using this apparatus. To do so, it is necessary to reduce the thermal noise of the mirror oscillator enough for its motion to be dominated by quantum fluctuations, which requires a highly optimized oscillator design that may also be cryogenic cooled. As with any quantum-limited experiment, a variety of other classical noises, such as seismic and acoustic vibrations, classical intensity and phase noise of the optical system must be reduced and controlled.