Quantum Optomechanics on Multiple Mass Scales

人类、网球和汽车等日常物体的运动都可以很好地用经典力学定律来描述，而经典力学定律最早是由艾萨克·牛顿提出的。但是原子及其组成粒子的微观世界可以用量子力学来更准确地描述。这两种物理体系——经典和量子——已经被很好地理解了，但是在这两种体系之间的边界会发生什么呢？是否有一个清晰、明确的界限？一个物体什么时候从经典行为过渡到量子行为？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、形状和成分的物体中，但通常被热噪声所掩盖，热噪声是构成物体的原子由于储存和释放热能而不断抖动。消除热噪声以揭示量子力学行为是非常困难的，即使对一小群原子来说也是如此，对更大的物体来说当然是非常困难的。但这是可以做到的，最初只是对一些特殊的原子，最近对越来越大的物体。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，而且有一些非常实际的应用。在这些应用中，产生奇异的光量子态，称为“压缩”态，可以用来比普通光进行更精确的测量。干涉式引力波探测器，比如发现黑洞碰撞引力波的先进LIGO探测器，依靠亚原子计（小于十亿分之一米的十亿分之一）精度来测量镜子的位置。为了进一步提高它们的灵敏度，人们可以向这些仪器中注入压缩态的光。这项工作是通过将激光与运动受量子力学效应而不是热噪声支配的可移动镜子强耦合来制造这些特殊的光态。为了制造一个压缩态，有必要找到一种方法将光的两个通常不相关的特性——振幅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，这种材料的折射率取决于电场的强度（光的振幅），这样，当光通过该材料时，振幅的波动就会在光的相位上留下印记。在这项工作中，PI使用了一种替代的，相对未开发的，使用光机械耦合的方法。一束强光照射在可移动的镜子上。由于辐射压力，光的振幅波动驱动反射镜的位置。反射镜位置波动被印在从反射镜反射的光的相位上，从而使振幅和相位波动相关联。该装置包括一个光学腔，其中一个镜子是由GaAlAs异质结构Bragg反射器制成的纳米制造悬臂振荡器。另一面镜子被烧蚀并涂在光纤尖端上。目的是利用这种装置在宽音频频带内产生压缩光。要做到这一点，有必要将镜像振荡器的热噪声降低到足以使其运动受量子涨落支配的程度，这需要高度优化的振荡器设计，也可以进行低温冷却。与任何量子限制实验一样，必须减少和控制光学系统的各种其他经典噪声，如地震和声学振动、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.