Cavity optomechanics: towards sensing at the quantum limit

试图将一个小型机械装置冷却到其量子基态的巨大挑战正在推动全球许多领先实验小组的激烈活动。仅仅在十年前，这似乎是一个不可行的目标，现在似乎非常接近：通过光学机械技术，微机械谐振器，如小镜子和杠杆已经冷却了几个数量级，下降到n~30的数量级。接近基态（n~1）的最终目标现在看来是一个现实的前景，尽管仍然存在严重的障碍;其中，与环境的热耦合是最严重的。（包括PI）独立提出了一个新的方案，该方案具有基本的新设计：介电纳米球，光学悬浮在空腔中，并通过由光场产生的偶极力冷却。在某种意义上，与腔结构的机械连接的缺乏使器件与热噪声的重要来源绝缘，并且使该方案相对于常规器件具有独特的优势。该项目汇集了来自伦敦和南安普顿的实验和理论小组，最终目标是首次成功地实验实施这一计划。此外，我们的目标是通过对光学冷却纳米球系统进行完整和现实的模拟，从理论上彻底理解其基础物理。一旦达到量子极限，主要目标是在此范围内操作设备。回报可能是巨大的。这是一种可实现的量子技术，它提供了无与伦比的测量灵敏度的前景，仅受海森堡不确定性原理的限制。例如，正是由于这个原因，这些设备被用于引力波探测器，这需要非常精确的位移检测。它们还提供了对量子-经典边界的基本洞察的可能性：有可能研究仅因宏观物体的位移而不同的叠加。一些实验小组正在研究使用BEC（玻色爱因斯坦凝聚体）作为机械振荡器的偶极力耦合腔光学力学。在这种情况下，目标已经处于基态，因此已经可以探索量子机制。我们还将从理论上研究这种机制，以确定是否量子效应，如压缩，（其改善量子机制中的感测）可以在这样的方案中可行地产生，因为共同申请人中的两个已经确定了潜在的有希望的机制。最后，从长远来看，我们注意到，与这项工作并行，正在积极地开发诸如微米尺寸的杠杆的小传感器用于生物传感应用（用于生物分子的超灵敏检测或作为力传感器）。伦敦大学学院，特别是LCN（伦敦纳米技术中心）是这一领域的领导者。另一方面，致力于将光学机械设备冷却到量子极限的小组（如Vahala的加州理工学院小组）已经在测试它们作为生物传感器的潜力。从长远来看，一个理想的目标是实现这两个方向的合并：量子极限检测和生物传感。我们将探索基于我们的介电纳米球的就业计划的可行性。

The grand challenge of attempting to cool a small mechanical device towards its quantum ground state is driving intense activity in many leading experimental groups worldwide. What seemed an unfeasible target only a decade ago, now appears tantalisingly close: by means of optomechanical techniques, micromechanical resonators such as small mirrors and cantilevers have been cooled by several orders of magnitude, down to occupation numbers of order n~30. The ultimate goal of approaching the ground state (n~1) now seems a realistic prospect, although serious obstacles remain; among these, thermal coupling to the environment is the most serious.However, within the last year, three groups (including the PI's) have independently proposed a novel scheme which has a fundamental new design: a dielectric nanosphere, optically levitated in a cavity and cooled by dipole forces arising from the optical field. The lack of mechanical connection to the cavity structure in a sense insulates the device from important sources of thermal noise and gives this scheme a unique edge in relation to conventional devices. The project brings together experimental and theory groups from London and Southampton with the ultimate goal of successfully implementing this scheme experimentally, for the first time. In addition, we aim to thoroughly understand the underlying physics theoretically by undertaking complete and realistic simulations of the optically cooled nanosphere system.Once the quantum limit is achieved, the main target is to operate the device in this regime. The rewards are potentially great. This is an attainable quantum technology which offers the prospect of unparalleled sensitivity in measurement, limited only by the Heisenberg uncertainty principle. For example, it is for this reason that these devices are used for gravitational-wave detectors, which require extraordinarily precise detections of displacement. They offer also the possibility of fundamental insights into the quantum-classical border: it may be possible to investigate superpositions which differ only by the displacement of a macroscopic object. Some experimental groups are investigating dipole-force coupling cavity optomechanics using a BEC (Bose Einstein Condensate) as the mechanical oscillator. In this case, the target is already in the ground state so it is already possible to explore the quantum regime. We will also investigate this regime theoretically, in order to establish whether quantum effects like squeezing (which improve sensing in the quantum regime) may be viably generated in such a scheme, as two of the co-applicants have already identified a potentially promising regime.Finally, taking the long view, we note that in parallel to this work, small sensors such as micron-sized cantilevers are actively being developed for biosensing applications (for ultra-sensitive detection of biomolecules or as force sensors). UCL, in particular the LCN (London Centre for Nanotechnology) is a leader in this field. On the otherhand, groups (such as the Caltech group of Vahala) working to cool optomechanical devices to the quantum limit are already testing their potential as biosensors.A desirable ambition, in the long-term would be to achieve a merger of these two directions: quantum limited detection and biosensing. We will explore the viability of employing schemes based on our dielectric nanospheres.