突破机械振子微弱力测量经典极限的声子压缩

High sensitive detection of weak forces is of great importance in both fundamental study and applied science. Micro-mechanical resonators, which are used as universal forces transducers, have been the critical component of many sensing units. They have many applications in fundamental research and high precision measurements, such as Casimir force measurement, mechanical detection of single nuclear spins, and verification of the inverse square law. The measurement precision of mechanical resonator can be enhanced by suppressing its thermal oscillation through cooling. Although it's feasible to depress the thermal noise of the resonators through cooling to increase the measurement precision, there still exits the zero-point vibrational fluctuation even if cooling a resonator to its ground state, which sets the classical limit of mechanical-based force detection. As the rapid progress of high precision measurements towards the classical limit, it has become an emergent and challenging problem to perform measurement beyond the limit. Similar with achieving quantum measurement using the squeezed light, realizing of the mechanical squeezing affords a practical approach to beat the classical limit of mechanical-based force detection strategies. Benefiting from our knowledge and experiences on optical cooling and coherent manipulation of mechanical resonators, in this proposal, force detection beyond the classical limit will be investigated by integrating optical cooling and squeezing of mechanical oscillation in a fiber-based cavity-optomechanical system. The cavity enhanced optomechanical back-action would yield a stronger squeezing of mechanical oscillation, which provides a high potential route in realizing quantum squeezing of mechanical resonator at room temperature and hence mechanical-based force detection beyond classical limit.

微弱力的精密测量对于基础研究与应用技术都十分重要。机械振子是一类广泛使用的力学传感器，构成了大量的传感元件。它在基础研究和精密测量的诸多领域都有着应用，如克什米尔力测量、单个核自旋探测、反平方定律验证。通过冷却振子可以大幅度降低其热噪声从而提高测量的精度。但即便将振子冷却到其振动量子基态，仍存在零点涨落现象，即力学测量的经典极限。随着精密测量向更高的精度发展，如何突破这一极限已经成为一个紧迫而具有挑战的问题。与基于压缩实现的量子测量类似，实现振子的压缩为突破振子力学探测的经典极限提供了可能。得益于我们在机械振子光学冷却和相干操控方面的研究基础，本项目拟在基于光纤腔的光力系统中将振子的光学冷却和光学压缩技术相集成，研究突破机械力探测经典极限的方法。通过光腔增强光力耦合实现更强的振子压缩，本项目研究为实现室温条件下振子的声子压缩，进而突破机械力探测的经典极限提供了一个可行的方法。

微纳机械振子作为力学传感器，因其力学灵敏度高、空间分辨率高以及能够与多物理场耦合的优点，被广泛应用于精密测量领域。在测量过程中，各物理量被转换成可直接测量的机械振子的位移，发展基于机械振子精密测量的重点在于提高机械振子位移的测量灵敏度。机械振子位移测量的噪声主要分为位移读出噪声（如光的散粒噪声），测量反作用噪声以及振子的热振动噪声。三类噪声之和的强度达到最小时，即为位移测量的标准量子极限（SQL，Standard Quantum Limit）。利用声子压缩的方法可以突破机械振子位移测量的SQL，从而突破机械振子微弱力测量的经典极限。为此，我们重点在实验上开展了声子压缩的研究。研究内容主要包括机械振子振动态测量，热振动压缩方法的研究，反馈辅助压缩增强技术的研究以及机械振子量子压缩四个方面。实验上我们构造了振子振动态测量系统，研究了相干脉冲压缩和参量下转换压缩方法，并利用两种方法都突破了压缩的3dB极限。进一步的，我们发展了一套矢量反馈技术，并利用该技术实辅助实现了压缩增强，使得振子的单一方向涨落低于反馈冷却极限6dB，较之振子的热态振动涨落降低了31dB，达到了国际先进水平。在量子压缩的研究中，我们通过微加工技术制备出了振动频率乘以品质因子（FQ值）接近10^11次方水平的微纳机械振子，使得振子满足4K环境下量子态制备的条件。通过构造高细度光纤微腔的方法，我们将系统的光学测量精度提高到了10^-14m/Sqrt(Hz)，达到微纳机械振子量子态的测量要求。在四年的研究过程中，项目组成员共计发表了7篇SCI论文。