宏观振子的亚开尔文冷却及飞秒牛顿力的测量
Sub-Kelvin Cooling of a Macroscopic Oscillator and femto-Newton Force Measurement
摘要：
测量非常小的力，特别是引力性质的力，一直都很有意义，因为这是对我们理解物理定律的基本检验。通常用于这种测量的超长周期机械振荡器由于能量的等分而将具有与每个自由度相关联的kT/2的热能。此外，低频带中的额外地震波动可以将该等效温度显著提高到10^5 K。最近，已经报道了使用光机械力的各种方法来降低MHz微悬臂梁振荡器的这种热能，从而有效地冷却它们。在这里，我们展示了一个克大小的宏观振子在等效温度下直接动态冷却到300 mK--噪声降低了10^6倍。通过精确测量扭转振子的位置，我们动态地提供了一个外部的“粘性”阻尼力。这种附加的耗散力基本上没有噪声，导致振荡器的快速冷却。此外，我们观察到的时间依赖性的冷却过程中，在各种冷却力参数。这种参数依赖性与我们提供的简单物理模型一致。我们进一步表明，该设备对小至<100 fN的力敏感-该力仅为单个生物分子通常施加的力或在典型重力实验中观察到的力的百分之几。我们还演示了动态控制振荡器的固有频率，在近二十年的跨度。该方法在微小力的精确测量中有重要的应用。
英文摘要：
Measuring very small forces, particularly those of a gravitational nature, has always been of great interest, as fundamental tests of our understanding of the physical laws. Ultra-long period mechanical oscillators, typically used in such measurements, will have kT/2 of thermal energy associated with each degree of freedom, owing to the equal-partition of energy. Moreover, additional seismic fluctuations in the low frequency band can raise this equivalent temperature significantly to 10^5 K. Recently, various methods using opto-mechanical forces have been reported to decrease this thermal energy for MHz, micro-cantilever oscillators, effectively cooling them. Here we show the direct, dynamical cooling of a gram-size, macroscopic oscillator to 300 mK in equivalent temperature - noise reduction by a factor of 10^6. By precisely measuring the torsional oscillator's position, we dynamically provide an external 'viscous' damping force. Such an added, dissipative force is essentially free of noise, resulting in rapid cooling of the oscillator. Additionally, we observe the time-dependent cooling process, at various cooling force parameters. This parameter dependence agrees well with a simple physical model which we provide. We further show that the device is sensitive to forces as small as <100 fN - a force only a few percent of that typically exerted by a single biological molecule or that observed in a typical gravity experiment. We also demonstrate the dynamic control of the oscillator's natural frequency, over a span of nearly two decades. The method may find important applications in precision measurements of very weak forces.
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