We present a joint theoretical and experimental analysis of thermorefractive noise in high-quality-factor ($Q$), small-mode-volume ($V$) optical microcavities. Analogous to well-studied stability limits imposed by Brownian motion in macroscopic Fabry-Perot resonators, we show that microcavity thermorefractive noise gives rise to a mode-volume-dependent maximum effective quality factor. State-of-the-art fabricated microcavities are found to be within one order of magnitude of this bound. By measuring the first thermodynamically limited frequency noise spectra of wavelength-scale high-$Q/V$ silicon photonic crystal cavities, we confirm the assumptions of our theory, demonstrate a broadband sub-$\mu \mathrm{K}/\sqrt{\mathrm{Hz}}$ temperature sensitivity, and unveil a new technique for discerning subwavelength changes in microcavity mode volumes. To illustrate the immediate implications of these results, we show that thermorefractive noise limits the optimal performance of recently proposed room-temperature, all-optical qubits using cavity-enhanced bulk material nonlinearities. Looking forward, we propose and analyze coherent thermo-optic noise cancellation as one potential avenue toward violating these bounds, thereby enabling continued development in quantum optical measurement, precision sensing, and low-noise integrated photonics.

中文翻译：

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光学微腔的基本热噪声限值

我们提出了高品质因数（$问$），小模式音量（$V$）光学微腔。类似于宏观法布里-珀罗谐振器中布朗运动所施加的经过充分研究的稳定性极限，我们表明，微腔热折射噪声会引起与模式体积有关的最大有效品质因数。发现最先进的微腔位于该边界的一个数量级之内。通过测量第一个热力学上受限制的频率噪声谱，其波长范围为$问/V$ 硅光子晶体腔，我们证实了我们的理论假设，证明了宽带亚$\mu \mathrm{ķ}/\sqrt{\mathrm{赫兹}}$温度敏感度，并揭示了一种新的技术来识别微腔模式体积中的亚波长变化。为了说明这些结果的直接含义，我们显示了使用腔增强的块状材料非线性，热折射噪声限制了最近提出的室温全光量子比特的最佳性能。展望未来，我们提出并分析相干热光噪声消除技术，作为消除这些界限的一种潜在途径，从而使量子光学测量，精密传感和低噪声集成光子学得以继续发展。