Understanding the thermally activated escape from a metastable state is at the heart of important phenomena such as the folding dynamics of proteins^1,2, the kinetics of chemical reactions³ or the stability of mechanical systems⁴. In 1940, Kramers calculated escape rates both in the high damping and low damping regimes, and suggested that the rate must have a maximum for intermediate damping⁵. This phenomenon, today known as the Kramers turnover, has triggered important theoretical and numerical studies⁶. However, as yet, there is no direct and quantitative experimental verification of this turnover. Using a nanoparticle trapped in a bistable optical potential, we experimentally measure the nanoparticle's transition rates for variable damping and directly resolve the Kramers turnover. Our measurements are in agreement with an analytical model that is free of adjustable parameters. The levitated nanoparticle presented here is a versatile experimental platform for studying and simulating a wide range of stochastic processes and testing theoretical models and predictions.

重要的现象（例如蛋白质^1,2的折叠动力学，化学反应的动力学³或机械系统的稳定性^4）的核心是理解从亚稳态热激活逸出。在1940年，Kramers计算了高阻尼和低阻尼两种情况下的逃逸率，并建议逃生率在中间阻尼时必须具有最大值⁵。这种现象，今天被称为克莱默斯周转率，已经引发了重要的理论和数值研究⁶。但是，到目前为止，还没有对此营业额进行直接和定量的实验验证。使用捕获在双稳态光学势中的纳米粒子，我们实验性地测量了纳米粒子的可变阻尼转换率，并直接解决了Kramers转换。我们的测量与无可调参数的分析模型相符。这里展示的悬浮纳米颗粒是一个多功能的实验平台，用于研究和模拟各种随机过程，并测试理论模型和预测。