Periodically driven Floquet quantum systems could provide a promising platform to investigate novel physics out of equilibrium¹, but the drive generically heats the system to a featureless infinite-temperature state^2,3,4. Fortunately, for high driving frequency, the heat absorption rate has been theoretically predicted to be exponentially small, giving rise to a long-lived prethermal regime that exhibits all the intriguing properties of Floquet systems^5,6,7,8. Here we experimentally observe Floquet prethermalization using NMR techniques and probe the heating rate. We first show the relaxation of a far-from-equilibrium initial state to a long-lived prethermal state, well described by a time-independent ‘prethermal’ Hamiltonian. By measuring the autocorrelation of this prethermal Hamiltonian we can further experimentally confirm the predicted exponentially slow heating rate. More strikingly, we find that, on the timescale at which the prethermal Hamiltonian picture breaks down, the Floquet system still possesses other quasiconservation laws. Our results demonstrate that it is possible to realize robust Floquet engineering, thus enabling the experimental observation of non-trivial Floquet phases of matter.

周期性驱动的 Floquet 量子系统可以提供一个很有前途的平台来研究失去平衡的新物理¹，但该驱动器通常会将系统加热到无特征的无限温度状态^2,3,4。幸运的是，对于高驱动频率，理论上预测吸热率呈指数级小，从而产生了一个长寿命的预热状态，它展示了 Floquet 系统^{5、6、7、8的所有有趣特性}. 在这里，我们使用 NMR 技术通过实验观察 Floquet 预热化并探测加热速率。我们首先展示了一个远离平衡的初始状态到一个长寿的热前状态的弛豫，这可以通过与时间无关的“热前”哈密顿量来很好地描述。通过测量这个热前哈密顿量的自相关，我们可以进一步通过实验确认预测的指数缓慢加热速率。更引人注目的是，我们发现，在热前哈密顿图分解的时间尺度上，Floquet 系统仍然具有其他准守恒定律。我们的结果表明，可以实现稳健的 Floquet 工程，从而能够对物质的非平凡 Floquet 相进行实验观察。