The physics of intense laser–matter interactions^1,2 is described by treating the light pulses classically, anticipating no need to access optical measurements beyond the classical limit. However, the quantum nature of the electromagnetic fields is always present³. Here we demonstrate that intense laser–atom interactions may lead to the generation of highly non-classical light states. This was achieved by using the process of high-harmonic generation in atoms^4,5, in which the photons of a driving laser pulse of infrared frequency are upconverted into photons of higher frequencies in the extreme ultraviolet spectral range. The quantum state of the fundamental mode after the interaction, when conditioned on the high-harmonic generation, is a so-called Schrödinger cat state, which corresponds to a superposition of two distinct coherent states: the initial state of the laser and the coherent state reduced in amplitude that results from the interaction with atoms. The results open the path for investigations towards the control of the non-classical states, exploiting conditioning approaches on physical processes relevant to high-harmonic generation.

^{通过经典地处理光脉冲来描述强激光-物质相互作用1,2}的物理特性，预计无需访问超出经典限制的光学测量值。然而，电磁场的量子特性始终存在³。在这里，我们证明了强烈的激光-原子相互作用可能导致产生高度非经典的光态。这是通过使用在原子中产生高次谐波的过程实现的^4,5，其中红外频率的驱动激光脉冲的光子被上转换为极紫外光谱范围内的更高频率的光子。当以高次谐波产生为条件时，相互作用后基模的量子态是所谓的薛定谔猫态，它对应于两种不同相干态的叠加：激光的初始态和相干态由于与原子的相互作用而导致振幅减小。结果为研究非经典状态的控制开辟了道路，利用与高谐波产生相关的物理过程的调节方法。