We demonstrate the emergence of nonreciprocal superradiant phase transitions and novel multicriticality in a cavity quantum electrodynamics system, where a two-level atom interacts with two counterpropagating modes of a whispering-gallery-mode microcavity. The cavity rotates at a certain angular velocity and is directionally squeezed by a unidirectional parametric pumping ${\chi }^{(2)}$ nonlinearity. The combination of cavity rotation and directional squeezing leads to nonreciprocal first- and second-order superradiant phase transitions. These transitions do not require ultrastrong atom-field couplings and can be easily controlled by the external pump field. Through a full quantum description of the system Hamiltonian, we identify two types of multicritical points in the phase diagram, both of which exhibit controllable nonreciprocity. These results open a new door for all-optical manipulation of superradiant transitions and multicritical behaviors in light-matter systems, with potential applications in engineering various integrated nonreciprocal quantum devices.

中文翻译：

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腔 QED 系统中的不可逆超辐射相变和多临界性

我们证明了空腔量子电动力学系统中不可逆超辐射相变和新型多临界性的出现，其中两能级原子与回音壁模式微腔的两种反向传播模式相互作用。腔体以一定的角速度旋转，并受到单向参数泵浦的定向挤压${\chi }^{（2）}$非线性。腔旋转和定向挤压的结合导致不可逆的一阶和二阶超辐射相变。这些转变不需要超强的原子场耦合，并且可以通过外部泵浦场轻松控制。通过系统哈密顿量的完整量子描述，我们识别了相图中的两种类型的多临界点，它们都表现出可控的非互易性。这些结果为光物质系统中超辐射跃迁和多临界行为的全光学操纵打开了一扇新的大门，在工程各种集成非互易量子器件中具有潜在的应用。