In recent years, one important experimental achievement was the strong coupling of quantum matter and quantum light. Realizations reach from ultracold atomic gases in high-finesse optical resonators to electronic systems coupled to THz cavities. The dissipative nature of the quantum light field and the global coupling to the quantum matter leads to many exciting phenomena such as the occurrence of dissipative quantum phase transition to self-organized exotic phases. The theoretical treatment of these dissipative hybrid systems of matter coupled to a cavity field is very challenging. Previously, often mean-field approaches were applied which characterize the emergence of self-organized phases as a zero-temperature transition for the particles, a ground-state Dicke transition. Here we develop a new approach which combines a mean-field approach with a perturbative treatment of fluctuations beyond mean-field, which becomes exact in the thermodynamic limit. We argue that these fluctuations are crucial in order to determine the mixed state (finite temperature) character of the transition and to unravel universal properties of the self-organized states. We validate our results by comparing to time-dependent matrix-product-state calculations.
Tuning the universality class of phase transitions by feedback: Open quantum systems beyond dissipation
我们将反馈控制的范式从量子态的控制转移到量子系统中相变的控制。我们表明，反馈允许调整的普适性类的相变通过修改其临界指数。我们扩展了以前的处理[D. A. Ivanov，T. Yu. Ivanova，S. F.卡瓦列罗-贝尼特斯和我B. Mekhov，Phys. Rev. Lett. 124，010603（2020）]，并超越了光场消隐近似。考虑自旋系综的线性和非线性模型。通过考虑涨落谱和系统在单量子轨道上的行为，解释了非平凡形状反馈对临界点附近量子涨落的可调谐性。
