Experimental realization of active nonlinear feedback control from hot rubidium vapor

Feedback control plays a crucial role in preparation and manipulation of quantum states, to evolve the quantum system towards a desired result. Here we report a novel feedback control system utilizing two four-wave mixing (FWM) processes, in which the first FWM process functions as an amplifier while the second FWM process serves as an active nonlinear controller. We experimentally investigate the classical properties of the output states, and demonstrate the manipulation of quantum states through the active nonlinear controller. Remarkably, we observe that the quantum correlation of the quantum states can be efficiently controlled and enhanced, even when the amplifier operates at a significantly low level of pump power. Furthermore, we identify an optimal intensity gain for the active nonlinear controller, which maximizes the quantum correlation of the system. These findings present a new strategy employing an active controller to enhance quantum correlation, which holds the potential to improve the communication fidelity of quantum information processing and enhance the measurement precision of quantum metrology in future applications.