Feedback loops based on real-time continuous measurements are commonly used for stabilisation purposes, and they have also been successfully applied to the stabilisation of quantum systems. Typically a system is continuously monitored via a probe electromagnetic field, and the acquired signal drives the actuator which in turn drives the system to the desired target. Here we demonstrate a novel closed-loop control scheme in which the actuator acts on the probe field itself in order to engineer its phase and amplitude fluctuations. The resulting feedback-controlled in-loop field is then exploited to manipulate the system and improve its performance. In-loop optical fields have already been employed in the negative feedback regime for noise suppression and stabilization, the so-called “squashing” . Here, instead, we operate in the “anti-squashing” regime of positive feedback and increased field fluctuations, and demonstrate the potentiality of this new technique by improving the sideband cooling of a nanomechanical membrane by 7.2 dB. In the fully quantum regime, feedback-controlled light would allow going below the quantum back-action cooling limit, similarly to what has been recently achieved by injecting squeezed vacuum fluctuation, but without the complications inherent to the use of quantum nonlinear devices. The proposed feedback architecture can be used in a broad range of applications, whenever a system of interest is linearly coupled to an electromagnetic field subject to a phase-sensitive measurement.
 Rossi M, Kralj N, Zippilli S, Natali R, Borrielli A, Pandraud G, Serra E, Di Giuseppe G and Vitali D arXiv:1704.04556