Stabilization of spin ensembles in hybrid quantum systems

Abstract

Hybrid quantum systems play an eminent role in the development of modern quantum information technology. They combine the advantages of complementary quan- tum systems, e.g., by using photons for transmitting quantum information and spins as long-lived quantum memories. Promising candidates in this regard are solid state spin ensembles coupled to a cavity, such as nitrogen-vacancy centers in diamond, which we consider in this thesis. In particular, we focus on inverted spin ensembles, which tend to be unstable systems, releasing their stored energy into the cavity. Being able to stabilize such an inverted spin ensemble enables a host of practical applications in modern quantum information technology ranging from quantum memories and quantum batteries to highly sensitive photo-detectors.In this thesis, we study various stabilization mechanisms for inverted spin ensembles coupled to a single cavity mode. First, we introduce a method of static stabilization, based on the inhomogeneous broadening and collective cooperativity of the ensemble. Here, we present a general framework, which allows us to determine a criterion for the stability of the inverted spin ensemble.We then extend this framework to periodically modulated systems by utilizing Floquet theory, which lets us determine a condition for stability with the help of the one-period propagator. While our derived formalism is very general, we test our dynamic stabilization method by simulating the dynamics of a concrete physical realization, based on nitrogen-vacancy centers in diamond. In this context, we also investigate the effect of an external probing pulse applied to a stabilized spin system. Finally, we study the stabilization mechanism in more detail, revealing a connection between stabilization and the phases of the cavity field and spins.