ABSTRACT
This entry presents in detail how quantum states of light can be engineered using atoms and optical nonlinearities, and how they can be stabilized. Many different strategies have been designed and proposed to counteract the decoherence associated with the interaction with the environment, which is able to spoil quantum communication protocols as quantum key distribution for public key cryptosystems. The entry focuses on schemes particularly suited for the protection of quantum states of cavity modes, which may be useful in quantum network schemes where the nodes consist of cavity QED systems made of high-Q cavity modes strongly coupled with single atoms. All the presented schemes try to mitigate the effects of the interaction with the environment either by using a closed-loop configuration in which the dynamics of the cavity mode is corrected in real time using the result of a continuous quantum measurement, or by using an open-loop configuration in which the effect of the environment is controlled by imposing a predetermined dynamics to the system operators involved in the interaction with the reservoir. If open-loop controls are at least as fast as the environmental dynamics, the decohering interaction can be effectively averaged to zero and decoherence suppression can be achieved. An example is provided where this fast modulation of the interaction with environment can be implemented using random, rather than deterministic, modulations.
