Quantum state protection using all-optical feedback

Summary form only given. Electromagnetic fields in cavities have already been used for quantum information processing. In particular, quantum information can be stored in high-Q electromagnetic cavities, and with this respect it is important to develop schemes able to increase the quantum information storage time as much as possible, and provide therefore a good quantum state protection against the effects of the decoherence due to cavity leakage. We have developed schemes for the preservation of generic quantum states in cavities based on feedback loops originating from homodyne measurements, or associated with direct photodetection supplemented with the injection in the cavity of an appropriately prepared atom. These schemes provide a significant increase of the decoherence time of an initially prepared quantum state, but are both characterized by some limitations. We show that a promising way to obtain perfect quantum state protection, that is, the preservation of an initially prepared generic quantum state for an arbitrarily large time, can be obtained by using an all-optical feedback scheme. In all-optical feedback schemes, the output light is not detected, but it is reflected around a feedback loop and sent into another cavity (the driven cavity) which is coupled to the first in some way. This scheme is an actual feedback scheme if the loop is one-way. All-optical feedback schemes are an example of cascaded quantum systems, in which the output from a source mode is used as an input for a second, driven mode. The new feature introduced by feedback is the presence of an interaction term between the two modes, so that the source mode dynamics are affected by the driven mode. We consider a Markovian treatment which enables us to derive a master equation for the dynamics of the source mode alone.