Non-Markovianity through accessible information

Memory effects govern the way a physical system responds to the action of the environment. It is a char- acteristic of non-Markovian quantum evolution that the environment affects the system relaxation and deco- herence through a back-flow of energy and information, respectively. To characterize the non-Markovianity of a given system evolution through a meaningful measure is therefore of utmost relevance. Here we propose an entanglement-based measure of non-Markovianity, employing the concept of assisted knowledge where the environment, E, acquires information about a system S by means of its measurement apparatus A. While the accessible information (the maximum amount of classical information) that E can acquire about S, via inter- action with A, is monotonously increasing for all (memoryless) Markovian quantum processes, it signals a back-flow of information in the case of non-Markovian processes. We present an experimental demonstration of this scenario using an optical-based setup that allows full access to the environmental degrees of freedom.