  Realistic quantum mechanical systems are always exposed to an external
environment. The presence of the environment often gives rise to a Markovian
process in which the system loses information to its surroundings. However,
many quantum systems exhibit a pronounced non-Markovian behavior in which there
is a flow of information from the environment back to the system, signifying
the presence of quantum memory effects [1-5]. The environment is usually
composed of a large number of degrees of freedom which are difficult to
control, but some sophisticated schemes for modifying the environment have been
developed [6]. The physical realization and control of dynamical processes in
open quantum systems plays a decisive role, for example, in recent proposals
for the generation of entangled states [7-9], for schemes of dissipative
quantum computation [10], for the design of quantum memories [11] and for the
enhancement of the efficiency in quantum metrology [12]. Here we report an
experiment which allows through selective preparation of the initial
environmental states to drive the open system from the Markovian to the
non-Markovian regime, to control the information flow between the system and
the environment, and to determine the degree of non-Markovianity by direct
measurements on the open system.
