Realistic quantum mechanical systems are always exposed to an external environment. This often induces Markovian processes in which the system loses information to its surroundings. However, many quantum systems exhibit nonMarkovian behaviour with a flow of information from the environment back to the system 1-5 . The environment usually consists of large number of degrees of freedom which are difficult to control, but some sophisticated schemes for reservoir engineering have been developed 6 . The control of open systems plays a decisive role, for example, in proposals for entanglement generation 7-9 and dissipative quantum computation 10 , for the design of quantum memories 11 and in quantum metrology 12 . Here we report an all-optical experiment which allows one 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 measurements on the open system.The standard approach to the dynamics of open quantum systems employs the concept of a quantum Markov process which is given by a semigroup of completely positive dynamical maps and a corresponding quantum master equation with a generator in Lindblad form 13,14 . Very recently, a toolbox for the engineering of such quantum Markov processes in a multi-qubit system of trapped ions has been realized experimentally 15 and technological developments have also allowed experimental studies of quantum correlations in open systems 16,17 . Within a microscopic approach, quantum Markovian master equations are usually obtained by means of the Born-Markov approximation, which presupposes a weak system-environment coupling and several further, mostly rather drastic approximations. However, in many processes occurring in nature these approximations are not applicable, a situation which occurs, in particular, in the cases of strong systemenvironment couplings, structured and finite reservoirs, and low temperatures, as well as in the presence of large initial systemenvironment correlations. In the case of substantial quantitative and qualitative deviations from the dynamics of a quantum Markov process one often speaks of a non-Markovian process, implying that the dynamics is governed by significant memory effects. Quite recently important steps towards the development of a general consistent theory of non-Markovian quantum dynamics have been made which try to rigorously define the border between Markovian The measure for quantum non-Markovianity constructed in ref.19 is based on the idea that memory effects in the open system dynamics can be characterized in terms of the flow of information between the open system and its environment. It has been used recently, for example, to describe this information flow in the energy transfer dynamics of photosynthetic complexes 2,4 , and to characterize memory effects of the dynamics of qubits in spin baths 5 . Here, we present the results of an experiment which enables one, through a careful preparation ...

The uncertainty principle, which bounds the uncertainties involved in obtaining precise outcomes for two complementary variables defining a quantum particle, is a crucial aspect in quantum mechanics. Recently, the uncertainty principle in terms of entropy has been extended to the case involving quantum entanglement 1 . With previously obtained quantum information for the particle of interest, the outcomes of both non-commuting observables can be predicted precisely, which greatly generalizes the uncertainty relation. Here, we experimentally investigated the entanglement-assisted entropic uncertainty principle for an entirely optical set-up. The uncertainty is shown to be near zero in the presence of quasi-maximal entanglement. The new uncertainty relation is further used to witness entanglement. The verified entropic uncertainty relation provides an intriguing perspective in that it implies the uncertainty principle is not only observabledependent but is also observer-dependent 2 .In quantum mechanics, the outcomes of an observable can be predicted precisely by preparing eigenvectors corresponding to the state of the measured system. However, the ability to predict the precise outcomes of two conjugate observables for a particle is restricted by the uncertainty principle. Originally observed by Heisenberg 3 , the uncertainty principle is best known as the Heisenberg-Robertson commutationwhere R ( S) represents the standard deviation of the corresponding variable R (S). It can be seen that the bound on the right-hand side is state-dependent and can vanish even when R and S are non-commuting. To avoid this defect, the uncertainty relation has been re-derived in terms of an information-theoretic model 5 in which the uncertainty relating to the outcomes of the observable is characterized by the Shannon entropy instead of the standard deviation. The entropic uncertainty relation for any two general observables was first given by Deutsch 6 . Soon afterwards, an improved version was proposed by Kraus 7 and then proved by Maassen and Uiffink 8 . The improved relation reads as follows:where H is the Shannon entropy, c = max i,j | a i |b j | 2 and represents the overlap between observables R and S, and |a i (|b j ) represents the eigenvectors of R (S).Although we cannot obtain the precise outcomes of both the two conjugate variables, even when the density matrix of the prepared state is known, the situation would be different if 1 Key Laboratory of Quantum Information, University of Science and Technology of China, CAS, Hefei, 230026, China, 2 Center for Quantum Technologies, National University of Singapore, 2 Science Drive 3, 117542, Singapore. † These authors contributed equally to this work. *e-mail: cfli@ustc.edu.cn.we invoked the effect of quantum entanglement. The possibility of violating the Heisenberg-Robertson uncertainty relation was identified early by Einstein, Podolsky, and Rosen in their famous paper, and was originally used to challenge the correctness of quantum mechanics (EPR paradox) 9 . Popper also p...

There is a minor error that a and b in the equations should be real numbers. Since one can always bring the state to the form ͑1͒ with real a and b via a local unitary operation. This error does not affect the result presented in the article. We thank Dr. Da-Wei Chang for bringing this to our attention.*

It is well known that many operations in quantum information processing depend largely on a special kind of quantum correlation, that is, entanglement. However, there are also quantum tasks that display the quantum advantage without entanglement. Distinguishing classical and quantum correlations in quantum systems is therefore of both fundamental and practical importance. In consideration of the unavoidable interaction between correlated systems and the environment, understanding the dynamics of correlations would stimulate great interest. In this study, we investigate the dynamics of different kinds of bipartite correlations in an alloptical experimental setup. The sudden change in behaviour in the decay rates of correlations and their immunity against certain decoherences are shown. Moreover, quantum correlation is observed to be larger than classical correlation, which disproves the early conjecture that classical correlation is always greater than quantum correlation. Our observations may be important for quantum information processing.

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Orbital angular momentum of light is a fundamental optical degree of freedom characterized by unlimited number of available angular momentum states. Although this unique property has proved invaluable in diverse recent studies ranging from optical communication to quantum information, it has not been considered useful or even relevant for simulating nontrivial physics problems such as topological phenomena. Contrary to this misconception, we demonstrate the incredible value of orbital angular momentum of light for quantum simulation by showing theoretically how it allows to study a variety of important 2D topological physics in a 1D array of optical cavities. This application for orbital angular momentum of light not only reduces required physical resources but also increases feasible scale of simulation, and thus makes it possible to investigate important topics such as edge-state transport and topological phase transition in a small simulator ready for immediate experimental exploration.

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multi-partite entangled states are important for developing studies of quantum networking and quantum computation. To date, the largest number of particles that have been successfully manipulated is 14 trapped ions. Yet in quantum information science, photons have particular advantages over other systems. In particular, they are more easily transportable qubits and are more robust against decoherence. Thus far, the largest number of photons to have been successfully manipulated in an experiment is six. Here we demonstrate, for the first time, an eight-photon Greenberger-Horne-Zeilinger state with a measured fidelity of 0.59 ± 0.02, which proved the presence of genuine eight-partite entanglement. This is achieved by improving the photon detection efficiency to 25% with a 300-mW pump laser. With this state, we also demonstrate an eight-party quantum communication complexity scenario. This eight-photon entangled-state source may be useful in one-way quantum computation, quantum networks and other quantum information processing tasks.