We report the creation of Greenberger-Horne-Zeilinger states with up to 14 qubits. By investigating the coherence of up to 8 ions over time, we observe a decay proportional to the square of the number of qubits. The observed decay agrees with a theoretical model which assumes a system affected by correlated, Gaussian phase noise. This model holds for the majority of current experimental systems developed towards quantum computation and quantum metrology.
Entanglement, its generation, manipulation and fundamental understanding is at the very heart of quantum mechanics. The phrase entanglement was coined by Erwin Schrödinger in 1935 for particles that are described by a common wave function where individual particles are not independent of each other but where their quantum properties are inextricably interwoven 1 . Entanglement properties of two and three particles have been studied extensively and are very well understood. Entanglement of four 2 and five 3 particles was demonstrated experimentally. However, both creation and characterization of entanglement become exceedingly difficult for multi-particle systems. Thus the availability of such multiparticle entangled states together with the full information on these states in form of their 1
* These authors contributed equally to this work.The control of quantum systems is of fundamental scientific interest and promises powerful applications and technologies. Impressive progress has been achieved in isolating the systems from the environment and coherently controlling their dynamics, as demonstrated by the creation and manipulation of entanglement in various physical systems. However, for open quantum systems, engineering the dynamics of many particles by a controlled coupling to an environment remains largely unexplored. Here we report the first realization of a toolbox for simulating an open quantum system with up to five qubits. Using a quantum computing architecture with trapped ions, we combine multi-qubit gates with optical pumping to implement coherent operations and dissipative processes. We illustrate this engineering by the dissipative preparation of entangled states, the simulation of coherent many-body spin interactions and the quantum non-demolition measurement of multi-qubit observables. By adding controlled dissipation to coherent operations, this work offers novel prospects for open-system quantum simulation and computation.Every quantum system is inevitably coupled to its surrounding environment. Significant progress has been made in isolating systems from their enviroment and coherently controlling the dynamics of several qubits [1][2][3][4]. These achievements have enabled the realization of highfidelity quantum gates, the implementation of small-scale quantum computing and communication devices as well as the measurement-based probabilistic preparation of entangled states, in atomic [5, 6], photonic [7] and solidstate setups [8][9][10]. In particular, successful demonstrations of quantum simulators [11, 12], which allow one to mimic and study the dynamics of complex quantum systems, have been reported [13].In contrast, controlling the more general dynamics of open systems amounts to engineering both the Hamiltonian time evolution of the system as well as the coupling to the environment. Although open-system dynamics in a many-body or multi-qubit system are typically associated with decoherence [14][15][16], the ability to design dissipation can be a useful resource. For example, controlled dissipation allows the preparation of a desired entangled state from an arbitrary state [17][18][19] or an enhanced sensitivity for precision measurements [20]. In a broader context, by combining suitably chosen coherent and dissipative time steps, one can realize the most general nonunitary open-system evolution of a many-particle system. This engineering of the system-environment coupling generalizes the concept of Hamiltonian quantum simulation to open quantum systems. In addition, this engineering enables the dissipative preparation and manipulation of many-body states and quantum phases [21], and also quantum computation based on dissipation [22].Here we provide the first experimental demonstration of a complete toolbox, through coherent and dissipative manipulations of a multi-qubit syst...
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