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In this paper, Djidjev et al. evaluate the performance of the D-Wave 2X quantum annealer on two NP-hard graph problems: clique finding and graph partitioning. Overall, they conclude that general problems which allow to be mapped onto the D-Wave architecture are typically still too small to show a quantum speedup (although the D-wave does provide similar quality solutions as the classical solvers). For simple simulated annealing algorithms, D-Wave is considerably faster and selected instances especially designed to fit D-Wave's particular chimera architecture can be solved orders of magnitude faster than with classical techniques.

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Quantum annealers such as the D-Wave 2X allow solving NP-hard optimization problems that can be expressed as quadratic unconstrained binary (QUBO) programs. However, the relatively small number of available qubits poses a severe limitation to the range of problems that can be solved. In this paper, Hahn et al. explore the suitability of preprocessing methods for reducing the sizes of the input programs and thereby the number of qubits required for their solution on quantum computers. Specifically preprocessing reductions are discussed for max. clique and max. cut problems.

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Superconducting qubits are most commonly measured by employing their off-resonant coupling to a readout resonator. Recent experimental results have shown readout times as low as 50ns for single qubits. In this paper, Heinsoo et al. experimentally show how multiple- and selective-readout of any (sub-)set of qubits can be realized via high-fidelity rapid multiplexed readout: probing several readout resonators coupled to a single feedline with a multi-frequency pulse. Applications can be found in quantum error correction, iterative quantum fourier transforms and entanglement-distillation or -swapping.

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In this paper, Gardas et al. propose the use of the quantum fluctuation theorem to benchmark the performance of quantum annealers in respect to computational errors caused by thermal noise. They experimentally test their proposal on 2 quantum annealing devices to illustrate the sensitivity of the fluctuation theorem to the smallest aberrations from ideal annealing.

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Realization of a universal set of quantum gates is a crucial milestone in the development of any qubit-technology aiming to become a platform for large scale quantum computing. In this paper, Gutiérrez et al. assess the capability of current trapped-ion architectures in the implementation of an entangling CNOT gate between encoded logical qubits.

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Topological data analysis (TDA) offers a robust way to extract useful information from noisy, unstructured data by identifying its underlying structure. In this paper, Huang et al. show an experimental proof-of-principle of a recently developed TDA quantum algorithm for calculating Betti numbers of data points (which count the number of topological holes of various dimensions in a scatterplot), using a six-photon quantum processor on a network of three data-points.

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Usually, the interaction of an open quantum system with a noisy environment causes an irreversible loss of quantum coherence, which can be described by the Born-Markov approximation. Contrary, a non-Markovian environment (e.g. strong system-environment coupling) shows a pronounced 'memory effect', which can be used to revive quantum coherence as an alternative from shielding the environment. In this paper, Dong et al. show experimental results for such a non-Markovianity-assisted high-fidelity implementation of a quantum algorithm.

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Commercially available hardware exists for Quantum Key Distribution (QKD) up-to a range of ~100 km. Range extension is being targeted using different approaches based on ground and satellite based trusted-nodes or quantum-repeaters. In this paper Liao et al. show experimental results of quantum key distribution between a low-Earth-orbit satellite and multiple ground stations located in China and Europe. A secret key is created between China and Europe at locations separated by 7600 km on Earth with ~kHz rate per passage of the satellite Micius over a ground station.

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Quantum states that violate Bell's inequality are all non-local states (entangled), but there are other states that do not violate a particular bell inequality but are still non-local. In this paper, Das et al. work with an inequality called I3322 (discovered by Collins et al.), which is inequivalent to the Bell-CHSH inequality (there are states that don't violate Bell-CHSH but do violate I3322), and construct a Bayesian game where a mixed entangled state provides higher individual payoffs than the classical equilibria and where the social welfare payoff is also increased beyond the upper limit for the classical scenario.

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Multi-qubit entanglement is a critical prerequisite for quantum supremacy (quantum computers offering tractability or speed-up compared to classical computing). So far, verified multipartite entanglement has been reported in qubit platforms up to 14 trapped ions, 10 photons and 10 superconducting qubits. In this paper, Wang et al. experimentally demonstrate an 18-qubit maximal (GHZ) entanglement by simultaneous exploiting three different degrees-of-freedom of six photons, including their paths, polarization, and orbital angular momentum.

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In this paper, Wang et al. present experimental results showing genuine multipartite entanglement of up to 16 qubits on the ibmqx5 device, a 16 transmon-qubit universal quantum computing device developed by IBM. Prior to these results entanglement had been reported up to 10 superconducting qubits.

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Quantum computers promise to reduce the computational complexity of simulating quantum many-body systems from exponential to polynomial. Much effort is being put in reducing the complexity of the necessary algorithms, to allow them to be run on noisy intermediate scale quantum computers. In this paper, Dumitrescu et al. report a quantum simulation of the deuteron binding energy on 2 such small-scale noisy cloud accessible quantum processors (the IBM QX5 and Rigetti 19Q).

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