Lu, C. - Y., Zhou, X. - Q., Gühne, O., Gao, W. - B., Zhang, J., Yuan, Z. - S., et al. (2007). Experimental entanglement of six photons in graph states. Nat. Phys., 3(2), 91–95.
Abstract: Graph states-multipartite entangled states that can be represented by mathematical graphs-are important resources for quantum computation, quantum error correction, studies of multiparticle entanglement and fundamental tests of non-locality and decoherence. Here, we demonstrate the experimental entanglement of six photons and engineering of multiqubit graph states. We have created two important examples of graph states, a six-photon Greenberger-Horne-Zeilinger state, the largest photonic Schrödinger cat so far, and a six-photon cluster state, a state-of-the-art `one-way quantum computer'. With small modifications, our method allows us, in principle, to create various further graph states, and therefore could open the way to experimental tests of, for example, quantum algorithms or loss- and fault-tolerant one-way quantum computation.
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Ursin, R., Tiefenbacher, F., Schmitt-Manderbach, T., Weier, H., Scheidl, T., Lindenthal, M., et al. (2007). Entanglement-based quantum communication over 144km. Nat. Phys., 3(7), 481–486.
Abstract: Quantum entanglement is the main resource to endow the field of quantum information processing with powers that exceed those of classical communication and computation. In view of applications such as quantum cryptography or quantum teleportation, extension of quantum-entanglement-based protocols to global distances is of considerable practical interest. Here we experimentally demonstrate entanglement-based quantum key distribution over 144km. One photon is measured locally at the Canary Island of La Palma, whereas the other is sent over an optical free-space link to Tenerife, where the Optical Ground Station of the European Space Agency acts as the receiver. This exceeds previous free-space experiments by more than an order of magnitude in distance, and is an essential step towards future satellite-based quantum communication and experimental tests on quantum physics in space.
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Pirandola, S., Mancini, S., Lloyd, S., & Braunstein, S. L. (2008). Continuous-variable quantum cryptography using two-way quantum communication. Nat. Phys., 4(9), 726–730.
Abstract: Quantum cryptography has recently been extended to continuous-variable systems, such as the bosonic modes of the electromagnetic field possessing continuous degrees of freedom. In particular, several cryptographic protocols have been proposed and experimentally implemented using bosonic modes with Gaussian statistics. These protocols have shown the possibility of reaching very high secret key rates, even in the presence of strong losses in the quantum communication channel. Despite this robustness to loss, their security can be affected by more general attacks where extra Gaussian noise is introduced by the eavesdropper. Here, we show a `hardware solution' for enhancing the security thresholds of these protocols. This is possible by extending them to two-way quantum communication where subsequent uses of the quantum channel are suitably combined. In the resulting two-way schemes, one of the honest parties assists the secret encoding of the other, with the chance of a non-trivial superadditive enhancement of the security thresholds. These results should enable the extension of quantum cryptography to more complex quantum communications.
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Zurek, W. H. (2009). Quantum Darwinism. Nat. Phys., 5(3), 181–188.
Abstract: Quantum Darwinism describes the proliferation, in the environment, of multiple records of selected states of a quantum system. It explains how the quantum fragility of a state of a single quantum system can lead to the classical robustness of states in their correlated multitude; shows how effective `wave-packet collapse' arises as a result of the proliferation throughout the environment of imprints of the state of the system; and provides a framework for the derivation of Born's rule, which relates the probabilities of detecting states to their amplitudes. Taken together, these three advances mark considerable progress towards settling the quantum measurement problem.
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Shor, P. W. (2009). Quantum information theory: The bits don't add up. Nat. Phys., 5, 247–248.
Abstract: A counterexample to the 'additivity question', the most celebrated open problem in the mathematical theory of quantum information, casts doubt on the possibility of finding a simple expression for the information capacity of a quantum channel.
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