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Raeisi, S., Tittel, W., & Simon, C. (2012). Proposal for Inverting the Quantum Cloning of Photons RID A-1600-2011. Phys. Rev. Lett., 108(12), 5 pp.
Abstract: We propose an experiment where a photon is first cloned by stimulated parametric down-conversion, making many (imperfect) copies, and then the cloning transformation is inverted, regenerating the original photon while destroying the copies. Focusing on the case where the initial photon is entangled with another photon, we study the conditions under which entanglement can be proven in the final state. The proposed experiment would provide a clear demonstration that quantum information is preserved in quantum cloning. It would furthermore allow a definitive experimental proof for micro-macro entanglement in the intermediate multiphoton state, which is still an outstanding challenge. Finally, it might provide a quantum detection technique for small differences in transmission (e.g., in biological samples), whose sensitivity scales better with the number of photons used than a classical transmission measurement.
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Leung, D., Mancinska, L., Matthews, W., Ozols, M., & Roy, A. (2012). Entanglement can Increase Asymptotic Rates of Zero-Error Classical Communication over Classical Channels. Commun. Math. Phys., 311(1), 97–111.
Abstract: It is known that the number of different classical messages which can be communicated with a single use of a classical channel with zero probability of decoding error can sometimes be increased by using entanglement shared between sender and receiver. It has been an open question to determine whether entanglement can ever increase the zero-error communication rates achievable in the limit of many channel uses. In this paper we show, by explicit examples, that entanglement can indeed increase asymptotic zero-error capacity, even to the extent that it is equal to the normal capacity of the channel.
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Borneman, T. W., Granade, C. E., & Cory, D. G. (2012). Parallel Information Transfer in a Multinode Quantum Information Processor. Phys. Rev. Lett., 108(14), 5 pp.
Abstract: We describe a method for coupling disjoint quantum bits (qubits) in different local processing nodes of a distributed node quantum information processor. An effective channel for information transfer between nodes is obtained by moving the system into an interaction frame where all pairs of cross-node qubits are effectively coupled via an exchange interaction between actuator elements of each node. All control is achieved via actuator-only modulation, leading to fast implementations of a universal set of internode quantum gates. The method is expected to be nearly independent of actuator decoherence and may be made insensitive to experimental variations of system parameters by appropriate design of control sequences. We show, in particular, how the induced cross-node coupling channel may be used to swap the complete quantum states of the local processors in parallel.
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Piani, M., & Adesso, G. (2012). Quantumness of correlations revealed in local measurements exceeds entanglement. Phys. Rev. A, 85(4), 5 pp.
Abstract: We analyze a family of measures of general quantum correlations for composite systems, defined in terms of the bipartite entanglement necessarily created between systems and apparatuses during local measurements. For every entanglement monotone E, this operational correspondence provides a different measure Q(E) of quantum correlations. Examples of such measures are the relative entropy of quantumness, the quantum deficit, and the negativity of quantumness. In general, we prove that any so-defined quantum correlation measure is always greater than (or equal to) the corresponding entanglement between the subsystems, Q(E) >= E, for arbitrary states of composite quantum systems. We analyze qualitatively and quantitatively the flow of correlations in iterated measurements, showing that general quantum correlations and entanglement can never decrease along von Neumann chains, and that genuine multipartite entanglement in the initial state of the observed system always gives rise to genuine multipartite entanglement among all subsystems and all measurement apparatuses at any level in the chain. Our results provide a comprehensive framework to understand and quantify general quantum correlations in multipartite states.
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Chen, J., Cubitt, T. S., Harrow, A. W., & Smith, G. (2011). Entanglement can Completely Defeat Quantum Noise. PHYSICAL REVIEW LETTERS, 107(25).
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Raeisi, S., Sekatski, P., & Simon, C. (2011). Coarse Graining Makes It Hard to See Micro-Macro Entanglement. PHYSICAL REVIEW LETTERS, 107(25).
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Meyer-Scott, E., Yan, Z., MacDonald, A., Bourgoin, J. - P., Huebel, H., & Jennewein, T. (2011). How to implement decoy-state quantum key distribution for a satellite uplink with 50-dB channel loss. PHYSICAL REVIEW A, 84(6).
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Berry, D. W., & Childs, A. M. (2012). BLACK-BOX HAMILTONIAN SIMULATION AND UNITARY IMPLEMENTATION. QUANTUM INFORMATION & COMPUTATION, 12(1-2), 29–62.
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Garnerone, S., Giorda, P., & Zanardi, P. (2012). Bipartite quantum states and random complex networks. NEW JOURNAL OF PHYSICS, 14.
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Moroder, T., & Gittsovich, O. (2012). Calibration-robust entanglement detection beyond Bell inequalities. PHYSICAL REVIEW A, 85(3).
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Passante, G., Moussa, O., & Laflamme, R. (2012). Measuring geometric quantum discord using one bit of quantum information. PHYSICAL REVIEW A, 85(3).
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Chen, J. X., Cubitt, T. S., Harrow, A. W., & Smith, G. (2011). Entanglement can Completely Defeat Quantum Noise. Phys. Rev. Lett., 107(25), 4 pp.
Abstract: We describe two quantum channels that individually cannot send any classical information without some chance of decoding error. But together a single use of each channel can send quantum information perfectly reliably. This proves that the zero-error classical capacity exhibits superactivation, the extreme form of the superadditivity phenomenon in which entangled inputs allow communication over zero-capacity channels. But our result is stronger still, as it even allows zero-error quantum communication when the two channels are combined. Thus our result shows a new remarkable way in which entanglement across two systems can be used to resist noise, in this case perfectly. We also show a new form of superactivation by entanglement shared between sender and receiver.
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Raeisi, S., Sekatski, P., & Simon, C. (2011). Coarse Graining Makes It Hard to See Micro-Macro Entanglement. Phys. Rev. Lett., 107(25), 5 pp.
Abstract: Observing quantum effects such as superpositions and entanglement in macroscopic systems requires not only a system that is well protected against environmental decoherence, but also sufficient measurement precision. Motivated by recent experiments, we study the effects of coarse graining in photon number measurements on the observability of micro-macro entanglement that is created by greatly amplifying one photon from an entangled pair. We compare the results obtained for a unitary quantum cloner, which generates micro-macro entanglement, and for a measure-and-prepare cloner, which produces a separable micro-macro state. We show that the distance between the probability distributions of results for the two cloners approaches zero for a fixed moderate amount of coarse graining. Proving the presence of micro-macro entanglement therefore becomes progressively harder as the system size increases.
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Childs, A. M., & Gosset, D. (2012). Levinson's theorem for graphs II. arXiv preprint, .
Abstract: We prove Levinson's theorem for scattering on an (m+n)-vertex graph with n semi-infinite paths each attached to a different vertex, generalizing a previous result for the case n=1. This theorem counts the number of bound states in terms of the winding of the determinant of the S-matrix. We also provide a proof that the bound states and incoming scattering states of the Hamiltonian together form a complete basis for the Hilbert space, generalizing another result for the case n=1.
Keywords: math-ph; math.MP; quant-ph
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Chen, L., & Djokovic, D. Z. (2012). Properties and construction of extreme bipartite states having positive partial transpose. arXiv preprint, .
Abstract: We investigate the set E of extreme points of the compact convex set of PPT states (i.e., the states having positive semidefinite partial transpose) of a bipartite M x N quantum system. Let E(M,N,r) denote the subset of E consisting of states of rank r which are supported on M x N. We show that for M,N>2 the sets E(M,N,M+N-2) are nonempty. On the other hand we show that for M,N>3 the sets E(M,N,N+1) are empty. It is known that the set E(M,N,MN) is empty, and we show that also the set E(M,N,MN-1) is empty. We divide the set of all states into the good and the bad states (the definition is too technical to be given here). We show that the good states have many good properties. In particular, we solve the separability problem for good M x N PPT states of rank M+N-2 when M=3 or 4. All pure bipartite states are good. We obtain a simple characterization of good PPT states.
Keywords: math-ph; math.MP; quant-ph
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