Antonio J. Lopez-Tarrida

Experimental Fully Contextual Correlations

Quantum correlations are contextual yet, in general, nothing prevents the existence of even more contextual correlations. We identify and test a noncontextuality inequality in which the quantum violation cannot be improved by any hypothetical postquantum theory, and use it to experimentally obtain correlations in which the fraction of noncontextual correlations is less than 0.06. Our correlations are experimentally generated from the results of sequential compatible tests on a four-state quantum system encoded in the polarization and path of a single photon.

Basic exclusivity graphs in quantum correlations

A fundamental problem is to understand why quantum theory only violates some noncontextuality (NC) inequalities and identify the physical principles that prevent higher-than-quantum violations. We prove that quantum theory only violates those NC inequalities whose exclusivity graphs contain, as induced subgraphs, odd cycles of length five or more, and/or their complements. In addition, we show that odd cycles are the exclusivity graphs of a well-known family of NC inequalities and that there is also a family of NC inequalities whose exclusivity graphs are the complements of odd cycles. We characterize the maximum noncontextual and quantum values of these inequalities, and provide evidence supporting the conjecture that the maximum quantum violation of these inequalities is exactly singled out by the exclusivity principle.

Environmental control of Tinto and Odiel river basins by PIXE

Abstract A study of the elemental concentrations of sediments of the rivers Tinto and Odiel, in Huelva, Spain, has been performed using PIXE. Thirteen samples have been collected, seven in the Tinto and six in the Odiel. Concentrations of 19 elements have been determined in each of them. The analysis of the data illustrates the environmental impact of the mining and fertilizer plants in the area.

Entanglement in eight-qubit graph states

Any 8-qubit graph state belongs to one of the 101 equivalence classes under local unitary operations within the Clifford group. For each of these classes we obtain a representative which requires the minimum number of controlled-Z gates for its preparation, and calculate the Schmidt measure for the 8-partite split, and the Schmidt ranks for all bipartite splits. This results into an extension to 8 qubits of the classification of graph states proposed by Hein, Eisert, and Briegel [M. Hein, J. Eisert, H.J. Briegel, Phys. Rev. A 69 (2004) 062311].

Quantum social networks

We introduce a physical approach to social networks (SNs) in which each actor is characterized by a yes–no test on a physical system. This allows us to consider SNs beyond those originated by interactions based on pre-existing properties, as in a classical SN (CSN). As an example of SNs beyond CSNs, we introduce quantum SNs (QSNs) in which actor i is characterized by a test of whether or not the system is in a quantum state |ψi� . We show that QSNs outperform CSNs for a certain task and some graphs. We identify the simplest of these graphs and show that graphs in which QSNs outperform CSNs are increasingly frequent as the number of vertices increases. We also discuss more general SNs and identify the simplest graphs in which QSNs cannot be outperformed.

Proposed experiment for the quantum "Guess My Number" protocol

An experimental realization of the entanglement-assisted “Guess My Number” protocol for the reduction of communication complexity, introduced by Steane and van Dam, would require producing and detecting threequbit GHZ states with an efficiency h . 0.70, which would require single photon detectors of efficiency s . 0.89. We propose a modification of the protocol which can be translated into a real experiment using present-day technology. In the proposed experiment, the quantum reduction of the multiparty communication complexity would require an efficiency h . 0.05, achievable with detectors of s . 0.47, for four parties, and h . 0.17 ss . 0.55d for three parties. DOI: 10.1103/PhysRevA.71.020301 One of the most impressive applications of quantum resources for information processing is the reduction of the communication complexity required for certain computations f1‐5g. Let us suppose that two or more separated parties need to compute a function of a number of inputs distributed among them. Using the best classical strategy, this would require a certain minimum amount of classical communication to be transmitted between the parties. However, if the parties initially shared some entangled states, then the amount of classical communication required for the computation would be a great deal smaller than if no entanglement were present. The quantum advantage usually grows with the number of parties involved f2g. Entanglement-assisted reduction of classical communication complexity has numerous potential applications in computer networks, VLSI circuits, and data structures f6g. A particularly attractive, thought-provoking, and stimulating way to show the quantum advantage was proposed by Steane and van Dam as a method for always winning the television contest “Guess My Number” sGMN df 5g. A team of three contestants sAlice, Bob, and Charlied, each of them isolated in a booth, is given an integer number n= nA+ nB + nC of apples swhere nj =0, 1 /2, 1, or 3 /2 d. One of the contestants must guess whether the number is odd or even just by receiving one bit from the other two contestants. The best classical strategy would allow the contestants to win in 75% of the cases. However, they can win in 100% of the cases if they initially share three-qubit Greenberger-HorneZeilinger sGHZd states f7,8g. The same game can be played with four contestants and the quantum versus classical advantage is the same: 100% vs 75%. Steane and van Dam stressed that “A laboratory demonstration of entanglementenhanced communication would be s⊃d a landmark in quantum physics and quantum information science” f5g. So far, however, the requirements for an experimental implementation of the quantum GMN protocol have impeded further progress. Some progress has been reported on simpler schemes of quantum reduction of classical communication