Bosonic indistinguishability-dependent contextuality

Producing contextual correlations in sequences of measurements on single quantum systems faces two major problems. One is the experimental difficulty of performing sequences of ideal measurements on highdimensional quantum systems, a problem that also affects other forms of quantum temporal correlations. The other is the simulability with classical light of existing contextuality experiments with photons. Here, we introduce a scheme that solves both problems. We show that, by encoding quantum information in n 2 indistinguishable bosons in m 2 modes, targeting observables exploiting the bunching-antibunching of bosons, and performing ideal measurements by dispersively coupling these systems with auxiliary qubits, it is possible to realize sequential quantum measurements on high-dimensional quantum systems and produce contextuality which cannot be simulated by classical light, as it relies on indistinguishability and higher-order interference

Pruebas algebraicas de imposibilidad de variables ocultas en mecánica cuántica

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física Teórica I, leída el 12-07-1996Se estudian las demostraciones algebraicas de los dos teoremas de imposibilidad de variables ocultas en mecánica cuántica más relevantes: el teorema de Bell-Kochen-Specker (BKS) de imposibilidad de variables ocultas no-contextuales, y el teorema de Bell-EPR de imposibilidad de variables ocultas locales. En particular: (a) se presenta la demostración más sencilla conocida del teorema de bks. (B) se propone un nuevo tipo de demostraciones del teorema de bks. (C) se proponen dos métodos para generalizar las demostraciones de bks a espacios de dimensión arbitraria mayor que tres. (C) se investiga la relación entre las demostraciones del teorema de bks y las demostraciones algebraicas del teorema de bell-epr (de reciente aparición en la literatura) y se muestra como estas se pueden obtener de aquellas. (d) se extiende el teorema de bell-epr al caso en el que solo se suponen predefinido los resultados de experimentos que se pueden predecir con certeza a partir de otros experimentos efectivamente realizados (teorema de bell-epr con elementos de realidad fuertes). (e) se estudia la posible relevancia las demostraciones algebraicas para lograr test experimentales de imposibilidad de variables ocultas mas decisivos que los conocidos.Depto. de Física TeóricaFac. de Ciencias FísicasTRUEpu

Device-independent quantum key distribution based on Bell inequalities with more than two inputs and two outputs

Device-independent quantum key distribution (DI-QKD) offers the strongest form of security against eavesdroppers bounded by the laws of quantum mechanics. However, a practical implementation is still pending due to the requirement of combinations of visibility and detection efficiency that are beyond those possible with current technology. This mismatch motivates the search for DI-QKD protocols that can close the gap between theoretical and practical security. In this work, we present two DI-QKD protocols whose security relies on Bell inequalities with more than two inputs and two outputs. We show that, for maximally entangled states and perfect visibility, a protocol based on a Bell inequality with three inputs and four outputs requires a slightly lower detection efficiency than the protocols based on Bell inequalities with two inputs and two outputs

Probing the limits of correlations in an indivisible quantum system

We employ a trapped ion to study quantum contextual correlations in a single qutrit using the 5-observable Klyachko,Can,Binicio˘glu,andShumovskyinequality,whichisarguablythemostfundamentalnoncontextuality inequality for testing quantum mechanics (QM). We quantify the effect of systematics in our experiment by purposely scanning the degree of signaling between measurements, which allows us to place realistic bounds on the nonclassicality of the observed correlations. Our results violate the classical bound for this experiment by up to 25 standard deviations, while being in agreement with the QM limit. In order to test the prediction of QM that the contextual fraction increases with the number of observables, we gradually increase the complexity of our measurements from 5 up to 121 observables. We find stronger-than-classical correlations in all prepared scenarios up to 101 observables, beyond which experimental imperfections blur the quantum-classical divide

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].Junta de Andalucía P06-FQM-02243Ministerio de Ciencia e Innovación FIS2008-05596Junta de Andalucía PAI-FQM-0239Junta de Andalucía P06-FQM-01649Ministerio de Educación y Ciencia MTM2008-05866-C03-01Junta de Andalucía PAI-FQM-016

Sustained state-independent quantum contextual correlations from a single ion

We use a single trapped-ion qutrit to demonstrate the quantum-state-independent violation of noncontextuality inequalities using a sequence of randomly chosen quantum nondemolition projective measurements. We concatenate 53 × 106 sequential measurements of 13 observables, and unambiguously violate an optimal noncontextual bound. We use the same data set to characterize imperfections including signaling and repeatability of the measurements. The experimental sequence was generated in real time with a quantum random number generator integrated into our control system to select the subsequent observable with a latency below 50 μs, which can be used to constrain contextual hiddenvariable models that might describe our results. The state-recycling experimental procedure is resilient to noise and independent of the qutrit state, substantiating the fact that the contextual nature of quantum physics is connected to measurements and not necessarily to designated states. The use of extended sequences of quantum nondemolition measurements finds applications in the fields of sensing and quantum information

Quantum randomness protected against detection loophole attacks

Device and semi-device-independent private quantum randomness generators are crucial for applications requiring private randomness. However, they are vulnerable to detection inefficiency attacks and this limits severely their usage for practical purposes. Here, we present a method for protecting semi-device-independent private quantum randomness generators in prepare-and-measure scenarios against detection inefficiency attacks. The key idea is the introduction of a blocking device that adds failures in the communication between the preparation and measurement devices. We prove that, for any detection efficiency, there is a blocking rate that provides protection against these attacks. We experimentally demonstrate the generation of private randomness using weak coherent states and standard avalanche photo-detectors

Experimental observation of quantum state-independent contextuality under no-signaling conditions

Contextuality, the impossibility of assigning context-independent measurement outcomes, is a critical resource for quantum computation and communication. No-signaling betweensuccessivemeasurementsisanessentialrequirementthatshouldbeaccomplishedin anytestofquantumcontextualityandthatisdifficulttoachieveinpractice.Here,weintroduce anoptimalquantumstate-independentcontextualityinequalityinwhichthedeviationfromthe classicalboundismaximal.Wethenexperimentallytestitusingsinglephotonsgeneratedfrom a defect in a bulk silicon carbide, while satisfying the requirement of no-signaling within the experimental error. Our results shed new light on the study of quantum contextuality under no-signalingconditions

Self-testing of a single quantum system from theory to experiment

Self-testing allows one to characterise quantum systems under minimal assumptions. However, existing schemes rely on quantum nonlocality and cannot be applied to systems that are not entangled. Here, we introduce a robust method that achieves self-testing of individual systems by taking advantage of contextuality. The scheme is based on the simplest contextuality witness for the simplest contextual quantum system—the Klyachko-Can-Binicioğlu-Shumovsky inequality for the qutrit. We establish a lower bound on the fidelity of the state and the measurements as a function of the value of the witness under a pragmatic assumption on the measurements. We apply the method in an experiment on a single trapped 40Ca+ using randomly chosen measurements and perfect detection efficiency. Using the observed statistics, we obtain an experimental demonstration of self-testing of a single quantum system