| 41. |
- Kleinmann, Matthias, et al.
(författare)
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Optimal Inequalities for State-Independent Contextuality
- 2012
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 109:25, s. 250402-
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Tidskriftsartikel (refereegranskat)abstract
- Contextuality is a natural generalization of nonlocality which does not need composite systems or spacelike separation and offers a wider spectrum of interesting phenomena. Most notably, in quantum mechanics there exist scenarios where the contextual behavior is independent of the quantum state. We show that the quest for an optimal inequality separating quantum from classical noncontextual correlations in a state-independent manner admits an exact solution, as it can be formulated as a linear program. We introduce the noncontextuality polytope as a generalization of the locality polytope and apply our method to identify two different tight optimal inequalities for the most fundamental quantum scenario with state-independent contextuality.
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| 42. |
- Kolenderski, Piotr, et al.
(författare)
-
Aharon-Vaidman quantum game with a Young-type photonic qutrit
- 2012
-
Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 86:1, s. 012321-
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Tidskriftsartikel (refereegranskat)abstract
- The Aharon-Vaidman (AV) game exemplifies the advantage of using simple quantum systems to outperform classical strategies. We present an experimental test of this advantage by using a three-state quantum system (qutrit) encoded in a spatial mode of a single photon passing through three slits. The preparation of a particular state is controlled as the photon propagates through the slits by varying the number of open slits and their respective phases. The measurements are achieved by placing detectors in the specific positions in the near and far fields after the slits. This set of tools allowed us to perform tomographic reconstructions of generalized qutrit states, and to implement the quantum version of the AV game with compelling evidence of the quantum advantage.
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| 43. |
- Larsson, Jan-Åke, et al.
(författare)
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Maximal violation of state-independent contextuality inequalities
- 2012
-
Ingår i: Quantum Theory: Reconsideration of Foundations - 6. - : American Institute of Physics (AIP). - 9780735411265 ; , s. 265-274
-
Konferensbidrag (refereegranskat)abstract
- The discussion on noncontextual hidden variable models as an underlying description for the quantum-mechanical predictions started in ernest with 1967 paper by Kochen and Specker. There, it was shown that no noncontextual hidden-variable model can give these predictions. The proof used in that paper is complicated, but recently, a paper by Yu and Oh [PRL, 2012] proposes a simpler statistical proof that can also be the basis of an experimental test. Here we report on a sharper version of that statistical proof, and also explain why the algebraic upper bound to the expressions used are not reachable, even with a reasonable contextual hidden variable model. Specifically, we show that the quantum mechanical predictions reach the maximal possible value for a contextual model that keeps the expectation value of the measurement outcomes constant.
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| 44. |
- Lisonek, Petr, et al.
(författare)
-
Kochen-Specker set with seven contexts
- 2014
-
Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 89:4, s. 042101-
-
Tidskriftsartikel (refereegranskat)abstract
- The Kochen-Specker (KS) theorem is a central result in quantum theory and has applications in quantum information. Its proof requires several yes-no tests that can be grouped in contexts or subsets of jointly measurable tests. Arguably, the best measure of simplicity of a KS set is the number of contexts. The smaller this number is, the smaller the number of experiments needed to reveal the conflict between quantum theory and noncontextual theories and to get a quantum vs classical outperformance. The original KS set had 132 contexts. Here we introduce a KS set with seven contexts and prove that this is the simplest KS set that admits a symmetric parity proof.
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| 45. |
- Marques, Breno, et al.
(författare)
-
Experimental Observation of Hardy-Like Quantum Contextuality
- 2014
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 113:25
-
Tidskriftsartikel (refereegranskat)abstract
- Contextuality is a fundamental property of quantum theory and a critical resource for quantum computation. Here, we experimentally observe the arguably cleanest form of contextuality in quantum theory [A. Cabello et al., Phys. Rev. Lett. 111, 180404 (2013)] by implementing a novel method for performing two sequential measurements on heralded photons. This method opens the door to a variety of fundamental experiments and applications.
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| 46. |
- Miklin, Nikolai, et al.
(författare)
-
Exponentially Decreasing Critical Detection Efficiency for Any Bell Inequality
- 2022
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 129:23
-
Tidskriftsartikel (refereegranskat)abstract
- We address the problem of closing the detection efficiency loophole in Bell experiments, which is crucial for real-world applications. Every Bell inequality has a critical detection efficiency η that must be surpassed to avoid the detection loophole. Here, we propose a general method for reducing the critical detection efficiency of any Bell inequality to arbitrary low values. This is accomplished by entangling two particles in N orthogonal subspaces (e.g., N degrees of freedom) and conducting N Bell tests in parallel. Furthermore, the proposed method is based on the introduction of penalized N-product (PNP) Bell inequalities, for which the so-called simultaneous measurement loophole is closed, and the maximum value for local hidden-variable theories is simply the Nth power of the one of the Bell inequality initially considered. We show that, for the PNP Bell inequalities, the critical detection efficiency decays exponentially with N. The strength of our method is illustrated with a detailed study of the PNP Bell inequalities resulting from the Clauser-Horne-Shimony-Holt inequality.
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| 47. |
- Mironowicz, Piotr, et al.
(författare)
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Quantum randomness protected against detection loophole attacks
- 2021
-
Ingår i: Quantum Information Processing. - : SPRINGER. - 1570-0755 .- 1573-1332. ; 20:1
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 48. |
- Nagali, Eleonora, et al.
(författare)
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Experimental Observation of Impossible-to-Beat Quantum Advantage on a Hybrid Photonic System
- 2012
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 108:9, s. 090501-
-
Tidskriftsartikel (refereegranskat)abstract
- Quantum resources outperform classical ones for certain communication and computational tasks. Remarkably, in some cases, the quantum advantage cannot be improved using hypothetical postquantum resources. A class of tasks with this property can be singled out using graph theory. Here we report the experimental observation of an impossible-to-beat quantum advantage on a four-dimensional quantum system defined by the polarization and orbital angular momentum of a single photon. The results show pristine evidence of the quantum advantage and are compatible with the maximum advantage allowed using postquantum resources.
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| 49. |
- Pokorny, Fabian, et al.
(författare)
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Tracking the Dynamics of an Ideal Quantum Measurement
- 2020
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 124:8
-
Tidskriftsartikel (refereegranskat)abstract
- The existence of ideal quantum measurements is one of the fundamental predictions of quantum mechanics. In theory, an ideal measurement projects a quantum state onto the eigenbasis of the measurement observable, while preserving coherences between eigenstates that have the same eigenvalue. The question arises whether there are processes in nature that correspond to such ideal quantum measurements and how such processes are dynamically implemented in nature. Here we address this question and present experimental results monitoring the dynamics of a naturally occurring measurement process: the coupling of a trapped ion qutrit to the photon environment. By taking tomographic snapshots during the detection process, we show that the process develops in agreement with the model of an ideal quantum measurement with an average fidelity of 94%.
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| 50. |
- Sadiq, Muhammad, et al.
(författare)
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Bell inequalities for the simplest exclusivity graph
- 2013
-
Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 87:1
-
Tidskriftsartikel (refereegranskat)abstract
- Which is the simplest logical structure for which there is quantum nonlocality? We show that there are only three bipartite Bell inequalities with quantum violation associated with the simplest graph of relationships of exclusivitywith a quantum-classical gap. These are the most elementary logical Bell inequalities. We showthat the quantum violation of some well-known Bell inequalities is related to them. We test the three Bell inequalities with pairs of polarization-entangled photons and report violations in good agreement with the quantum predictions. Unlike other experiments testing noncontextuality inequalities with pentagonal exclusivity, the ones reported here are free of the compatibility loophole.
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