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- Smania, Massimiliano, et al.
(författare)
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Avoiding apparent signalling in Bell tests for quantum information applications
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Bell tests have become a powerful tool for checking security, quantifying randomness, detecting entanglement, and many other applications, as well as for investigating fundamental physical limits. Most Bell experiments make the assumptions of fair sampling and equal detection efficiency, and some also the assumption of setting reproducibility. Here, we point out that there are typical experimental imperfections and practices that lead to the violation of these assumptions and can go unnoticed. This is a problem that can invalidate the conclusions of many past and future Bell experiments and allow for malicious attacks. Detecting, quantifying, and fixing this problem is therefore of fundamental importance, especially for modern applications where the experimental values are used to reach quantitative conclusions about security, randomness, or entanglement. To illustrate the issue and its causes, we present a set of Bell experiments using polarization-entangled photons, where we identify common imperfections and practices that cause the failure of the assumptions. These experiments tell us which methods we should avoid and to which aspects of the setup we should pay special attention. We show that the failure of these assumptions results in a violation of a fundamental requirement in any Bell experiment, namely, non-signalling. We present a test based on the deviation from the non-signalling conditions that quantify and help us to fix the problem. We emphasize that adopting the measures and conducting the tests suggested here is necessary in order to obtain reliable conclusions in modern quantum information applications based on Bell tests.
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- Smania, Massimiliano, 1990-
(författare)
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Bell tests and the no-signalling assumption
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Bell's theorem was originally meant for testing fundamental properties of Nature, namely local realism. However, through the years it has become a powerful device for certifying encryption security, randomness, and entanglement among other properties. Especially after the series of loophole-free violation papers at the end of 2015, these applications are becoming more and more relevant. In most of these scenarios, additional assumptions - as fair-sampling - are set forth in order to achieve the required near-optimal violations. However, it turns out that many of the experiments realised so far suffer from apparent signalling. We say “apparent” because we do not believe that the issue comes from actual communication between different measurement stations, but rather, as we show in this work, from systematic issues related to the particular experimental realisation.After making a point for the importance of correcting these errors, we identify and address some of the most common sources of signalling in a set of experiments based on single photon polarisation qubits. Finally, we report a reliable CHSH violation which is free from apparent signalling.While the core results are contained in the attached Paper I, we first provide the reader with a brief introduction to the concepts involved in this work, in addition to supplementary unpublished experimental work.
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| 4. |
- Smania, Massimiliano, et al.
(författare)
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Experimental certification of an informationally complete quantum measurement in a device-independent protocol
- 2020
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Ingår i: Optica. - 2334-2536. ; 7:2, s. 123-128
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Tidskriftsartikel (refereegranskat)abstract
- Minimal informationally complete positive operator-valued measures (MIC-POVMs) are special kinds of measurement in quantum theory in which the statistics of their d(2)-outcomes are enough to reconstruct any d-dimensional quantum state. For this reason, MIC-POVMs are referred to as standard measurements for quantum information. Here, we report an experiment with entangled photon pairs that certifies, for what we believe is the first time, a MIC-POVM for qubits following a device-independent protocol (i.e., modeling the state preparation and the measurement devices as black boxes, and using only the statistics of the inputs and outputs). Our certification is achieved under the assumption of freedom of choice, no communication, and fair sampling.
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| 5. |
- Smania, Massimiliano, et al.
(författare)
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Experimental quantum multiparty communication protocols
- 2016
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Ingår i: npj Quantum Information. - : Springer Science and Business Media LLC. - 2056-6387. ; 2
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Tidskriftsartikel (refereegranskat)abstract
- Quantum information science breaks limitations of conventional information transfer, cryptography and computation by using quantum superpositions or entanglement as resources for information processing. Here we report on the experimental realisation of three-party quantum communication protocols using single three-level quantum system (qutrit) communication: secret-sharing, detectable Byzantine agreement and communication complexity reduction for a three-valued function. We have implemented these three schemes using the same optical fibre interferometric setup. Our realisation is easily scalable without compromising on detection efficiency or generating extremely complex many-particle entangled states.
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| 6. |
- Smania, Massimiliano, 1990-
(författare)
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Photonic multipartite entanglement : Generation, measurement and applications
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We are currently witnessing a fundamental change in the field of quantum information, whereby protocols and experiments previously performed in university labs are now being implemented in real-world scenarios, and a strong commercial push for new and reliable applications is contributing significantly in advancing fundamental research. In this thesis and related included papers, I first look at a keystone of quantum science, Bell's theorem. In particular, I will expose an issue that we call apparent signalling, which affects many current and past experiments relying on Bell tests. A statistical test of the impact of apparent signalling is described, together with experimental approaches to successfully mitigate it. Next, I consider one of the most refined ideas that recently emerged in quantum information, device-independent certification. Device-independent quantum information aims at answering the question: "Assuming we trust quantum mechanics, what can we conclude about the quantum systems or the measurement operators in a given experiment, based solely on its results, while making minimal assumptions on the physical devices used?". In my work, the problem was successfully approached in two different scenarios, one based on entangled photons and the other on prepare-and-measure experiments with single photons, with the aim of certifying informationally-complete quantum measurements. Finally, I conclude by presenting an elegant and promising approach to the experimental generation of multi-photon entanglement, which is a fundamental prerequisite in most modern quantum information protocols.
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| 7. |
- Tavakoli, Armin, et al.
(författare)
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Self-testing nonprojective quantum measurements in prepare-and-measure experiments
- 2020
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 6:16
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Tidskriftsartikel (refereegranskat)abstract
- Self-testing represents the strongest form of certification of a quantum system. Here, we theoretically and experimentally investigate self-testing of nonprojective quantum measurements. That is, how can one certify, from observed data only, that an uncharacterized measurement device implements a desired nonprojective positive-operator valued measure (POVM).We consider a prepare-and-measure scenario with a bound on the Hilbert space dimension and develop methods for (i) robustly self-testing extremal qubit POVMs and (ii) certifying that an uncharacterized qubit measurement is nonprojective. Our methods are robust to noise and thus applicable in practice, as we demonstrate in a photonic experiment. Specifically, we show that our experimental data imply that the implemented measurements are very close to certain ideal three- and four-outcome qubit POVMs and hence non-projective. In the latter case, the data certify a genuine four-outcome qubit POVM. Our results open interesting perspective for semi-device-independent certification of quantum devices.
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