41. 
 Marques, Breno, et al.
(författare)

Experimental Observation of HardyLike Quantum Contextuality
 2014

Ingår i: Physical Review Letters.  00319007 . 10797114. ; 113:25

Tidskriftsartikel (refereegranskat)abstract
 <p>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.</p>


42. 
 Nagali, Eleonora, et al.
(författare)

Experimental Observation of ImpossibletoBeat Quantum Advantage on a Hybrid Photonic System
 2012

Ingår i: Physical Review Letters.  00319007 . 10797114. ; 108:9, s. 090501

Tidskriftsartikel (refereegranskat)abstract
 <p>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 impossibletobeat quantum advantage on a fourdimensional 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.</p>


43. 
 Pokorny, Fabian, et al.
(författare)

Tracking the Dynamics of an Ideal Quantum Measurement
 2020

Ingår i: Physical Review Letters.  00319007 . 10797114. ; 124:8

Tidskriftsartikel (refereegranskat)abstract
 <p>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%.</p>


44. 
 Sadiq, Muhammad, et al.
(författare)

Bell inequalities for the simplest exclusivity graph
 2013

Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics.  10502947 . 10941622. ; 87:1

Tidskriftsartikel (refereegranskat)abstract
 <p>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 quantumclassical gap. These are the most elementary logical Bell inequalities. We showthat the quantum violation of some wellknown Bell inequalities is related to them. We test the three Bell inequalities with pairs of polarizationentangled 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. DOI: 10.1103/PhysRevA.87.012128</p>


45. 
 Smania, Massimiliano, et al.
(författare)

Avoiding apparent signalling in Bell tests for quantum information applications
 ????

Annan publikation (övrigt vetenskapligt)abstract
 <p>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 polarizationentangled 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, nonsignalling. We present a test based on the deviation from the nonsignalling 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.</p>


46. 
 Smania, Massimiliano, et al.
(författare)

Experimental certification of an informationally complete quantum measurement in a deviceindependent protocol
 2020

Ingår i: Operator Theory : Advances and Applications.  10044469 . 23342536. ; 7:2, s. 123128

Tidskriftsartikel (refereegranskat)abstract
 <p>Minimal informationally complete positive operatorvalued measures (MICPOVMs) are special kinds of measurement in quantum theory in which the statistics of their d(2)outcomes are enough to reconstruct any ddimensional quantum state. For this reason, MICPOVMs 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 MICPOVM for qubits following a deviceindependent 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.</p>


47. 
 Tavakoli, Armin, et al.
(författare)

Quantum Clock Synchronization with a Single Qudit
 2015

Ingår i: Scientific Reports.  20452322 . 20452322. ; 5

Tidskriftsartikel (refereegranskat)abstract
 <p>Clock synchronization for nonfaulty processes in multiprocess networks is indispensable for a variety of technologies. A reliable system must be able to resynchronize the nonfaulty processes upon some components failing causing the distribution of incorrect or conflicting information in the network. The task of synchronizing such networks is related to Byzantine agreement (BA), which can classically be solved using recursive algorithms if and only if less than onethird of the processes are faulty. Here we introduce a nonrecursive quantum algorithm, based on a quantum solution of the detectable BA, which achieves clock synchronization in the presence of arbitrary many faulty processes by using only a single quantum system.</p>


48. 
 Tavakoli, Armin, et al.
(författare)

Quantum predictions for an unmeasured system cannot be simulated with a finitememory classical system
 2018

Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information.  24699926 . 24699934. ; 97:3

Tidskriftsartikel (refereegranskat)abstract
 <p>We consider an ideal experiment in which unlimited nonprojective quantum measurements are sequentially performed on a system that is initially entangled with a distant one. At each step of the sequence, the measurements are randomly chosen between two. However, regardless of which measurement is chosen or which outcome is obtained, the quantum state of the pair always remains entangled. We show that the classical simulation of the reduced state of the distant system requires not only unlimited rounds of communication, but also that the distant system has infinite memory. Otherwise, a thermodynamical argument predicts heating at a distance. Our proposal can be used for experimentally ruling out nonlocal finitememory classical models of quantum theory.</p>


49. 
 Vallone, Giuseppe, et al.
(författare)

Bell scenarios in which nonlocality and entanglement are inversely related
 2014

Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics.  American Physical Society.  10502947 . 10941622. ; 89:1, s. 012102

Tidskriftsartikel (refereegranskat)abstract
 <p>We show that for twoqubit chained Bell inequalities with an arbitrary number of measurement settings, nonlocality and entanglement are not only different properties but are inversely related. Specifically, we analytically prove that in absence of noise, robustness of nonlocality, defined as the maximum fraction of detection events that can be lost such that the remaining ones still do not admit a local model, and concurrence are inversely related for any chained Bell inequality with an arbitrary number of settings. The closer quantum states are to product states, the harder it is to reproduce quantum correlations with local models. We also show that, in presence of noise, nonlocality and entanglement are simultaneously maximized only when the noise level is equal to the maximum level tolerated by the inequality; in any other case, a more nonlocal state is always obtained by reducing the entanglement. In addition, we observed that robustness of nonlocality and concurrence are also inversely related for the Bell scenarios defined by the tight twoqubit threesetting I3322 inequality, and the tight twoqutrit inequality I3.</p>

