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Register a new userEquivalence of the quantumness of sequential correlations and spatial correlations
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Quantum information protocols can be realized using the `prepare and measure setups which do not require sharing quantum correlated particles. In this work, we study the equivalence between the quantumness in a prepare and measure scenario involving independent devices, which implements quantum random number generation, and the quantumness in the corresponding scenario which realizes the same task with spatially separated correlated particles. In particular, we demonstrate that quantumness of sequential correlations observed in the prepare and measure scenario gets manifested as superunsteerability, which is a particular kind of spatial quantum correlation in the presence of limited shared randomness. In this scenario consisting of spatially separated quantum correlated particles as resource for implementing the quantum random number generation protocol, we define an experimentally measurable quantity which provides a bound on the amount of genuine randomness generation. Next, we study the equivalence between the quantumness of the prepare and measure scenario in the presence of shared randomness, which has been used for implementing quantum random-access codes, and the quantumness in the corresponding scenario which replaces quantum communication by spatially separated quantum correlated particles. In this case, we demonstrate that certain sequential correlations in the prepare and measure scenario in the presence of shared randomness, which have quantumness but do not provide advantage for random-access codes, can be used to provide advantage when they are realized as spatial correlations in the presence of limited shared randomness. We point out that these spatial correlations are superlocal correlations, which are another kind of spatial quantum correlations in the presence of limited shared randomness, and identify inequalities detecting superlocality.
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The quantification of quantum correlations (other than entanglement) usually entails laboured numerical optimization procedures also demanding quantum state tomographic methods. Thus it is interesting to have a laboratory friendly witness for the nature of correlations. In this Letter we report a direct experimental implementation of such a witness in a room temperature nuclear magnetic resonance system. In our experiment the nature of correlations is revealed by performing only few local magnetization measurements. We also compare the witness results with those for the symmetric quantum discord and we obtained a fairly good agreement.
Random access codes are important for a wide range of applications in quantum information. However, their implementation with quantum theory can be made in two very different ways: (i) by distributing data with strong spatial correlations violating a Bell inequality, or (ii) using quantum communication channels to create stronger-than-classical sequential correlations between state preparation and measurement outcome. Here, we study this duality of the quantum realization. We present a family of Bell inequalities tailored to the task at hand and study their quantum violations. Remarkably, we show that the use of spatial and sequential quantum correlations imposes different limitations on the performance of quantum random access codes. We also show that there exist random access codes for which spatial quantum correlations offer no gain over classical strategies, whereas sequential quantum correlations can yield an advantage. We discuss the physics behind the observed discrepancy between spatial and sequential quantum correlations.
We address the problem of whether parties who cannot communicate but share nonsignaling quantum correlations between the outcomes of sharp measurements can distinguish, just from the value of a correlation observable, whether their outcomes were produced by sequential compatible measurements on single systems or by measurements on spatially separated subsystems. We show that there are quantum correlations between the outcomes of sequential measurements which cannot be attained with spatially separated systems. We present examples of correlations between spatially separated systems whose quantum maximum tends to the sequential maximum as the number of parties increases and examples of correlations between spatially separated systems whose quantum maximum fails to violate the noncontextual bound while its corresponding sequential version does.
Nuclear Magnetic Resonance (NMR) was successfully employed to test several protocols and ideas in Quantum Information Science. In most of these implementations the existence of entanglement was ruled out. This fact introduced concerns and questions about the quantum nature of such bench tests. In this article we address some issues related to the non-classical aspects of NMR systems. We discuss some experiments where the quantum aspects of this system are supported by quantum correlations of separable states. Such quantumness, beyond the entanglement-separability paradigm, is revealed via a departure between the quantum and the classic
We provide experimental evidence of quantum features in bi-partite states classified as entirely classical according to a conventional criterion based on the Glauber P-function but possessing non-zero Gaussian quantum discord. Their quantum nature is experimentally revealed by acting locally on one part of the discordant state. Adding an environmental system purifying the state, we unveil the flow of quantum correlations within a global pure system using the Koashi-Winter inequality. We experimentally verify and investigate the counterintuitive effect of discord increase under the action of local loss and link it to the entanglement with the environment. For a discordant state generated by splitting a state in which the initial squeezing is destroyed by random displacements, we demonstrate the recovery of entanglement highlighting the role of system-environment correlations.
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