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Register a new userExperimental evidence for bounds on quantum correlations
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We implemented the experiment proposed by Cabello [arXiv:quant-ph/0309172] to test the bounds of quantum correlation. As expected from the theory we found that, for certain choices of local observables, Cirelsons bound of the Clauser-Horne-Shimony-Holt inequality ($2sqrt{2}$) is not reached by any quantum states.
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In contrast with software-generated randomness (called pseudo-randomness), quantum randomness is provable incomputable, i.e. it is not exactly reproducible by any algorithm. We provide experimental evidence of incomputability --- an asymptotic property --- of quantum randomness by performing finite tests of randomness inspired by algorithmic information theory.
Quantum mechanics admits correlations that cannot be explained by local realistic models. Those most studied are the standard local hidden variable models, which satisfy the well-known Bell inequalities. To date, most works have focused on bipartite entangled systems. Here, we consider correlations between three parties connected via two independent entangled states. We investigate the new type of so-called bilocal models, which correspondingly involve two independent hidden variables. Such models describe scenarios that naturally arise in quantum networks, where several independent entanglement sources are employed. Using photonic qubits, we build such a linear three-node quantum network and demonstrate non-bilocal correlations by violating a Bell-like inequality tailored for bilocal models. Furthermore, we show that the demonstration of non-bilocality is more noise-tolerant than that of standard Bell non-locality in our three-party quantum network.
A unifying principle explaining the numerical bounds of quantum correlations remains elusive despite the efforts devoted to identifying it. Here we show that these bounds are indeed not exclusive to quantum theory: for any abstract correlation scenario with compatible measurements, models based on classical waves produce probability distributions indistinguishable from those of quantum theory and, therefore, share the same bounds. We demonstrate this finding by implementing classical microwaves that propagate along meter-size transmission-line circuits and reproduce the probabilities of three emblematic quantum experiments. Our results show that the quantum bounds would also occur in a classical universe without quanta. The implications of this observation are discussed.
Alternative theories to quantum mechanics motivate important fundamental tests of our understanding and descriptions of the smallest physical systems. Here, using spontaneous parametric downconversion as a heralded single-photon source, we place experimental limits on a class of alternative theories, consisting of classical field theories which result in power-dependent normalized correlation functions. In addition, we compare our results with standard quantum mechanical interpretations of our spontaneous parametric downconversion source over an order of magnitude in intensity. Our data match the quantum mechanical expectations, and do not show a statistically significant dependence on power, limiting on quantum mechanics alternatives which require power-dependent autocorrelation functions.
Quantum correlations represent a fundamental tool for studies ranging from basic science to quantum technologies. Different non-classical correlations have been identified and studied, as entanglement and discord. In view of future applications in this letter we explore experimentally the rich geometry of Bell-diagonal states, measuring the values of entanglement and discord and highlighting the effect of decoherence in real experiments.
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