Towards a Loophole-Free Bell's Inequality Experiment

Carlos Henrique Monken, P. H. Souto Ribeiro, Sebastião Pádua

We show that the photon pair collection efficiency in coincidence measurements can be maximized when the two-photon state is generated in the process of spontaneous parametric down-conversion in a thin crystal. This effect can be used to close part of the so-called detection loophole in Bell's inequalities experiments. The effect is also demonstrated experimentally.

Jean Marie Vigoureux

Gonzalo Carvacho, Jaime Cariñe, Gabriel Saavedra +9 more

Device-independent quantum communication will require a loophole-free violation of Bell inequalities. In typical scenarios where line of sight between the communicating parties is not available, it is convenient to use energy-time entangled photons due to intrinsic robustness while propagating over optical fibers. Here we show an energy-time Clauser-Horne-Shimony-Holt Bell inequality violation with two parties separated by 3.7 km over the deployed optical fiber network belonging to the University of Concepción in Chile. Remarkably, this is the first Bell violation with spatially separated parties that is free of the postselection loophole, which affected all previous in-field long-distance energy-time experiments. Our work takes a further step towards a fiber-based loophole-free Bell test, which is highly desired for secure quantum communication due to the widespread existing telecommunication infrastructure.

Mario Iannuzzi, Vittorio Merlo

M. A. Rowe, D. Kielpinski, V. Meyer +4 more

Jaskaran Singh, Adán Cabello

There are bipartite quantum nonlocal correlations requiring very low detection efficiency to reach the loophole-free regime but that need too many measurement settings to be practical for actual experiments. This leads to the general problem of what can be concluded about loophole-free Bell nonlocality if only a random subset of these settings is tested. Here we develop a method to address this problem. We show that, in some cases, it is possible to detect loophole-free Bell nonlocality by testing only a small random fraction of the settings. The consequence is a higher detection efficiency. The method allows for the design of loophole-free Bell tests in which, given a quantum correlation that violates a Bell inequality, one can calculate the minimum fraction of contexts needed to reach the detection-loophole-free regime. The results also enforce a different way of thinking about how local realistic models or classical communication can be used to simulate quantum nonlocal correlations, as it shows that the amount of resources that are needed can be made arbitrarily large simply by considering more contexts.