Physicists address loophole in tests of Bell's inequality

Physicists address loophole in tests of Bell's inequality using 600-year-old starlight
February 7, 2017 by Jennifer Chu

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Physicists from MIT, the University of Vienna, and elsewhere have presented a strong demonstration of quantum entanglement even when vulnerability to the freedom-of-choice loophole is significantly restricted. Credit: Christine Daniloff/MIT
Quantum entanglement may appear to be closer to science fiction than anything in our physical reality. But according to the laws of quantum mechanics—a branch of physics that describes the world at the scale of atoms and subatomic particles—quantum entanglement, which Einstein once skeptically viewed as "spooky action at a distance," is, in fact, real.

Imagine two specks of dust at opposite ends of the universe, separated by several billion light years. Quantum theory predicts that, regardless of the vast distance separating them, these two particles can be entangled. That is, any measurement made on one will instantaneously convey information about the outcome of a future measurement on its partner. In that case, the outcomes of measurements on each member of the pair can become highly correlated.

Read more at: https://phys.org/news/2017-02-physicists-loophole-bell-inequality-year-old.html#jCp

 

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the paper:
https://arxiv.org/pdf/1611.06985v2.pdf

Cosmic Bell Test: Measurement Settings from Milky Way Stars


Bell’s theorem states that some predictions of quantum mechanics cannot be reproduced by a local-realist theory. That conflict is expressed by Bell’s inequality, which is usually derived under the assumption that there are no statistical correlations between the choices of measurement settings and anything else that can causally affect the measurement outcomes. In previous experiments, this “freedom of choice” was addressed by ensuring that selection of measurement settings via conventional “quantum random number generators” was space-like separated from the entangled particle creation. This, however, left open the possibility that an unknown cause affected both the setting choices and measurement outcomes as recently as mere microseconds before each experimental trial. Here we report on a new experimental test of Bell’s inequality that, for the first time, uses distant astronomical sources as “cosmic setting generators.” In our tests with polarization-entangled photons, measurement settings were chosen using real-time observations of Milky Way stars while simultaneously ensuring locality. Assuming fair sampling for all detected photons, and that each stellar photon’s color was set at emission, we observe statistically significant & 7.31σ and & 11.93σ violations of Bell’s inequality with estimated p-values of . 1.8 × 10−13 and . 4.0 × 10−33, respectively, thereby pushing back by ∼600 years the most recent time by which any local-realist influences could have engineered the observed Bell violation.

 

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