Cosmic Bell Test: Measurement Settings from Milky Way Stars

Bells theorem states that some predictions of quantum mechanics cannot be reproduced by a local-realist theory. That conflict is expressed by Bells 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 Bells 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 photons color was set at emission, we observe statistically significant $gtrsim 7.31 sigma$ and $gtrsim 11.93 sigma$ violations of Bells inequality with estimated $p$-values of $ lesssim 1.8 times 10^{-13}$ and $lesssim 4.0 times 10^{-33}$, respectively, thereby pushing back by $sim$600 years the most recent time by which any local-realist influences could have engineered the observed Bell violation.