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 $\gtrsim 7.31 \sigma$ and $\gtrsim 11.93
\sigma$ violations of Bell's 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.
