Cosmic Bell Test: Measurement Settings from Milky Way Stars
Johannes Handsteiner, Andrew S. Friedman, Dominik Rauch, Jason Gallicchio*, Bo Liu, Hannes Hosp, Johannes Kofler, David Bricher, Matthias Fink, Calvin Leung, Anthony Mark, Hien T. Nguyen, Isabella Sanders, Fabian Steinlechner, Rupert Ursin, Soren Wengerowsky, Alan H. Guth, David I. Kaiser, Thomas Scheidl, and Anton Zeilinger
Physical Review Letters 118, 060401 (2017)
Abstract
Bell鈥檚 theorem states that some predictions of quantum mechanics cannot be reproduced by a local-realist theory. That conflict is expressed by Bell鈥檚 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 鈥渇reedom of choice鈥� was addressed by ensuring that selection of measurement settings via conventional 鈥渜uantum random number generators鈥� was spacelike 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鈥檚 inequality that, for the first time, uses distant astronomical sources as 鈥渃osmic setting generators.鈥� 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鈥檚 color was set at emission, we observe statistically significant 鈮�7.31鈦� and 鈮�11.93鈦� violations of Bell鈥檚 inequality with estimated values of 鈮�1.8脳10鈭�13 and 鈮�4.0 脳10鈭�33, respectively, thereby pushing back by 鈭�600 years the most recent time by which any local-realist influences could have engineered the observed Bell violation.