title: Cosmic Bell Test: Measurement Settings from Milky Way Stars.
abstract: 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 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'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.
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.
authors.forename: Johannes
authors.initials: J
authors.lastname: Handsteiner
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Andrew S
authors.initials: AS
authors.lastname: Friedman
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.
authors.forename: Dominik
authors.initials: D
authors.lastname: Rauch
authors.affiliation: Department of Physics, Harvey Mudd College, Claremont, California 91711, USA.
authors.forename: Jason
authors.initials: J
authors.lastname: Gallicchio
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.|School of Computer, NUDT, 410073 Changsha, China.
authors.forename: Bo
authors.initials: B
authors.lastname: Liu
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.
authors.forename: Hannes
authors.initials: H
authors.lastname: Hosp
authors.affiliation: Max Planck Institute of Quantum Optics, Hans-Kopfermann-Straße 1, 85748 Garching, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Kofler
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.
authors.forename: David
authors.initials: D
authors.lastname: Bricher
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.
authors.forename: Matthias
authors.initials: M
authors.lastname: Fink
authors.affiliation: Department of Physics, Harvey Mudd College, Claremont, California 91711, USA.
authors.forename: Calvin
authors.initials: C
authors.lastname: Leung
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Anthony
authors.initials: A
authors.lastname: Mark
authors.affiliation: NASA Jet Propulsion Laboratory, Pasadena, California 91109, USA.
authors.forename: Hien T
authors.initials: HT
authors.lastname: Nguyen
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Isabella
authors.initials: I
authors.lastname: Sanders
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.
authors.forename: Fabian
authors.initials: F
authors.lastname: Steinlechner
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.|Vienna Center for Quantum Science & Technology (VCQ), Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria.
authors.forename: Rupert
authors.initials: R
authors.lastname: Ursin
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.
authors.forename: Sören
authors.initials: S
authors.lastname: Wengerowsky
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: Alan H
authors.initials: AH
authors.lastname: Guth
authors.affiliation: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: David I
authors.initials: DI
authors.lastname: Kaiser
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Scheidl
authors.affiliation: Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria.|Vienna Center for Quantum Science & Technology (VCQ), Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Vienna, Austria.
authors.forename: Anton
authors.initials: A
authors.lastname: Zeilinger