Cosmic Bell Experiments
An important fundamental issue in all previous Bell experiments is the quality of the random numbers, which are used to decide the setting parameter on both photons independently. In our “loophole-free” experiment , we did this with high quality random number generators , which, to the best of our knowledge, are among the best random number generators currently available. Yet, as has been pointed out in the early days of this experiment, such random number generators could in principle still be influenced by some very recent event in their common past. In that sense, it is important to look for the most independent possible sources of randomness available in the Universe.
It has been suggested very early to use the light from quasars at opposite locations in the sky to choose between the different measurement settings in a Bell experiment and a related explicit proposal for such a “Cosmic Bell” test was published in 2014 . One of the ideas is that there are regions of the Universe, which, due to the inflationary early phase of the expansion of the Universe, are causally disconnected in the sense that they could not have been in any causal connection except at the time before inflation [Alan Guth, private communication].
In a recent publication , our group now reports on the first experimental implementation of the proposed „Cosmic Bell“ test. In the experiment, which was conducted in collaboration with the MIT group of Kaiser and Guth, we performed tests with polarization-entangled photons while using distant astronomical sources as “cosmic setting generators.” Thereby, measurement settings were chosen based on the color of photons detected during real-time observations of Milky Way stars. While we simultaneously ensured locality we assumed fair sampling for all detected photons, and that each stellar photon’s color was set at emission. We observed statistically significant violations of Bell’ inequality, thereby pushing back by approximately 600 years the most recent time by which any local-realist influences could have engineered the observed Bell violation.
Our recent experiment is the first along a cosmic distance ladder, starting with stars in our galaxy, then using light from nearby galaxies etc., all the way to the faintest and farthest quasars which can reasonably be employed.
 M. Giustina, M. A. M. Versteegh, S. Wengerowsky, J. Handsteiner, A. Hochrainer, K. Phelan, F. Steinlechner, J. Kofler, J.-Å. Larsson, C. Abellán, W. Amaya, V. Pruneri, M. W. Mitchell, J. Beyer, T. Gerrits, A. E. Lita, L. K. Shalm, S. W. Nam, T. Scheidl, R. Ursin, B. Wittmann, and A. Zeilinger, Significant-Loophole-Free Test of Bell’s Theorem with Entangled Photons, Phys. Rev. Lett. 115, 250401 (2015).
 M. W. Mitchell, C. Abellan, W. Amaya, Strong experimental guarantees in ultrafast quantum random number generation, Phys. Rev. A 91, 012314 (2015).
 J. Gallicchio, A. S. Friedman, D. I. Kaiser, Testing Bell’s Inequality with Cosmic Photons: Closing the Setting-Independence Loophole, Phys. Rev. Lett. 112, 110405 (2014).
 J. Handsteiner, A. S. Friedman, D. Rauch, J. Gallicchio, B. Liu, H. Hosp, J. Kofler, D. Bricher, M. Fink, C. Leung, A. Mark, H. T. Nguyen, I. Sanders, F. Steinlechner, R. Ursin, S. Wengerowsky, A. H. Guth, D. I. Kaiser, T. Scheidl, and A. Zeilinger, Cosmic Bell Test: Measurement Settings from Milky Way Stars, Phys. Rev. Lett. 118, 060401 (2017).