Phys. Rev. Lett. 118, 140402 (2017)

Cosmic photons emitted by celestial objects can be used to create a highly effective optical random number generator (RNG) and perform improved tests of Bell's inequalities. Scientists in China pointed a 1-m-diameter telescope at a variety of cosmic radiation sources, ranging in brightness (magnitude between 4.85 and 13.5) and distance (between 756 and 7.49 × 108 light years), and used a CCD camera and a single-photon avalanche diode to detect the emission and measure the arrival time. As the generation time of the photons is random, it dictates that arrival time is as well. The tests took place at the Astronomy Observatory at Xinglong, China. Photon-counting signal rates were sufficient to generate raw random bits at a rate exceeding 106 s−1 that pass the standard NIST statistical tests, demonstrating that the method is not only effective but is as efficient as laser-based RNGs. By using two such telescope set-ups in different labs and combining their outputs it is possible to make a cosmic-based Bell test with greater confidence than usual against loopholes, a feat accomplished in a separate experiment by scientists in Vienna (J. Handsteiner et al., Phys. Rev. Lett. 118, 060401; 2017)10.1103/PhysRevLett.118.060401.