View article

We describe the physical randomness generation strategy used in the loophole-free Bell tests of 2015. A system consisting of telecommunications lasers, detectors, interferometry, and fast analog and digital electronics produces analog signals with a large contribution traceable to spontaneous emission events, and bounded contribution from all other sources. Fast parity-bit randomness extraction is used to produce output bits with excess predictability below 10-5 due to spontaneous-emission events less than 36 ns in the past. This randomness generation strategy satisfies for the first time the stringent requirements identified in modern statistical analyses of loophole-free Bell tests.