Einstein haaated the way entanglement worked, because it allowed interactions to travel much faster than light. He argued this made quantum mechanics an incomplete theory, because it violated at least one of two fundamental assumptions: locality, or the idea that to get from point A to B you have to pass all points in between, and realism, that things continue to exist when you’re not interacting with them.
Proving Einstein wrong has been a huge challenge, as the easiest approaches demand that entanglement persist over large enough distances to make measurement practical, and that a very high detection rate is maintained. While quite a few experiments have come close, only two seem to have dodged every loophole.
We present a loophole-free violation of local realism using entangled photon pairs. We ensure that all relevant events in our Bell test are spacelike separated by placing the parties far enough apart and by using fast random number generators and high-speed polarization measurements. A high-quality polarization-entangled source of photons, combined with high-efficiency, low-noise, single-photon detectors, allows us to make measurements without requiring any fair-sampling assumptions.
Jennifer Ouellette also has an excellent write-up. It looks like there are no more objections left to save local realism.
Using a hypothesis test, we compute p-values as small as 5.9×10−9 for our Bell violation while maintaining the spacelike separation of our events. We estimate the degree to which a local realistic system could predict our measurement choices. Accounting for this predictability, our smallest adjusted p-value is 2.3 × 10−7. We therefore reject the hypothesis that local realism governs our experiment.